The living planet: putting our knowledge of plate tectonics to work

Impact N o . 145

3 Comment

5 Plate tectonics: a framework for understanding our living planet

José Achache

21 Heat flow of the earth and geothermal resources

Valiya M. Hamza

33 Continental growth and collision, and mineral prospection, in Southeast Asia Charles S. Hutchison

45 Sedimentary basins, plate tectonics and oil fields Bruce Sellwood

55 Volcanoes and m e n Claude Jaupart

63 Seismicity of Japan: earthquakes and tsunamis

Katsuyuki Abe

75 Earthquake-proof construction and architecture Ananá S. Ar y a

89 Deep continental drilling on the Kola peninsula and the structure of the Earth's crust Olea L. Kuznetsov

97 Readers' forum

99 Erratum—Impact 142

Reminder to readers

Impact of science on society is also published in Arabic, Chinese, French, Korean, Russian and Spanish. Information about these editions can be obtained by writing to the following:

Arabic: Unesco Publications Centre in Cairo, 1 Talaat Harb Street, Cairo, Arab Republic of Egypt.

Chinese: Institute of Policy and Management, Chinese Academy of Sciences, P . O . Box 821, Beijing, People's Republic of China.

French: Editions Eres, 19 rue Gustave-Courbet, 31400 Toulouse, France. Korean: Republic of Korea National Commission for Unesco, P . O . Box Central 84,

Seoul, Republic of Korea. Russian: The U S S R State Committee for Publishing, c/o the U S S R National

Commission for Unesco, 9 Prospekt Kalinina, M o s c o w G-19, U S S R . Spanish: Universidad de Salamanca, Secretariado de Publicaciones, Intercambio

Cientifico, Apartado 325, Salamanca, Spain.

Authors are responsible for the choice and the presentation of the facts contained in signed articles and for the opinions expressed therein, which are not necessarily those of Unesco and do not commit the organization.

Published texts may be reproduced and translated free of charge (except when reproduction or translation rights are reserved), provided that mention is made of the author and source. An entire issue may not be reproduced as a whole without the authorization of Unesco.

Comment

For centuries, m a n has observed seismic and volcanic activity and frequently suffered from it. Early reports of natural disasters m a y , in fact, be considered as being a m o n g the first scientific geophysical observations. It was not until 50 years ago, however, that the mechanisms responsible for this activity began to be understood. Until then, it had seemed as if natural disasters could occur anywhere at any m o m e n t on earth.

But m a n has also lived and produced at the earth's expense. In the nineteenth century, the development of industry underlined the need for a more systematic extraction of mineral resources. Prospecting at that time was still largely performed by trial and error methods, since there were few guidelines on the distribution of ore deposits and coal measures.

This situation has changed after the emergence during the second half of this century of the basic principles of plate tectonics. Earth sciences have evolved toward a global understanding of our planet, from the kinematics and dynamics of surface phenomena to the thermal, mechanical and chemical couplings between the core, the mantle, the crust, the ocean and the atmosphere. With this new perspective, the distribution of active zones and natural resources at the surface of the earth can be accounted for by simple mechanisms.

As a consequence, the numerous disciplines of geophysics and geology could no longer work side by side and ignore one another. They had become the tools of every earth scientist willing to study and understand natural phenomena on earth. For instance, understanding the structure and, hence, the mechanical behaviour of the lithosphère, which is the first step towards earthquake risk assessment, requires the analysis of m a n y different parameters such as seismic velocities, gravity, topography, tectonic analyses on various scales, heat flux and magnetics.

In the first article of this issue, we present the basic principles of plate tectonics in the context of their discovery. W e show that, on a global scale, this theory provides the appropriate framework for the study of natural phenomena.

V . M . H a m z a pictures the earth as a large heat engine—a nuclear power plant would be more accurate—which provides the required energy to drive plate tectonics and all its related surface phenomena. H e , then, gives a global presentation of geothermal resources which can be diverted for man ' s use. The fundamental importance of heat on earth is further illustrated by its role in the formation of mineral resources.

After they have formed, mineral deposits are transported by plate motion. C . S. Hutchison shows h o w plate tectonics leads to the concentration of these deposits along belts such as subduction-related volcanic arcs and intracontinental collision zones, which are particularly developed in Southeast Asia.

3 Impact of science on society, no. 145. 3 4

Comment

B . Sellwood describes the conditions necessary for the formation of hydrocarbon resources. Although the succession of these conditions is rather stringent, they can be accounted for by plate tectonics in several environments. Such a theory, therefore, provides a guideline to assess the global distribution of such reserves.

Volcanic eruptions are the most spectacular evidence of the internal activity of the earth. In the fifth article C . Jaupart describes the main characteristics of volcanoes on earth and analyses their impact on society. Surprisingly, he shows that volcanoes are not solely a source of destruction but m a y sometimes promote the development of society.

K . A b e gives a detailed account of the seismicity of Japan and the occurrence of tsunamis generated, in some instances, by underwater earthquakes. H e shows h o w this activity is distributed with respect to subduction zones, a fundamental feature of plate tectonics.

O u r current understanding of plate tectonics gives us a clear knowledge of the global distribution and frequency of earthquakes. It is, however, still not possible to accurately forecast individual events, particularly in continental areas. A . S. Arya shows that, while prediction can be of little use in earthquake mitigation, w e n o w have the capacity to design and build earthquake-proof buildings.

In the final article, O . L . Kuznetsov reports on the deep drilling project undertaken by the Soviet Union in the Kola Peninsula. This work illustrates the basic limitation of direct investigations within the earth. However, it shows that even at rather shallow depth, in-situ observations m a y sometimes depart from remote sensing and modelling results universally used in the earth sciences.

José Achache

4

Plate tectonics: a framework for understanding our living planet

José Achache

The development of new concepts in earth sciences has grown rapidly in the course of the second half of this century and has shaken most of our previous ideas about the earth. It eventually led to the layout of the theory of plate tectonics. Seismic, volcanic and tectonic features observed at the surface of the planet are now seen as a consequence of intense internal activity, and their investigation can no longer be separated from the study of the internal structure of the earth. Such a global approach provides a powerful framework for the understanding of catastrophic natural phenomena and the carrying out of prospecting for natural resources in a more efficient way.

The earth has long been seen as an ageing planet with localized, random signs of activity, its geological structure inherited from the past and fixed for eternity. This static view of the earth stemmed from the difficulty of prospecting the whole surface of the planet, in particular ocean basins, and of observing it on a global scale. O n the contrary, the picture emerging today is that of a slowly but continuously evolving planet. The first evidence for this maintained internal activity is given by the distribution of altitudes over the surface of the earth. Indeed, topography results from the combined action of internal activity, which creates relief, and erosion. If the earth were no longer active, mountains would tend to be eroded, thus filling valleys and ocean basins with sediments, and in time the average altitude would tend to be zero. The reality is quite different, since an analysis of the globe shows a bimodal distribution of m e a n altitudes, with two m a x i m a at —4500 metres (the m e a n depth of the oceans) and + 100 metres (the m e a n elevation of continents) (see figure 1). This topography must then be dynamically maintained and attests to the earth being a living planet.

But observers do not have access to the interior of the earth and all the studies of the internal processes and structures must rely on indirect observations. This, too, has been a major obstacle to the understanding of our planet, and has often linked progress in the earth sciences with technological advances. The discovery of large-scale structures such as the tectonic plates appeared as a direct consequence of the extensive survey of the ocean bottom performed after the Second World W a r by modern and well-equipped océanographie ships. The worldwide distribution of earthquake epicentres

José Achache is Chargé de Recherches at the Instituí de Physique du Globe de Pans. His current work

involves the analysis of satellite measurements of the earth"s magnetic field, with a particular emphasis on the

field of crustal origin and its implications for the determination of the deep structure of the continental crust.

H e m a y be contacted at the following address: Department of Geomagnetism and Paleomagnetism, Institut

de Physique du Globe de Paris. 4, place Jussieu, 75252 Paris, France.

5 Impact of science on society, no. 145. 5 19

José Achachc

lii/urc I.

Graph showing percentage of the surface of the earth at various altitudes. Peak A corresponds to the mean altitude of the continents, Peak B to the mean depth of the oceans. After Allèque (1984).

was obtained with the development of a new generation of highly sensitive seis­mometers. M o r e recently, the advent of space techniques has m a d e possible the observation of the earth on a truly global scale. Space techniques provide a remarkable insight into the deep interior of the earth, allow the constant monitoring of crustal motions and constitute a new means of prospecting for natural resources, on a planetary scale.

T h e stable and stratified body pictured by nineteenth-century scientists in n o w replaced by a unified system in which surface motions are coupled with internal processes on large scales both in space and time. O n e cannot understand surface phenomena on earth without studying the interior of the planet. This intimate relationship between surface geology and internal geophysics and geochemistry has emerged as the basic principle of mode rn earth sciences.

F r o m continental drift to plate tectonics

Continental drift

At the beginning of this century, Alfred Wegener, a G e r m a n meteorologist, proposed that all the continents were once grouped as a single supercontinent that he named Pangaea1 (see figure 2). S o m e 300 million years ago, Pangaea started to break up. North and South America drifted away from Africa, opening the Atlantic Ocean. In a similar way, the Indian Ocean resulted from the separation of Africa, India, Australia and Antarctica. This suggestion of large horizontal displacements of continents at the surface of the earth (by several thousands of kilometres) was very m u c h at odds with all the geological theories prevailing at that time. Indeed, geological processes such as mountain building were interpreted as the result of small (i.e. of a few kilometres) local vertical displacements of the crust. T h e debate between fixist and mobilist views of the history of the earth was opened.

Wegener's hypothesis was first based on the striking similarities in shape of the coastlines of Africa and South America. But Wegener pursued his idea, accumulating evidence for the existence of Pangaea from palaeontological observations, sedimen-tology, mineralogy and m a n y other disciplines. Similar species are observed at the same

6

Percentage of the Earth's surface Sea

level

20%

10% I t.

Eve

rest

s y ^ S '—w

'<

/ 1

/ 1 / 1 ' 1

1 • -

B

X 1 •

uril

Tren

ch

\ ^

100m -4500m Altitude

Plate tectonics understanding our planet

20°E

20°E

Figure 2. Wegener's reconstruction of the landmass Pangaea, approximately 200 million years ago. Panthalassa (meaning 'all seas') evolved into the Pacific Ocean, and the Mediterranean is a remnant of the Tethys Sea. Shading represents the polar glacier thought to have flowed over Southern Gondwanaland during Permian time, and explaining the glacial deposits found in South America, Africa, India and Australia.

epoch in the palaeontological record of the continents on both sides of the south Atlantic. Since m a n y of these species arc strictly continental lifeforms (like the reptile Mesosaurus), it indicates that land connections must have existed in the past. T h e Glossopteris flora of the Carboniferous age (about 300 million years ago), which equally cannot be expected to have crossed an ocean, is nevertheless widespread over all continents in the Southern Hemisphere. Glacial deposits of the same age define a continuous polar cap w h e n all southern continents are placed in their position according to Wegener's reconstruction (figure 2).

Wegener's theory of continental drift was able to account for m a n y more poorly understood observations, but above all it provided the first satisfactory explanation for mountain building. However , Wegener was not able to determine the forces that powered the motion of continents over such distances and that had sufficient energy to build mountains. Led by Sir Harold Jeffreys, a distinguished British geophysicist, the majority of earth scientists refuted Wegener's theory, and it eventually fell into oblivion after twenty years of controversy.

Past recordings of the magnetic field

Like the burial, the revival of continental drift theory c a m e from the United K i n g d o m , in the late 1950s, with the study of the natural magnetization of rocks.

A strong magnetic field exists at the surface of the earth and is generated in the core of the earth. Between 2900 and 5000 k m depth, the outer core is predominantly m a d e of

7

José Achache

iron and behaves like a liquid. It is therefore a good electric conductor. The magnetic field of the earth is believed to be produced by a self-sustained d y n a m o process, driven by convective fluid motion in the electrically conducting outer core. Crustal rocks containing magnetic minerals are magnetized by the core field. In s o m e instances, this magnetization can be 'frozen' for millions of years, thus creating a permanent magnetization in the rocks parallel to the direction of the ambient field existing at the time of formation of the rock. This ability of rocks of the crust to retain the m e m o r y of the earth's magnetic field in the past has been fundamental to the revival of the concept of continental drift.

The main field of the earth (the core field) has two remarkable properties. First, it is almost dipolar; that is, it is similar to the field that would be created by a bar magnet located at the centre of the earth (figure 3). For this reason, the needle of a magnetic compass always points to the magnetic north pole. Its second interesting property is that it undergoes polarity reversals, during which the north and south poles are switched. Using the first property one can determine the position of the north pole by measuring the direction of the magnetization in crustal rocks anywhere on the surface of the earth. W h e n performing such measurements on rocks from various continents around the globe, S. K . Runcorn and E . Irving,2 two British geophysicists, observed systematic discrepancies between the magnetic pole positions deduced from rocks of the same age but of different origins. Furthermore, rocks of different ages sampled in a given location showed a regular migration of the pole position with time. This apparent wander of the pole implied that either the pole or the continent had actually drifted. All their observations led them to conclude that the continents had been continously

Geographic Magnetic North \ /North /

Pole \ / Pole / \

Figure 3. The Eartrfs magnetic field is much like that which would be produced if a giant bar magnet were placed at the Earth's centre and slightly inclined from the axis of rotation.

Plate tectonics—understanding nur planet

drifting. In addition, they were able to show that the movements of the continents thus predicted brought them in a position close to that proposed by Wegener in his Pangaea reconstruction.

Sea-floor spreading

But the major obstacle remained. W h a t could be the cause of this motion and what is the force which drives continental drift? The answer was to come from the study of the ocean floor.

Several striking features are observed on the topographic m a p of the ocean bottom, foremost being a network of ridges 2000 to 4000 metres high and about 2000 kilometres wide which runs continuously through the Atlantic, Indian and Pacific Oceans. These ridges appear to be rifted along their axis and are similar to the rift valley of East Africa. The second most important topographic features are the deep trenches which girdle the north and west Pacific Ocean along the Aleutians, Japan, the Marianas and the Philippines.

Performing geophysical measurements of all kinds, research vessels have been able to assess m a n y fundamental properties of the oceanic crust. Seismic reflection profiles showed this crust to be m u c h thinner than under the continents and to be mainly composed of basaltic rocks rather than granites. The thickness of sediments was also surprisingly small, given the age of the oceans and the observed rate of sedimentation, and it was seen to increase away from the ridges. Gravity measurements revealed strong anomalies above the trenches and to a lesser extent above the ridges. Mid-ocean ridges were also observed to be regions of anomalously high heat flow, indicating the presence of volcanic activity. All these observations led to the formulation of the sea-floor spreading hypothesis3 in a paper referred to by its o w n author as an essay in geopoetry. According to this hypothesis, the mid-ocean ridges are accreting zones where the ocean floor is constantly generated from upwelling mantle material. The newly formed sea-floor then moves away from the volcanic ridges, across the ocean basins. At trenches, it sinks and returns into the mantle, drawing d o w n the sediments deposited during its travel through the ocean. The sea-floor is seen as constantly moving at the surface of the earth and recycling through the mantle in less than 200 million years. This model was confirmed by the planetary distribution of earthquakes plotted in the early sixties (figure 4). It shows that the vast majority of earthquakes occur along ridges and trenches, the ones in the latter being m u c h stronger and located deeper in the mantle (down to 700 k m ) .

The obstacle to Wegener's theory could then be easily overcome. The continents, instead of drifting on the underlying mantle, were entrained in the motion of a thicker surface layer involving the crust of both the oceans and the continents and the upper part of the mantle4. This layer, called the lithosphère, is created at mid-ocean ridges where upwelling mantle material cools and solidifies, then drifts at the earth's surface and eventually returns to the mantle at trenches. In fact, as early as 1931, Holmes 5 had proposed that continental drift was associated with mantle convection and thus driven by thermal forces. Indeed, the earth's mantle is being heated by the decay of radioactive isotopes of uranium, thorium and potassium. Because the temperature increases with depth in the mantle, the hot rocks at depth are gravitationally unstable with respect to colder and denser rocks near the surface. This results in a convective motion in which colder rocks descend into the deep mantle and hotter rocks ascend toward the surface.

9

José Achache

1961-7 having focal depths between 0 and 700 k m . Epicentres by the U . S . Coast and Geodetic Survey. Computer plot by M . Barazangi and J. Dormán, Columbia University. Compare with figure 5.

This long-term, fluid-like behaviour remained a puzzle for a long time since the mantle was k n o w n beyond doubt to be in a solid state.

The most convincing evidence for sea-floor spreading is given by the magnetic anomalies observed in the oceans6. Basaltic rocks from the oceanic crust are rich in ferromagnetic minerals and can therefore acquire strong magnetization. At spreading centres, the basalt cools in the ambiant magnetic field and is magnetized in the direction parallel to this field. B y this process the direction of the field at a given period is 'frozen' in the portion of the crust created during this period. Since the oceanic crust is continuously created, if this direction changes (in particular if it reverses) the magnetization in the crust will change accordingly, the sea-floor acting as a magnetic tape recorder of the evolution (i.e. the successive reversals) of the earth's magnetic field. Magnetic anomaly profiles obtained in the oceans consistently reveal the same pattern of successive reversals of the magnetic field, thus confirming that reversals have occurred (it was not obvious at the time) and that the sea-floor is spreading symmetrically on both sides of ridges. In addition, since the reversals of the field can be dated by the analysis of sedimentary columns, it provides a means of measuring the rate of sea-floor spreading.

Plate tectonics: a unifying model

According to plate tectonics theory78 ' ' , the outer shell of the earth, the lithosphère, is m a d e of 13 thin, contiguous, rigid plates (figure 5) in motion with respect to each other, with velocities of the order of a few centimetres per year. The fundamental aspect of this n e w theory is that the lithosphère is rigid and therefore the motion of each plate follows simple geometrical laws. Once the parameters of the motion are determined (and very

10

Plate tectonics—understanding our planet

Figure 5. Distribution of the major surface plates. After Turcotte and Schubert (1982).

few measurements are necessary to do this) the relative displacements of the plates can be predicted anywhere along the plates' boundaries. In addition, this property accounts for the fact that all the tectonic activity on earth—the majority of earthquakes, volcanic eruptions and mountain building—occurs at plate boundaries. Seismic studies have shown the existence of a layer of partial melting in the mantle, immediately below the lithosphère. This layer, called the asthenosphere, allows the movement of the lithosphère over the deeper solid mantle.

A new view of the earth

Created at ridges—the accreting boundaries—the plates converge at trenches where one of them bends and descends into the mantle in a process called subduction. In some instances, two plates can slide past each other without diverging nor converging. The boundary is then is transform fault, an example of which is given by the San Andreas fault in California, which separates the Pacific and the North American plates. Further to the north, the boundary between these two plates becomes a subduction zone, illustrating the fact that the nature of a boundary between two given plates depends on its orientation with respect to the relative motion of the plates.

Since the majority of catastrophic phenomena occur at plate boundaries, they must be related to processes taking place there. In this respect, accreting boundaries are not of major interest, since only a mild and continuous activity is observed there. Furthermore, ridges are situated in the middle of ocean basins, the only two exceptions being Iceland and the Republic of Djibouti. Their study is, however, of interest for mineral prospecting, for they are the site of important mineralizations associated with volcanic and hydrothermal activity (see article by H a m z a ) . Transform faults are also of rare occurrence on land. These faults are associated with intense seismic activity (the 1906 earthquake destroyed the city of San Francisco). Large transcurrent faults similar to these transform faults are observed within some continental plates suggesting a more

11

José Achoche

complicated behaviour of these plates (the Altyn Tagh fault in China, the north Anatolian fault in Turkey, etc.) and, like transform faults, they can be the cause of damaging earthquakes.

Subduction zones tend to be located closer to inhabited areas and therefore represent a major cause of destructive earthquakes or volcanic eruptions. Converging plate boundaries are also where processes creating large mineral deposits are located. A typical example of a subduction zone is found in Central and South America, where the Cocos and Nazca plates are subducted below the Caribbean and South American plates (figure 5). T h e Mexican-Peru-Chile trench and the Andes are the main topographic features associated with this subduction. It is also.the locus of the greatest earthquakes ever recorded, as well as deadly volcanic eruptions. In 1986 alone, two major earthquakes hit Mexico and San Salvador, and the eruption of the volcano N e v a d o Del Ruiz destroyed the city of Armero in Colombia. Shallow earthquakes occur on the fault zone which separates the downgoing slab from the overriding lithosphère. Deeper earthquakes occur within the descending plate d o w n to a depth of 700 k m (fi gure 6). Vulcanism along subduction zones has a peculiar geometry. Volcanoes are regularly spaced along a line which trends parallel to the subduction. T h e distance between this line and the trench depends on the angle at which the plate descends into the mantle, because the melting process which generates m a g m a can only occur at a given depth. The slope of the subducting slab is also related to the nature of the tectonic structures that develop at the leading edge of the overriding plate. W h e n the dip angle is small (less than 451), the subduction is bordered by a mountain range,

Subduction mélanges:

high-pressure, low-temperature metamorphism

a> c o o

»'1

Porphyry copper deposits

Asthenosphere ̂ í^^ífes

Figure 6. Diagram showing the features and activities associated with Andean-type subduction (not to scale). Earthquakes indicated by black stars.

12

Plate tectonics—understanding our planet

like the Andes in South America. Otherwise, a marginal basin is created between the trench and the continent, as in Japan or-the Marianas where the dip of the slab is larger than 75° (see figure 7 and the article by Abe).

Vulcanism is also observed in the interior of plates, as for instance in Hawaii, which is located in the middle of the Pacific plate (see figure 5). M o r g a n 1 0 proposed an explanation of this vulcanism by the existence of ascending plumes of hot material, generated in the deep mantle, which produce partial melting near the surface. This observation challenges the model of large convective cells in the mantle previously admitted and leads to the consideration of at least two scales of convection in the mantle.

Plate tectonics and earthquake hazards assessment

The study of the pattern of earthquake occurrence was a fundamental clue to the discovery of plate tectonics. It was observed that they concentrate on mid-ocean ridges, transform faults and subduction zones. In the case of subduction, earthquake hypocentres (or foci) are seen to be located in the plane of the downgoing slab d o w n to a depth of 700 k m (figure 6).

A n earthquake results from the sudden release of elastic energy stored in the lithosphère by the continuous motion of plates. Rupture usually occurs along pre­existing faults w h e n the amount of energy accumulated exceeds the frictional bond of the rocks along these faults. For this reason, the initial step in earthquake prediction, in particular in continental areas, is to perform a detailed mapping of all pre-existing faults. It is also important to try and date the last event that occurred on each fault. Indeed, earthquake risk assessment relies mainly on the determination of possible spatial or temporal patterns in the distribution and frequency of shocks. For example, one has observed the progressive westward migration of epicentres along the North Anatolian fault in Turkey since the beginning of the century. In addition, these large

Extension Rifting

TO t=

.y m Volcanic > o sediments

Ocean

Folded sediments

(copper sulphides)

Partial melting of subducted oceanic crust to form m a g m a Partial

melting of upper mantle

Continental sediments

Old continental crust

Mantle

Figure 7. A marginal basin often opens behind volcanic island arcs where oceanic crust is being subducted under a continental margin.

13

José Achuche

earthquakes occurred with a near-periodicity of about 10 years. Along subduction zones the distribution of seismic activity sometimes reveals segments with a consider­ably reduced seismic activity. Along such segments, referred to as gaps, elastic energy presumably accumulates (since it seems not to be released by microseismicity). The probability of large earthquakes occurring in such seismic gaps is thus higher.

O n e speaks more frequently of risk assessment than prediction since no technique has yet proved efficient for the prediction of earthquakes. Chinese geophysicists have successfully predicted a few events and in particular the great earthquake in Hai Cheng in 1975, for which a warning was issued only five hours before the main shock. Such a prediction was only m a d e possible by the remarkable organization of Chinese society in rural areas where the observation of natural phenomena can be efficiently performed and reported. Indeed, the Tangshan earthquake which occurred about a year later in an urban area could not be predicted and left a death toll of more than 700000.

Several properties of the soil seem to undergo modification before earthquakes and can be used as precursors. Fissures and cracks in the rocks open and dilate, thus producing local deformation of the ground over large distances. The electrical properties of the soil also seem to be altered in relation to earthquakes. Greek scientists have observed strong variations in the electric signal recorded in the ground prior to earthquakes. However, no causal relationship has been demonstrated between earthquakes and these precursors. Water seems to play an important role in these phenomena. It has been observed in Denver, U S A , that the number of small earthquakes correlates with the amount of waste water injected into the ground through a deep well. Water m a y act as a lubricant on faults, thus preventing large energy storage in the ground, the energy released through microseismicity instead of large earthquakes.

W h e n looking at figure 4, one observes that a small percentage of earthquakes occurs within continental plates, in particular, in the Alpine-Mediterranean region and in the Himalaya-Tibet area. These earthquakes, which in some instances can be quite destructive, are the consequence of the collision of two continents (figure 8). Because of its lower density, a continent cannot be subducted when it reaches a subduction zone. As a consequence, the convergence of the two plates proceeds by deformation of one or both continental masses until the subduction shifts to a more favourable position (energetically speaking). A collision between two continents like India and Eurasia results in a more diffuse plate boundary than observed in the oceans. The India-Eurasia collision was initiated some 50 million years ago. Surprisingly, India is still moving northward today at more than 2 cm/year. Such a continuous movement has produced about 2000 k m of crustal shortening within Asia. Tapponnier and his colleagues1 ' have shown that this convergence induces large-scale deformations as far north within Asia as Lake Baikal. Large strike-slip faults several thousands of kilometres long have developed through China, along which large crustal blocks escape sideways, allowing India to drift further north. Such faults developed by propagating through previously unfaulted crust. The prediction of the earthquakes associated with this phenomenon is therefore a very difficult task.

Plate tectonics and mineral resources

As the population on earth increases and more countries are developing industrial capacities, the world demand for mineral resources can only increase. Indeed, the rate

14

Plate tectonics—understanding our planet

Figure Ü.

The possible stages of plate collision: (a) Convergence between plates with continental and oceanic lithosphère at leading edges; (b) Collision of continents, producing a mountain range, magmatic belt and a thickened continental crust. Plate motions may be brought to a halt at this point; (c) Alternatively, the plate m a y break off and a new subduction zone m a y be started elsewhere. A n extinct subduction zone may remain to form a mountain belt within a continent (e.g. the Ural mountains).

Volcanic chain

á Subduction zone

Oceanic crust

(a)

Mountain range

• y - : - x . y . y - : : : . y

Extinct subduction zone

•™TsiP " %

N e w subduction zone

y

of consumption of these resources is increasing faster than the growth of the population in all industrialized countries.

T h e transformation of mineral resources and the production of manufactured products requires ever larger amounts of energy. Unlike mineral resources which are only recycled and dispersed at the earth's surface by h u m a n activity, energy from whatever origin (wood, coal, oil, geothermal or radiogenic) is irretrievably lost to the universe once it is used.

M a n has been extracting mineral resources from the ground for centuries and this process has dramatically increased with time. Thus , most of the surface of the continents has been explored by geologists and most outcropping deposits are likely to have been discovered. Future exploration will therefore have to focus o n more remote and less accessible areas like the ocean floor, the polar continents, the deeper crust and, later, other planets. This trend is s h o w n by the emphasis which is today being put on oceanic and polar exploration and seismic profiling of the deep crust of the continents by consortia co-sponsored by scientific and industrial institutions (e.g. C O C O R P in the U S A and E C O R S in France). In this process, geological methods will be progressively outdated and replaced by exploration techniques and concepts pertaining to geo­physics and geochemistry. First of all. it will require a better understanding of the

15

José Achache

internal processes at the origin of mineralization. In order to better locate deposits, a precise knowledge of the relation between plate tectonics and mineral deposition will be a crucial factor.

The exploitation of these deposits will also become more costly, and future mining operations both at sea and on the continents will require ever more energy supplies. This factor, together with the rapid industrialization of developing countries, will increase the energy demand, and renewed oil prospection is therefore likely to be of strategic interest in the near future.

The formation of oil

The formation and accumulation of oil depends on a particular succession of events and the existence of precise conditions. First, organic matter must be produced in large quantities and stored in an environment deficient in oxygen. It must then be buried under sediments where it can remain trapped. Thus, the environment should not be subjected to strong tectonic deformation that would allow escape or alteration of the maturing oil stored in porous sediments. Although rather stringent, all these conditions are met in the appropriate sequence in two classical types of formation which are both consequences of plate motion12: marginal basins and the passive margins of continents.

W h e n subduction occurs along the margin of a continent with a deep enough angle, a marginal sea develops behind the arc formed by the chain of volcanoes (see above and figure 7). Such basins are particularly developed along the east coast of Asia: the Bering Sea, the Sea of Okhotsk, the Sea of Japan, the Yellow Sea, the East China Sea and the South China Sea. The marginal basin environment is favourable for the accumulation and maturation of oil since, first, organic material is trapped behind the island arc: the trench deflects the deep ocean circulation thus preventing oxidation. The rate of sedimentation is very high in these basins and can easily trap the organic matter deposited. Finally, the extensional tectonic forces produce only minor folds, which together with salt layers can create the appropriate conditions for the storage of oil over geological time.

Similar conditions are observed during the formation of passive margins, i.e. at the early stage of fragmentation of a continent. A narrow and shallow ocean develops between the two separating land masses after the initial phase of rifting (see figure 9). Intense terrigeneous sedimentation occurs and the main tectonic forces are extensional. The progressive subsidence of the margins favours the formation of salt deposits which form efficient traps for oil formations.

Minerals

Hydrothermal mineral deposits, and in particular sulphides, constitute a major part of k n o w n metallic ores. T h e distribution of these deposits on the earth's surface bears a close relationship with plate boundaries. Indeed, most of the sulphides are located along past or active convergent plate boundaries (see figures 7 and 9). Even gold-bearing deposits are often found with sulphides in extinct convergent zones. As shown by the example of the Red Sea where the richest submarine metallic deposits were found, mid-oceanic ridges and ocean basins are also the site of significant mineraliz­ation (see figures 7 and 9). It seems that the concentration of metals extracted from mantle material at ridges is produced by hydrothermal processes. Sediments deposited

16

Plate tectonics—understanding our planet

Asthenosphere

(a) -»- Atlantic Sea -*-

Rock salt Organic matter

Metallic minerals

(b)

South American plate • ^ - - ^ Atlantic Ocean

African plate

* -

Petroleum Metallic minerals Sulphides

(0

Figure 9.

\.l~:J>¿VJ ,-•

'Salt d o m e s , : , ' X .

Mid-Atlantic Ridge

The accumulation of mineral deposits and oil at plate boundaries, as exemplified by the development of the South Atlantic Ocean, (a) Pangaea is rifted into two continents (Africa and South America) about a divergent plate boundary, (b) Sea floor spreading occurs, and South America moves away. Thick layers of rock salt, organic materials (which lead to oil formation) and metallic minerals accumulate in the Atlantic Sea. (c) The spreading from the mid-Atlantic ridge continues, widening the Sea into an Ocean. Metallic metals continue to accumulate about the ridge. Salt originating in the thick layers of rock salt buried under the sediments of the continental margins rises in large dome-shaped masses that trap the oil and gas generated from organic matter.

17

José Achoche

near active spreading centres are then enriched with iron, manganese, copper, nickel, lead, cobalt, uranium, chromium, mercury, vanadium, cadmium and bismuth. In addition to the well-known polymetallic nodules, pure copper and manganese are also found in the oceanic crust.

W h e n an ocean closes during the collision of two continental masses, as shown in figure 8, small scraps of oceanic lithosphère m a y be trapped between the continents in a process called obduction. These scales of oceanic lithosphère deposited on top of the continental crust along suture zones are called ophiolites. The discovery of these ophiolites has been fundamental to the study of the structure of the oceanic lithosphère and to the understanding of the processes at ridges. Naturally, ophiolites contain all the mineral deposits characteristic of the oceanic crust but are m u c h more accessible for extraction, since they are always located on continents.

A large number of such suture zones and associated ophiolite sequences are observed in Southeast Asia (sec article by Hutchinson). This is a consequence of the mosaic structure of this continent, which appears to be built of several juxtaposed continental blocks of different origins that collided and were then accreted to Asia during the last 200 million years. The study of continental drift in the past, a discipline k n o w n as palaeogeography, allows us to determine the past position of continents and extinct convergent zones and therefore to locate ophiolite sequences. This provides another guide for mineral prospection in continental areas. The best known examples of such ophiolites are given by the Troodos massif in Cyprus and the O m a n series, but their occurrence seems to be widespread over continents.

Conclusion: a look to the future

As illustrated in the first part of this article, progress in our knowledge of the earth has been linked with technological improvements. M o r e accurate instruments have led to the discovery of new phenomena that have triggered the further work of scientists.

But, because our planet is also a place of conflicts between nations, the observation of the earth has become a strategic necessity. M a n y steps forward in the development of the theory of plate tectonics were made possible by observations gathered for strategic purposes. The survey of the ocean floor was extensively performed in the aftermath of World W a r II, during which the allied forces and shipping had been constantly attacked by enemy submarines. It has revealed the supremacy of submarines in modern conflict and provided the incentive for the accurate determination of the bathymetry of the ocean floor. Later, in the early 1960s the U S A and the U S S R started building huge nuclear arsenals and initiated talks on arms control. In order to detect nuclear explosion experiments, and in particular underground tests, both countries had to develop highly sensitive seismometers. These instruments led to the establishment of the m a p of the world distribution of earthquakes that helped in the discovery of the different types of plate boundaries.

T h e Deep Sea Drilling Project ( D S D P ) was also a major source of information on the structure of the oceanic crust. This project was made possible through the construction of a remarkable vessel, the Glomar Challenger. This ship allowed scientists to drill several kilometres into the crust in the middle of ocean basins and to recover cores of this crust for laboratory analysis, an operation requiring a remarkably accurate means of position-holding. T h e ship was initially financed and constructed on

18

Plate tectonics—understanding our planet

the initiative of the millionaire H o w a r d Hughes to try and recover a Soviet submarine that had sunk in the Atlantic Ocean and was lying 5000 metres below the surface.

Today, with the development of remote-sensing satellites, earth observation is an even more strategic issue, both from the military and economic points of view. A s stated in the introduction, space techniques are also a unique tool for the global observation of natural phenomena and they have already led to fundamental progress in the earth sciences. In addition, because of the international policy of free access to all data gathered from space, remote sensing should become highly profitable to all nations, even those without space capabilities of their o w n , and it should become in the near future a powerful technique for both fundamental research, the prospection of mineral resources and the monitoring of natural disasters of meteorological or internal origin.

Notes

1. A . W E G E N E R , The Origins of Continents and Oceans, London, Methuen, 1924. 2. S. K . R U N C O R N , Continental Drift, N e w York, Academic Press, 1962. 3. H . H . H E S S , History of ocean basins, in A . E . J. Engel, H . L . James and B. F. Leonard (eds.).

Petrologic Studies: a volume in honor of A. F. Buddington, Boulder, Geological Society of America, 1962.

4. R. S. D I E T Z , Continent and ocean basin evolution by spreading of the sea-floor, Nature, Vol. L90, 1961, pp. 854-857.

5. A . H O L M E S , Principles of Physical Geology, London, Nelson, 1945. 6. F. J. V I N E and D . M . M A T T H E W S , Magnetic anomalies over oceanic ridges, Nature, Vol. 199.

1963. pp. 947-949. 7. D . P. M C K E N Z I E and R . L . P A R K E R , The north-pacific: an example of tectonics on a sphere.

Nature, Vol. 216, 1967, pp. 1276-1280. 8. W . J. M O R G A N , Rises, trenches, great faults and crustal blocks. Journal of Geophysical

Research, Vol. 73, 1968, pp. 1959-1982. 9. X . L E P I C H Ó N , Sea-floor spreading and continental drift, Journal of Geophysical Research.

Vol. 73, 1968, pp. 3661-3697. 10. W . J. M O R G A N , Deep mantle convection plumes and plate motions, American Association of

Petroleum Geologists Bulletin, Vol. 56, 1972, pp. 203-213. 11. P. T A P P O N N I E R , F. P E L T Z E R , A . - Y . L E D A I N , R. A R M I J O and P. C O B B O L D , Propagating

extrusion tectonics in Asia, new insights from simple experiments with plasticine. Geology. Vol. 10, 1982, pp. 611-616.

12. P. A . R O N A , Plate tectonics and mineral resources, Scientific American, 1943, pp. 86-95.

To delve more deeply

C . J. A L I . E Q U E , L'écume de la Terre, Paris, Fayard, 1982. F. PRESS and R. SIEVER. Earth, San Francisco, W . H . Freeman, 1978. D . L. T U R C O T T E and G . S C H U B E R T , Geodynamics, N e w York, John Wiley, 1982. S. U Y E D A , The New View of the Earth, San Francisco, W . H . Freeman, 1978.

19

Heat flow of the earth and geothermal resources

Valiya M . Hamza

The very powerful sources of energy that exist in the interior of our planet produce major flows of heat in the surface layers. Under certain conditions such flows can lead to the formation of thermal energy reservoirs in the earth, and play a significant role in the mobilization of mineral and hydrocarbon deposits. Our understanding of the thermal régime of the earth is likely to become crucial to the future exploration and exploitation of energy and mineral resources.

Grieving over a fallen flower, K u m a r a n Ashan, a poet from Kerala, once lamented:

Today this is your fate T o m o r r o w w e follow C o m e to think of it Nothing is Permanent Even the High Mountains A n d the Deep Seas Perish one Day .

Writing several decades before modern theories of sea-floor spreading and plate tectonics were proposed, the poet in his attempt to console himself over the tragedy that befell a fallen flower has cited one of the unique characteristics of the planet on which w e live: the eternal rejuvenation of its surface features. Recent planetary studies using deep space probes point out that, unlike other terrestrial planets of the solar system, earth is at present an active planet. Global geological investigations confirm that internal activities of the planet have been modifying the surface rock formations during the last 3500 million years, the period over which geological records exist. S o m e of the surface features of the earth are obviously the result of interactions with the hydrosphere and the atmosphere, but there is n o doubt that large-scale morphological features are related to processes taking place in its interior.

D r Valiya M . H a m z a is currently head and research co-ordinator of the Geothermal Laboratory at the Institute of Technological Research of the State Government of Sào Paulo, Brazil. H e held the post of Associate Professor at the University of Sao Paulo from 1974 to 1981, and before that studied in India and Canada. His current research interests include geothermics, the thermal maturation of hydrocarbons, heat control in underground mines and thermal aspects of underground nuclear waste storage. H e m a y be contacted through the Institute of Geosciences, University of Sào Paulo, Caixa Postal 20889, 05508 Sào Paulo, Brazil.

2 1 Impacl til science on .socit'fv. no. 145, 21 32

Valiva M . Hamza

T h o u g h w e k n o w very little about the nature of forces that have been at work in the interior of our planet, it is clear that very powerful energy sources, capable of operating over long time-spans of billions of years, are necessary to meet the energy requirements. Several sources of energy have been discussed, but presently most geoscientists agree that radiogenic heat produced in the interior of the earth, since its formation as a solid planetary body, has contributed significantly to its energy budget. All rocks contain small quantities of naturally occurring radioactive elements, mainly uranium, thorium and potassium. During radioactive decay of these elements a part of their mass is converted to energy, most of which is eventually transformed into heat. T h e quantity of thermal energy produced by radioactive decay per unit mass of c o m m o n rocks is small, but the total a m o u n t released over long time intervals can be quite substantial. For example, one cubic kilometre of granitic-type rock is capable of producing on average an equivalent thermal energy of nearly 900 gigajoules (21 billion calories) in one year. It is reasonable to suppose that release of such large quantities of heat over geological times of the order of billions of years could produce high temperatures inside the earth, leading to the establishment of a substantial heat flux towards the surface. If the rate of heat loss through the surface is less than the rate of heat generation, a continuing rise in temperature will result in melting and the eventual onset of thermal convection. O n e is naturally inclined to ask the question: did the earth go through such a sequence of internal heating? O u r knowledge of the thermal history of the earth is indeed rudimentary, but m u c h can be learnt by examining its present internal structure, as deduced from geophysical data and geological evidence. A cursory examination of the earth's internal structure, presented schematically in figure 1, is enough to realise that the earth is indeed a hot planet, with temperatures in excess of 4000 C in its central parts. Volcanic activity and lava flows are strong, direct evidence for the existence of high temperatures at relatively shallow depths. Seismic studies show that some of the internal layers constituting nearly 15% of the earth's volume are in molten liquid state. Geomagnetic investigations s h o w that the earth's magnetic field is generated within this liquid core, most probably a result of circulatory movements of conducting fluids set up by active thermal convection.

6370 >4000

Figure 1.

Schematic representation of the internal structure of the Earth and inferred temperatures.

22

Heat flow of the earth and geothermal resources

Between the relatively cold surface layer and the molten liquid core is the mantle, with temperatures in the range 1000-3000ÜC. W h a t kind of movements can be expected in the mantle? Seismologists tell us that the mantle behaves like a solid as far as the passage of seismic waves is concerned. O n the other hand, geophysical studies on the response of mantle to long-period surface load variations show that it is incapable of supporting stress changes over time periods of more than 1000 years. Recent laboratory studies have provided valuable clues to this apparently strange behaviour. At high temperatures the mechanical strength of the mantle is reduced to such a level that it acts like a solid under short-period stress changes, while its true behaviour over long periods can better be described as that of a highly viscous fluid. It is only logical to conclude that under such conditions the internal motions are bound to be relatively slow, but there will be considerable viscous drag on the confining boundary layers. W e m a y expect a higher degree of viscous drag on the cold upper boundary, which could be pushed around, thrust up or drawn d o w n in response to movements taking place in the mantle. This is the essence of plate tectonics theory, according to which the outermost skin of the earth, the lithosphère, m a y be considered as consisting of several rigid plates whose movements are affected to a large extent by dynamic processes taking place in the underlying mantle. Since heat and temperature exert profound influences on the characteristics of such movements, measurement of heat flux through the lithosphère should provide important clues to the nature of processes taking place in the interior of the earth.

The nature of the geothermal flux

The total amount of thermal energy stored in the earth is quite large; a rough estimate would put it at about 12 teraquads (about 3 x 10 3 0 calories). However, the low-conducting crustal rocks, acting like a thermal blanket, allow only an extremely tiny fraction, estimated at 100 quads per year, to escape to t he surface. Nevertheless, study of the nature and characteristics of this heat flux and its spatial and temporal variations can throw light on thermal processes taking place in the interior of our planet and h o w it interacts with the passive outer shells on which w e live. This is geothermics, a relatively young but fascinating branch of the geosciences that deals with the problems of terrestrial heat flow and the thermal state of the earth's interior. The history of geothermics is rather short, the earliest scientific attempts dating back only to the seventeenth century. It was the time when the budding industrial revolution of the western world was putting up increasing demands for natural resources, prompting mineral exploration companies to open up underground mines. The hostile thermal environments encountered in deep mines provided the early impetus for studying the nature of geothermal heat flux. Initial attempts carried out in coal mines were, however, unsuccessful, partly because of the difficulties encountered in devising suitable measuring techniques and partly because of the lack of a clear understanding of the physical nature of heat. In spite ofthese difficulties some of the early investigators were successful in deducing a reasonable good picture of thermal conditions inside the earth. In this context the following passage written by Robert Boyle in 1671 as part of his discussions on the origin of terrestrial heat is quite remarkable:

I shall add as a conjecture, that the positive cause of the actual warmth m a y proceed from those deeper parts of the subtcrraneal Region, which ly beneath

23

Valiya M. Hamza

those places which m e n have yet had occasion and ability to dig. For it seems probable to m e that in these yet impenetrated Bowells of the Earth, there are great store-houses of either actual fires, or places considerably hot, or, (in some regions) of both, from which reconditorics (if m a y so call them) or magazines of hypogeall heat, that quality is communicated, especially by subterranean channells, clefts, fibres or other conveyences, to the less deep parts of the Earth, either by a propagation of heat through the substance of the interposed part of the soil...

A n d m u c h less have w e any certain knowledge of the temperature of the more inward, and (if I m a y so speak) the m o r e centrall parts of the earth; in which, whether there be not a continued solidity or great tracts of Fluid matter...

Systematic scientific studies on terrestrial heat flow began in the early nineteenth century while collective efforts on a global scale got under w a y only during the last few decades. Recent compilations of published data show that heat flow measurements have been carried out in more than 5000 localities on the surface of the globe. Though large areas still remain uncovered, the available data, a s u m m a r y of which is presented in table 1, reveal some of the important characteristics of geothermal heat flux.

T h e most striking observation is that geothermal heat is escaping at a higher rate through the floor of oceans than through continents, the excess value in relation to the average for continental regions being nearly 40%. Consequently w e can say that more heat is being lost through the southern rather than the northern hemisphere. Another interesting feature is that heat flow is not uniform over the surface of the globe. In continental regions the geologically younger and tectonically active regions are characterized by relatively higher heat flow than those observed in older tectonically inactive areas. Likewise in oceanic regions high heat flows are encountered in areas of

Table 1. Summary of heat flow measurements and heat loss estimates for continental and oceanic regions. {After Sclater et al.. 1980.)

Region

Continental Africa and Madasgascar South America North America Australasia Antarctica Europe and Asia

All continents

Oceanic North Pacific South Pacific Indian Ocean North Atlantic South Atlantic Marginal basins

All oceans

Global

Mean 1

m W / m 2

49 53 54 64 54 60

57

95 77 83 67 59 71

78

70

leat flow

cal/m2day

1028 1089 1123 1313 1123 1244

1184

1970 1598 1719 1391 1218 1469

1616

1443

Area (10" km2)

37-8 22-3 33-9 18-8 17-6 71-1

201-5

62-5 76-7 69-5 361 369 26-9

308-6

5101

Heat loss (10" cal/s)

448 282 442 286 229

1024

2711

1425 1419 1383 581 520 457

5771

8519

24

Heat flow of the earth and geothermal resources

mid-ocean ridges, while old ocean basins and trench areas are characterized by low heat flow. Detailed analyses of the data indicate that there is in fact a systematic decrease of heat flow with age in both oceanic and continental regions, an observation of far-reaching importance in understanding the nature of deep-seated thermal processes. Let us see h o w this observation fits into the present dynamic-earth model.

According to the plate tectonics theory, ocean ridges are zones of upwelling flow, ocean trenches zones of downwelling flow and intermediate regions zones of lateral flow in the mantle. In ridge areas the upwelling m a g m a cools and solidifies on contact with sea water, forming a rigid surface boundary later. A s this layer is carried away from the ridge due to lateral flow it cools and thickens, a result of progressive downward solidification. In trench areas this rigid surface layer is drawn downwards back into the mantle. The observed distribution of high, intermediate and low heat flow values are found to be in surprisingly good agreement with the thermal model predicted by plate tectonics theory. In fact, the plate tectonics model and the observed heat flow pattern allow us to reconstruct the temperature distribution in the lithosphère and the underlying mantle. A n example of idealized temperature distributions in upwelling and downwelling limbs of a mantle convection cell is shown in figure 2. Interesting features in this model are the appearance of plume-like shapes of high temperature isotherms under ridge areas and the sharp draw-down of low-temperature isotherms deep into mantle under the trench areas.

Another interesting observation emerging from recent geothermal studies is that the mechanism of heat loss is more diversified in younger, tectonically active areas than in older, stable areas, even though conduction is still the most c o m m o n and the dominant m o d e of heat transfer. It appears that in areas where the supply of heat from depth is high, its dissipation by conduction is insufficient and other mechanisms c o m e into play. Volcanic eruption and lava flows are spectacular examples of heat transport associated with mass flow. Fumeroles, steam vents, geysers and thermal springs are examples of surface manifestations of geothermal heat flux associated with fluid flows. In m a n y areas fluid flow induced by active thermal convection transports substantial quantities of heat but their surface manifestations are limited due to the presence of impermeable cap rocks. In certain regions such as fissured crystalline massifs and sedimentary basins groundwater flows, driven by topography, can carry and redis­tribute heat over large areas.

It is clear that in order to arrive at a realistic picture of global heat loss w e must consider the relative influence of the different mechanisms of heat transport and their respective areal extents. This is not an easy task because in m a n y areas the necessary information is lacking and subjective estimates have to be made . W e can nevertheless arrive at an approximate picture of the earth's overall heat loss by combining conductive heat flow data with information on the relevant geological and geophysical characteristics of continental and oceanic regions. A n example of such an attempt is shown in figure 3, where the earth's suface is divided into zones of high, intermediate and low geothermal flux. In zones of high heat flux, which include mainly ridge areas in oceanic regions and young tectonically active areas in continental regions, thermal energy is being lost by convection and conduction. The zones of intermediate heat flow include mainly sedimentary basins where thinning of low conducting crust allows higher-than-normal heat loss, but redistribution of heat by advection affects the conductive heat flow pattern. Finally w e have the tectonically stable Precambrian areas, where heat flux is low and conduction is the dominant m o d e of heat transfer.

25

Valiya Ai. Hamza

Figure 2. Idealized distribution of surface heat flow and lithospheric temperatures due to a mantle convection cell.

Let us n o w look into s o m e of the consequences of heat flux through crustal layers. Under the action of tectonic stresses low-temperature crust exhibits brittle behaviour while high-temperature crust allows ductile deformation to take place. Consequently, within comparable tectonic environments, seismic risk is low in high-temperature crust relative to low-temperature crust.

In certain regions where favourable conditions exist, flow of heat can lead to formation of thermal energy reservoirs. If the reservoir is located in near-surface layers it can be exploited for its heat content. In other words w e are talking about geothermal energy resources. It is only logical to assume that the probability of encountering such reservoirs is high in regions of high heat flux. Understanding the characteristics of geothermal energy reservoirs on a global scale is clearly of fundamental importance for the planning of energy development schemes in m a n y countries. Because of the importance that energy resources represent to the progress of our modern society it is worthwhile to consider this aspect of geothermics in a little more detail.

Geothermal energy resources

Throughout recorded time m a n has striven to divert for his o w n use the natural sources of energy, and geothermal energy is no exception. In fact, it is an old concept that has

26

Heat flow of the earth and geothermal resources

Figure 3.

Approximate outlines of principal global heat loss zones, drawn on the basis of conductive heat flow Pattern and geological characteristics.

ZONES OF HEAT LOSS

HIGH • INTERMEDIATE • LOW (>80mw/m2) (60-80 mw/m 2 ) (<60 m w / m 2 )

assumed new importance in the light of the rapidly increasing energy demands of our modern society. Geothermal energy is present everywhere as the natural heat of the earth, but it is usually dispersed in the outer crust and extraction for economic use presents formidable technological barriers. Only w h e n it is trapped or concentrated close to the surface is exploitation possible or potentially feasible, and then w e m a y regard it as a geothermal resource.

The basic components essential for a geothermal resource are: ( 1 ) a heat source, (2) a confined but permeable reservoir, and (3) a fluid to transfer heat to the surface. In the case of some resources permeability must be created artificially, while in some others recycling of fluids is necessary for continuous operation. O n the basis of the characteristics of these components geothermal resources can be divided into several types. In the classification scheme given in table 2 the resources are grouped into two basic types: petrothermal and hydrothermal. In petrothermal resources most of the heat is stored in the rock mass while in hydrothermal resources pore fluids carry a substantial fraction of the total energy in situ. Further subdivisions of these types can be m a d e on the basis of characteristics of geothermal and geologic environments, mechanisms of heat transfer and reservoir temperature.

Petrothermal resources are considered to hold substantia] potential for future exploration, but success in exploitations has so far been limited because of the technological difficulties encountered in devising suitable mechanisms for extraction of heat from the reservoir and transferring it to the surface. Only hydrothermal resources have so far been developed commercially and even here successful exploitation depends on locating permeable hot zones than in finding high temperature areas.

S o m e idea of the global distribution of geothermal resources can be obtained by calculating the thermal energy stored in the first 3 k m (considered to be the depth limit for economic drilling) of the crust. T h e main problem here is the lack of sufficiently

27

Valiya M. Hamza

Table 2. Type.

Type

Petrothermal

Hydrothermal

s of geothermal resources and their characteristics.

Sub-type

Magma (partly molten)

H o t dry rock

Radiogenic

Convective

Advective

Environment

Geothermal (*)

H

H

I/L

H/I

I/L

Geological

Volcanic

Intrusive bodies

Crystalline massifs

Tectonized areas

Sedimentary basins

Mechanism of heat

transport

Injection of external fluids

Injection of external fluids

Injection of external fluids

Recycling of natural fluids Recycling of

natural fluids

Reservoir temperature

(°C)

>650

90-650

30-150

90-350

30-150

!H—high heat flow ( > 8 0 m W / m 2 ) . I—intermediate heat flow (60-80 m W / m 2 ) . L—low heat flow ( < 6 0 m W / m 2 ) .

detailed geothermal and geological data, and meaningful estimates for m a n y regions,

especially those covered by oceans, are difficult if not impossible. For continental

regions the availability of geological data is better, but for most areas calculations have

to be based on subjective approximation of the extent of geothermal zones.

Nevertheless, preliminary calculations m a d e by subdividing the areas into low- and

high-grade geothermal zones reveal some important characteristics of geothermal

resources. Estimates of global geothermal resource base for 3 k m depth given in table 3

show that the magnitude of geothermal energy is quite substantial w h e n compared with

world energy production from conventional fuels, hydroelectricity and nuclear energy.

Another conspicuous feature to be noted is that low-grade heat at temperatures of less

Table 3. Comparison of geothermal resource base estimates for different continental regions with the 1975 energy production rates. Data from Rowley (1982); McRae and Dudas (1977).

Region

North America Central and

South America Western Europe Eastern Europe Asia Africa Oceania

Worldwide

Geothermal resource I (106 quads)

Low-grade (<150°C)

8-30

5.67 1-57 6-95 8-29 5-53 3-56

39-9

High-grade (>150"C)

0-34

0-27 002 006 0-22 008 018

1-2

jase

Total

8-64

5-94 1 59 7-01 8-51 5-61 3-74

41-0

1975 Energy production

Conventional fuels

63-96

11-61 17-36 5211 26-39 13-02

3-91

188-4

rate (quads/year)

Hydroelectricity and nuclear

2-45

0-48 1-80 0-58 0-40 013 0-51

6-4

Total

66-41

1209 1916 61-31 26-80 1315 4-42

264-4

28

Heat flow of the earth and geothermal resources

than 150 'C m a k e up a major part of the total geothermal resource base. Though not evenly distributed, such resources are more widespread and c o m m o n . O n the other hand high-grade resources constitute only a small fraction of the total resource and their distribution is highly uneven.

H o w can such resources be put to work for the benefit of mankind? Experience gained in the world-wide use of geothermal energy during the last few decades show that it is indeed cheaper than other conventional energy sources. However , unless transformed into other suitable forms it cannot easily be transported over large distances. Because of the sharp decrease in efficiency of geothermal power plants at low temperatures, conversion to electricity is advisable only for high-grade geothermal resources having temperatures greater than 150°C. O n the other hand efficiency is m u c h higher for direct-use applications at temperatures less than 150°C. A s most of the geothermal resources are of low-temperature type they m a y be considered as ideally suitable for applications in agriculture, industry and general servicing. A summary of the possible uses of low-temperature geothermal fluids is given in figure 4.

The role of geothermal heat in mobilization of minerals and hydrocarbons

Geothermal resources have c o m e to be commonly identified as concentrations of thermal energy stored underground. This is, however, a rather restricted outlook, and

Figure 4.

Spectrum of possible applications for the heat content of low temperature geothermal resources.

INDUSTRIAL

te z> c/i Q UJ z o

: 5 *

il

II

¡i

29

Valiya M. Hamza

the role of geothermal heat in the mobilization of mineral and hydrocarbon resources, so vital to the progress of our modern society, has often been overlooked. Before the development of the dynamic-earth model of plate tectonic theory, mineral exploration programmes were planned without a clear understanding of the distinct features in the geographical and age distributions of mineral deposits. Results of deep-sea drilling programmes, indicating that ocean basins contain a host of mineral deposits, brought about substantial changes in the conventional theories of metallogenesis. Attempts to explain the origin of ocean mineral deposits in the framework of plate tectonic theory has contributed to a m u c h better understanding of ore-forming processes and the occurrence of mineral belts on a global scale.

Let us look at the processes that lead to the formation of mineral deposits on the sea floor. As upwelling m a g m a approaches the ocean bed near mid-ocean ridges its temperature drops below the liquidus level and a hot surface boundary layer is formed. As the temperature drops further this layer turns brittle, becoming unable to resist the development oflarge thermal contraction stresses. The result is an extensive network of fracture systems and penetration of cold sea water into the hot subsurface m a g m a environment. Heat and the ensuing chemical reactions transform the downflowing alkaline sea water into a hot acidic solution that aggressively dissolves metals existing even at low concentrations. Sea water loses certain elements and compounds, notably magnesium and sulphates whilst absorbing elements such as lithium, potassium, calcium, barium, copper, iron, manganese and zinc. Thermal convection drives this metal-rich hot brine back through permeable channels to the sea floor. U p o n contact with cold sea water the dissolved metals precipitate from the brine, forming mineral deposits. Concentrations of economically exploitable deposits, however, occur only under favourable geothermal and geological conditions that include large networks of permeable fracture systems, high thermal gradients to drive convection cells and the formation of surface layers that protect against post-depositional changes. A schematic representation of the ocean crust mineralization process is shown in figure 5 and the geographic distribution of mineral deposits formed by sea-floor hot springs in figure 6.

Geothermal heat also plays a significant role in the mobilization of oil and gas deposits. Recent advances in the study of thermal maturation of sedimentary rocks show that transformation of organic matter to oil and gas is a process that depends on time and temperature. T o drive the large-scale generation of hydrocarbons, the organic matter has to be 'cooked' within a reasonably well-defined range of temperatures. In sedimentary basins such temperatures are attained as a result of subsidence and burial, as well as enhanced flux resultingfrom the rise of hot asthenospheric material to the base of the crust, Analysis of hydrocarbon deposits on a global scale show that most of the giant deposits are found in high heat flow basins. Even within an oil-prone basin commercial deposits are frequently found to be associated with areas which are 'hot spots'.

Q u o vadis?

Research and development work carried out in geothermics over the last few decades has contributed enormously to our knowledge of the dynamic processes taking place in the interior of earth. Analysis of geothermal data on the basis of the framework provided by plate tectonic theory has permitted the development of models capable of explaining the principal features in the thermal regime of the lithosphère. Global maps

30

Heat flow of the earth and geothermal resources

DEPOSITS ON PRESENT

SEA FLOOR

COPPER IRON-

MANGANESE ZINC

SULFUR SILICON

COPPER IRON MANGANESE ZINC ULFUR LICON

H £ f t T AND DISSOLVED G 4 S e s

\ ^ J I t

l'ii/itre 5. Schematic representation of the mineralization process by hydrothermal circulation in the ocean crust.

^ ^

• Sea-floor deposits

Figure 6.

I Continental deposits

Global distribution of hydrothermal ore deposits on the sea-floor and the continents. Those presently on the continents are believed to have formed originally in a sea-floor setting. (After Rona , 1986.)

31

Heat flow of the earth and geothermal resources

of geothermal heat loss are providing important clues for the setting up of exploration programmes for geothermal energy and hydrothermal ore deposits.

Having gained a basic understanding of thermotectonic activities on a global scale the international geothermal community is geared up for major efforts in elucidating the nature of thermal processes taking place on regional and local scales. Improve­ments in the knowledge of processes taking place in the mantle should help us to not only better understand the regional characteristics of volcanic and seismic activities, but also to provide auxiliary information for assessing the hazards posed by such natural phenomena.

Detailed information on the subsurface thermal regime is of considerable value in setting realistic goals in exploration and exploitation of geothermal energy and hydrothermal mineral resources, both at regional and local levels. Such efforts are of immediate interest to development work under progress in m a n y countries, and should be carried out with international support and cooperation. In this context it should be noted that the environmental consequences of the exploitation of natural resources and the hazards posed by natural activities often transcend our present political boundaries. •

To delve more deeply

J. W . E L D E R , Geothermal Systems, Academic Press, London, 1981. H . D . K L E M M E , Geothermal gradients, heat flow and hydrocarbon recovery, In Petroleum and

Global Tectonics (Edited by Alfred G . Fischer and Sheldon Judson). Princeton University Press, pp. 251-304, 1975.

W . H . K . Lift, Terrestrial Heat Flow, A m . Geophys. Univ. Monograph, no. 8, 1965. A . M C R A I : . and J. L. D U D A S , Energy Source Book, Aspen Systems Corp.. pp. 240-242. 1977. P. A . R O N A , Mineral deposits from sea-floor hot springs. Scientific American, vol. 254,

pp. 66 74, 1986. J. C . R O W L E Y , Worldwide geothermal resources, in Handbook of Thermal Energy, Gulf

Publishing Company. Houston, Texas, pp. 44-176, 1982. J. G . Sc L A T E R , C . J A U P A R T , and D . G A L S O N , The heat flow through oceanic and continental crust

and the heat loss of the Earth, Renews of Geophysics and Space Physics, vol. 18, pp. 289-311, 1980.

D . T U R C O T T E , and G . S C H U B E R T . Geodynamics Applications of Continuum Physics to Geological Problems, John Wiley & Sons, N e w York, 1982.

32

Continental growth and collision, and mineral prospection in Southeast Asia

Charles S. Hutchison

The continental plates of Southeast Asia have grown by accretion of low-density sediments along active convergent plate margins, and by collision of microcontinents which rifted from large ancestral continents and were brought along by sea-floor spreading. Prospection for mineral deposits is focused on two geological settings: (1) volcanic arcs, which resulted from subduction along active convergent plate margins, yield important copper, gold and silver deposits, and (2) granite bodies, which resulted from crustal melting in complex collision zones, yield important tin and tungsten deposits. Rifting of the continental crust resulted in subsiding depressions which infilled with sediments brought in by major rivers over the past 50 million years. Their burial increased the temperature and converted the contained plant and plankton accumulations to coal, oil and gas.

Introduction

The continental crust of Southeast Asia has grown episodically through time. The most important process was the transportation of microcontinents, which split from large ancestral continents by oceanic sea-floor spreading to collide with and weld onto the previous Southeast Asian continent. India rifted from Gondwanaland in Jurassic time—120 million years ( M a ) ago—and was carried along by sea-floor spreading of the Indian Ocean to eventually collide with Tibet in Eocene time (45 M a ago), resulting in the massive wrinkling of the earth's crust w e k n o w as the Himalayas. Other microcontinents collided in Late Triassic time (210 M a ago) and Australia is n o w beginning its collision with the volcanic arcs of Indonesia: at its present northwards velocity of 6 c m per year, it will come to rest 30 M a from n o w when the Indonesian, Philippine and western Melanesian arcs are squashed between Australia and southeast China to form a spectacular n e w mountain belt. Southeast China and eastern Vietnam have lost m u c h of their c o m m o n continental shelf because of rifting and splitting into m a n y microcontinents, a process which began about 60 M a ago. M a n y of the microcontinents were carried southwards by sea-floor spreading of the South China Sea to collide with and enlarge Borneo about 20 M a ago.

Charles S. Hutchison is Professor of Applied Geology at the University of Malaya, Kuala Lumpur . H e is the foremost authority on the geology of Southeast Asia and has carried out field research in Malaysia, Indonesia, Thailand, Philippines, south China and Taiwan. In addition to more than 80 papers in geological journals, he has authored books on petrology and economic geology and his most recent entitled Geological Evolution of Southeast Asia is due to be published early in 1987. Professor Hutchison m a y be contacted at the following address: Department of Geology, University of Malaya, 59100 Kuala Lumpur . Malaysia.

33 Impact of science on society, no. 145. }?> 44

Charles S. Hutchison

The continental area also grows by enlargement of the accretionary prism along convergent plate margins. This is formed of scraped-up sediments which do not descend at the oceanic trenches because of their relatively low density. The underlying oceanic basalt floor descends or subducts, but the overlying unconsolidated sediments pile up and form a highly deformed prism parallel to the trench. This is well illustrated by the islands lying offshore western Sumatra. Continental growth by this method is a slow continuing process and is not as spectacular or on the same scale as a continental collision. The processes of accretion and collision m a y , however, occur together. This has happened in Sarawak, west Borneo, where the Luconia carbonate province microcontinent, which rifted from the continental shelf of southeast China-Vietnam, has collided with the older continent of West Borneo and its wide accretionary prism, built up slowly over the timespan 75 to 40 M a ago, became compressed in a buffer zone between the two.

Growth of the accretionary prism

Continuing subduction results in the outward and upward growth of a highly deformed pile of poorly consolidated sedimentary material. In addition to scraped-up sediments, the accretionary prism commonly contains fragments of the underlying ocean crust volcanic rocks and strongly hydrated (serpentinized) mantle rocks, brought up by faulting in the unstable trench region.

As the accretionary prism grows in height, it becomes unstable and unconsolidated material slumps outwards to establish its equilibrium surface slope. The prism therefore grows continuously upwards and outwards, and the trench therefore has to migrate oceanwards. Because the accretionary prism represents a highly deformed mixture of rocks, it is commonly referred to as a mélange wedge.

The example of Nias Island

Nias Island, lying off the shore of west Sumatra, provides an excellent example of continuous growth of the accretionary prism.

Spectacular erosion of the Himalayas of north India and Tibet causes the Ganges and Brahmaputra rivers to carry the greatest sediment load of any river system in the world. The sediments reach the Bay of Bengal through the combined delta system in Bangladesh and are channelled into the Bay of Bengal through a deep submarine canyon called the Swatch of N o Ground (figure 1). These sediments are then carried by submarine turbidity currents far into the deep ocean. At about 1CTN, the turbidity currents are diverted by a prominent north-south submarine volcanic ridge known as the Ninetyeast Ridge to split into two lobes, the Bengal Fan to the west and the Nicobar Fan to the east. The fans have a sediment thickness of more than 10 k m in the north but progressively thin southwards to 'feather-out' or gradually reach zero thickness at about latitude 5ÛS1.

Because of the narrow gap between the northern extremity of the Ninetyeast Ridge and Sumatra, sediment can reach the Nicobar Fan only by coursing along the Sunda Trench. West of Nias Island, there is therefore a plentiful supply of ocean-floor unconsolidated sediment overlying the oceanic volcanic crust.

The oceanic crust of the Indian Ocean adjacent to Nias is known to be as old as 48 M a . It is now being pushed in a north-north-east direction at about 6 c m per year

34

Tectonics and minerals in Southeast Asia

from the presently active Southeast Indian Ridge spreading axis. The Indian Ocean crust does not arrive perpendicular to the Sunda Trench (figure 1) and the strongly oblique incidence m a y be resolved into two components, one perpendicular to the trench (subduction) and another parallel to the trench (transform). T h e important transform component has resulted in the major S e m a n g k o Fault, which extends the

Figure 1. M a p of Peninsular S. E . Asia showing how sediment from the' Himalayas is brought by the Ganges-Brahmaputra rivers to the sea to be transported into the deep ocean by turbidity currents. NNE-directed sea-floor spreading causes subduction of Indian Ocean crust at the Sunda Trench and continues to push continental India beneath Tibet. Modified after Curray et al.1.

35

Charles S. Hutchison

length of the Barisan Mountains of Sumatra. All regions on the south-west side of the fault are moving towards the north-west relative to the regions which lie on the north­east side.

T h e component of sea-floor spreading perpendicular to the Sunda Trench results in a slow subduction of the Indian O c e a n crust beneath the continental margin of Sumatra (figure 2). Although it is n o w k n o w n that s o m e sediments which overlie oceanic crust are able to subduct, o n the whole their low density prevents them from going d o w n on the 'conveyor belt' of subducting oceanic crust. This is especially true along the Sunda Trench, where the large amoun t of Nicobar Fan sediments cannot be

Figure 2. Subduction of the Indian Ocean crust beneath continental Sumatra and the growth of the accretionary prism with time. For line of section, see figure 1. Simplified after Karig et al.2.

36

Tectonics and minerals in Southeast Asia

subducted. They therefore become scraped-up to form a continuously deforming accretionary prism which has grown above sea level to form the islands of Nias and Mentawai (figure 2). Detailed studies of Nias Island have shown that the accretionary prism has grown in height and width from about 21 M a ago to the present day. As the accretionary prism grows, the position of the Sunda Trench is forced to slowly migrate oceanwards.

Another characteristic feature of a convergent plate margin is the fore-arc basin (figure 2). This is a trough-shaped depression lying between the growing accretionary prism and the volcanic arc, which also grows because of active vulcanism. As a result the basin also grows with time. It occurs in a very stable position and becomes infilled with undeformed sediments, eroded from the rising volcanic arc. This has been an important target for oil company exploration. Unfortunately, despite m u c h effort and cost, fore-arc basins have proved to be too cold to have caused the planktonic and plant material trapped in the sediments to be converted to oil and gas on a commercial scale.

Accretionary prism squashed by a microcontinent

It m a y occasionally happen that sea-floor spreading carries along a continent or a microcontinent as an integral part of the predominantly oceanic crust and underlying mantle. Thus, the Indian Ocean Plate is presently moving at about 6 c m per year in a north-north-east direction sway from the south-east Indian Ocean Ridge spreading axis, and is carrying along with it the huge continent of Australia, which split from Antarctica about 95 M a ago3. The island of Australia has n o w reached the Sunda Trench but, because of its size and continental nature, it refuses to subduct. It is therefore colliding with the trench and pushing the accretionary prism and volcanic arc ahead of it. The collision complex is seen on the island of Timor. North of Timor, the volcanic arc has been extinguished and is uplifting because of the collision.

The example of Northwest Borneo

Northwest Borneo and the adjacent part of the South China Sea offers an excellent example of an accretionary prism squashed between the continent against which it formed and a microcontinent which was brought along by sea-floor spreading of the South China Sea4 (figure 6).

During Oligocène time (34 M a ago), the Borneo continent did not extend as far north as its does today. The plate margin between the old China Sea ocean crust and the continent of West Borneo was active and subduction resulted in growth of the Rajang Accretionary Prism from about 75 M a to 30 M a ago (figure 3). Active subduction also resulted in a volcanic arc. Igneous activity in this arc has resulted in important gold, antimony and mercury deposits, especially in the Bau mining district south-west of Kuching, the capital of Sarawak.

The rocks of the Rajang Accretionary Prism are of deep water origin deposited by turbidity currents, and I believe the source of the sediments to have been the great M e k o n g River, which flows into the South China Sea through Indochina. The turbidity fan and accretionary prism were built upon oceanic crust, fragmented parts of which are n o w seen at the surface as a mélange deposit.

The c o m m o n continental shelf of Vietnam and south-east China began to rift and split up into many microcontinents about 60 M a ago4. Sea-floor spreading of the South

37

Charles S. Hutchison

Figure 3. Cross sections through the geological provinces of northwest Borneo coastal and offshore retion in Oligocène times (34 M a ago), and the present day, to show h o w the Rajang accretionary prism has been squashed against the West Borneo Basement by the Luconia Platform, which is a microcontinent rifted from the continental shelf of S.E. China-Vietnam. Modified after James5.

China Sea from 38 to 14 M a ago carried m a n y of the microcontinents far to the south. A s a result of the rifting and sea-floor spreading, the South China Sea contains a widespread scatter of microcontinents, most of which lie just below sea level and present a hazard to shipping. O n e of them is the Luconia Platform.

A s s h o w n in figure 3 the Luconia Platform microcontinent collided with the accretionary prism about 20 M a ago and squashed against the West Borneo continent in a collision fold-belt. This collision brought to an end subduction activity along the margin of Borneo, terminated the igneous activity in Sarawak, and resulted in considerable growth of the landmass size of Borneo.

T h e Luconia Platform is entirely beneath sea level. It contains a thick sequence of Miocene (7 to 20 M a old) limestone platform and coral reefs, typical of a continental shelf6. T h e limestones have provided an important offshore gas field for Malaysia. The gas is piped ashore at Bintulu to be liquified for export. T h e major oil field of the B a r a m Delta of Sarawak and Brunei is of a totally different character, and represents a major delta formed by the ancient Ba ram River flowing from continental Borneo to build out the Miocene delta into the South China Sea of the same age as the Luconia Platform limestones. However , the sequence of the delta is of m u d s and sands and is devoid of limestone. Plant material brought into the delta m u d s by the river as long as 15 M a ago, has been converted to oil because of burial heating, and has accumulated in the delta sands. T h e oil is produced in the Miri and Seria fields of Sarawak and Brunei.

38

Tectonics and minerals in Southeast Asia

Continent-to-continent collision

The split-up of the southern super-continent of Gondwanaland 7 can n o w be documented with a fair degree of accuracy (figure 4).

India began to separate from Australia and Antarctica about 128 M a ago, and Australia from Antarctica and N e w Zealand about 95 M a ago3 . The separation of Australia from Antarctica was slow. T h e separation of India from Australia and Antarctic was initially slow, then from 90 to 45 M a ago it m a d e a spectacular flight northwards at a velocity reaching 20 c m per year.

The suturing of India onto Asia

The beginning of collision of India against Asia is dated aroung 45 M a ago. The

position of the southern part of Asia to the immediate north of the suture zone (the

Tiyure 4. The evolution of the northeastern Indian Ocean, modified after Curray et al.1, showing separation of India from Gondwanaland about 126 M a ago and Australia from Antarctica about 90 M a ago and the northwards flight of India to collide with Asia about 45 M a ago.

39

Charles S. Hutchison

Lhasa Block) has been determined by palaeomagnetic measuremenis on Early Cretaceous (100 M a old) continental red beds near Lhasa8. The data demonstrate that the sample areas (present latitude 30 N ) lay 100 M a ago at a palaeo-latitude of around 12°N. The latitude differences imply that the Lhasa Block has been displaced northwards by about 2000 k m . At 100 M a ago, when southern Tibet lay 12°N, India would have been about 30" south of the equator3.

The Lhasa Block has been progressively pushed northwards since the Eocene collision 45 M a ago. Effects of the collision are indicated by the numerous occurrences of volcanic and intrusive rocks of different ages, the flysch formations and the intricate pattern of faults. Continental India has underthrust Asia, and the Himalayan region is characterized by a series of thrust planes all dipping towards the north. The planes dip steeply near the surface but become shallow at depth. The sediments, which formed the northern continental shelf of India before collision, have been thrust upwards towards the south over more ancient rocks. The underthrusting of India beneath Asia has resulted in continental crustal thickening from the normal 35 k m to about 70 k m in the Himalayas.

The 6 c m per year motion of the Indian Ocean plate away from the south-east Indian Ocean Ridge continues to push India northwards, and the subcontinent continues to underthrust beneath Asia. Most of the northwards convergence between India and Eurasia at a rate of 2 c m per year occurs n o w along the Main Boundary Thrust9.

Crustal thickening has resulted in extremely complex structures in the Himalayas and partial melting of the crust along the thrust zones has led to the formation of granite m a g m a s which have risen high into the M o u n t Everest region. The age of these granites has been demonstrated by radiometric dating to be as young as Miocene ( 13 to 21 M a old)10.

By contrast, the Gandise or Transhimalayan igneous belt to the north of the Yarlung-Zangbo suture zone is m u c h older. It contains both volcanic and plutonic rocks whose ages are as old as 485 extending to as young as 45 M a 1 0 . This belt therefore has a long history, most of which represents a Cordilleran or Andean-type volcanic-plutonic arc formed along the southern Asian margin before the arrival of India. Only the Eocene latest phase (45 M a old) is related to the actual collision.

Geological reasoning suggests that the granites of the Himalayas should be rich in an association of tin and tungsten deposits, but prospecting in the region has given disappointing results.

Older granites and tin-tungsten deposits

Southeast Asia contains a much older collisional suture along which two continents collided in Late Triassic time (220 M a ago). The suture zone extends northwards through Peninsular Malaysia, beneath the Gulf of Thailand, and then trends north-east near Urraradit and N a n in northern Thailand. Subduction of ocean crust before the collision resulted in volcanic rocks and granites to the cast of the suture and the final collision resulted in the 198 to 210 M a old Main Range granite of Peninsular Malaysia11 (figure 5).

These granites have been the source of more than 70% of the tin mined this century worldwide12. The tin deposits occur closely at the contact between the granite bodies and the sedimentary rocks into which they have intruded. The tin metal is in the form of

40

Tectonics and minerals in Southeast Asia

L E G E N D

Eastern Belt, epizonal cale-oik senes gabbro-granodionte-gronite Late

^ 4 Permian-Late Triassic

Central Belt, No-rich granitoid and metomorphic rocks Permian-Late " v 1

^—^ Triassic

Main Rang». Belt granites- Predomi­nantly late Triassic. Deep-seated and sheared on east, becoming epizonal in the direction of the arrow -4 west of Penang

North Thailand granites of continental crustal derivation Predominantly Late Triassic

j ^ Western Belt Cretaceous epizonal granites

2 2 0 Major radiometric ages, M a

Main areas of late Paleozoic-eorly Mesozoic volcanism

l i i I Ophiolite line (Suture Zone)

^— — Main major fault lines

Structural lines in the Gulf of Thailand

Tertiary shear-rift basin

Figure 5. The tin-tungsten granites of Peninsular Southeast Asia. Their ages, determined by radiometric measurements, are shown. After Hutchison1 ' .

41

Charles S. Hutchison

cassiterite (tin oxide) deposited along with quartz from hot fluids close to the contact zones. Deep weathering and erosion of the border zones during the Early Quaternary (1 to 2 M a ago) has removed the quartz and cassiterite and deposited them by major river systems into extensive alluvial fans, from which the cassiterite has been mined by opencast or dredging methods. T h e distribution of the tin fields is strongly hetero­geneous, with six major centres accounting for over 70% of the mined output12: the Kinta Valley (Ipoh), Bangka, Kuala L u m p u r , Phuket, Billiton, and the east coast of Peninsular Malayasia (Kuantan) (see figure 5). Usually the same granites which are associated with tin also have an association with tungsten. B u r m a formerly w a s a major producer from regions near Tavoy Point and west of M a e L a m a .

Mineral deposits within volcanic arcs

The other major types of mineral deposit in Southeast Asia are related to active and recently extinct volcanic arcs12. T h e trench system, which extends from Sumatra to the Banda Sea and northwards through the Philippines, is paralleled by very active arcs of volcanoes, characteristic of convergent plate margins. T h e volcanic activity occurs on

Figure 6. The principal mineral deposits of Southeast Asia in relation to plate boundaries. The size of the symbols bears no relation to relative importance of the deposits. The outline of Sundaland represents the region of older pre-Mesozoic (older than 150 M a ) continental crust. Tin, tungsten and antimony deposits are confined to Sundaland, After Hutchison13.

42

Tectonics and minerals in Southeast Asia

the overlying plate at a point where oceanic crust of the subducting plate has descended to a depth of 100 to 150 k m . In the Philippines there are two opposed trench systems (figure 6).

The recently extinct volcanoes contain disseminations and fine veins containing copper and iron sulphides associated with gold and silver. Deep erosion of the volcanic cones is necessary to expose the mineral concentrations, so that volcanoes which have been extinct for more than 10 M a offer the best exploration target. The greatest concentration of copper deposits (porphyry copper) is in the Philippines, with a smaller deposit at M a m u t in Borneo. During the present-day economic recession, emphasis is being placed on the discovery and exploitation of gold and silver deposits. These are mined in the Philippines and Indonesia, with formerly mined deposits at Bau in Borneo and Raubin Peninsular Malaysia (figure 6). N e w gold deposits are being discovered and exploited in Sumatra and Kalimantan.

Figure 6 shows the great contrast in distribution of mineral deposits. Older continental crust is necessary for the occurrence of tin, tungsten and antimony deposits12. They arc confined to the Sundaland region of continental Southeast Asia. Copper, iron, gold, silver, chromium and nickel occur in economic concentrations where continental crust is absent. Mercury appears not to be selective.

In summary , the localization of mineral, oil and gas deposits is largely the result of collision of microcontinents onto the margin of large continental masses at convergent plate boundaries. These processes induce large-scale deformation by underthrusting, folding and faulting. This emphasizes the need for a better understanding of these tectonized areas of the world within the framework of plate tectonics. •

Notes

1. J. R . C U R R A Y , F. J. E M M E L , D . G . M O O R E , and R. W . R A I T T , Structure, tectonics, and

geological history of the northeastern Indian Ocean. A . E . M . Nairn and F. G . Stehli (eds.). The Ocean basins and margins, Vol. 6, The Indian Ocean. Plenum Press, N e w York, pp. 399-450, 1982.

2. D . E . K A R I G , M . B. L A W R E N C E , G . F. M O O R E , and J. R . C U R R A Y , Structural framework of the

fore-arc basin, N . W . Sumatra. Journal of the Geological Society of London, Vol. 137, pp. 77-91, 1980.

3. J. J. V E E V E R S , (editor), Phanerozoic Earth Historv of Australia. Oxford Geological Sciences Series 2, Clarendon Press, Oxford, 1986.

4. B . T A Y L O R , and D . E . H A Y E S , Origin and history of the South China Sea Basin. D . E . Hayes (ed.) The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands: Part 2. Geophysical monograph 27, Amer. Geophysical Union, Washington, 1983.

5. D . M . D . J A M E S , (editor), The Geology and Hydrocarbon Resources of Negara Brunei Darusslam. Brunei M u s e u m and Brunei Shell Petroleum Co., Bandar Seri Bagawan, Brunei, 1984.

6. C . S. H U T C H I S O N , Tertiary basins of S. E . Asia-their disparate tectonic origins and eustatic stratigraphical similarities. Geological Society of Malaysia Bulletin, Vol. 19, pp. 109-122, 1986.

7. Named by Sir L. L. Fermor after the Gond, a central India hill tribe which lives north of Nagpur, and Van (or Wan), Sanskrit for forest. Thus, Gondwanaland, the supercontinent of which India was part more than 130 M a ago.

8. J. A C H A C H E , V. C O U R T I L L O T , and Y . X . Z H O U , Paléographie and tectonic evolution of southern Tibet since middle Cretaceous time: new paleomagnetic data and synthesis. Journal of Geophysical Research, Vol. 89, pp. 10311-10339, 1984.

9. J. L. M E R C I E R , and Li G U A N G C E N (editors). Mission Franco-Chinoise au Tibet 1980, Editions du centre national de la recherche scientifique, Paris, 1986.

43

Tectonics and minerals in Southeast Asia

10. F. D E B O N , P. L E F O R T , S. M . F. S H E P P A R D , and J. S O N E T , The four plutonic belts of the Transhimalaya-Himalaya: a chemical, mineralogical, isotopic, and chronological synthesis along a Tibet-Nepal section. Journal of Petrology, Vol. 27, pp. 219-250, 1986.

11. C S . H U T C H I S O N , Multiple Mesozoic Sn-W-Sb granitoids of Southeast Asia. Circum-Pacific plutonism terranes. Geological Society of America Memoir, Vol. 159, 35-60, 1983.

12. C . S. H U T C H I S O N , and D . T A Y L O R , Metallogenesis in SE Asia. Journal of the Geological Society of London, Vol. 135, pp. 407^428, 1978.

13. C . S. H U T C H I S O N , The distribution and origin of Southeast Asian ore deposits. Malayasian geographers. Vol. 1, pp. 13-36, 1978.

To delve more deeply

C . S. H U T C H I S O N , Geological evolution of Southeast Asia. Oxford, Clarendon Press, Oxford Geological Sciences Series, (in press).

44

Sedimentary basins, plate tectonics and oil fields

Bruce Sellwood

Whilst plate tectonics has not led directly to the discovery of specific oilfields, the concept has helped to explain why certain types of sedimentary basin in the world are more suitable than others as sites of oil and gas generation and containment, and provides a framework for future exploration.

Hydrocarbons are distributed in sedimentary basins around the world. These basins are crustal downwarps of all sizes in which sequences of sandstone, mudstone, limestone and salts have accumulated, sometimes in vast thicknesses of m a n y kilometres. Basins o w e their origins to the large-scale processes that govern the dynamic motions of the earth's crust and the movement of plates. They are generated on either continental or oceanic crust. The largest basins are quite simply the oceans themselves, but as w e shall see, oceans are unpromising sites for hydrocarbon formation and entrapment.

The type of sedimentary basin generated, and its subsequent geological evolution, depends upon interplay between a complex of variables, probably the most important of which are: whether the underlying crust is continental or oceanic in type; whether the tectonic regime is divergent (tensional) or convergent (compressional) (some basins c o m m e n c e as tensional regimes and subsequently experience strong compressional stresses, e.g. the Gulf area); and over what time period ensuing stress regimes operated. Climate controls the rate at which adjacent uplands are weathered, and thus to some extent the nature and maturity of deposited sediments.

Plate tectonics provides the petroleum geologist with a global framework within which to review the occurrence of oil and gas1. This framework also allows us to understand some of the changes that have taken place in the geographic distributions of lands and seas during geological time (changing palaeogeographic patterns). Such a broad-brush comprehension allows us to predict the global habitat of hydrocarbons in terms of likely areas of generation, migration and entrapment. But plate tectonics has not led to the discovery of specific oil fields; instead, the concept has helped to explain

Bruce Sellwood is Lecturer in Geology at the University of Reading in the United Kingdom. His principal professional interests centre upon the evolution of sedimentary basins, and his research work is carried out in southern England, the Paris Basin, the North Sea and the Middle East. D r Sellwood has held visiting professorships in Texas, U S A and Malaysia and has written many articles and contributions to textbooks on his subject. His address is: Department of Geology, University of Reading, Whiteknights, Reading R G 6 2 A B , United Kingdom.

45 Impact of science on society, no. 145, 45-53

Bruce Sellwood

w h y certain types of basin are more favourable than others as generation and entrapment sites.

The main requirements for regional hydrocarbon accumulation in a sedimentary basin are:

Potential source rocks: usually fine-grained sediments (shales, mudstones) with a high content of total organic carbon ( T O C ) . The presence of such sediments means that the waters in the basin were periodically stagnant. Gentle simmering of such sediments during burial allows hydrocarbons to be generated.

Potential reservoir rocks: usually sandstones or limestones with high porosities and permeabilities. Such sediments usually accumulate on shallow, well-agitated shelf areas or in high-energy channels.

Potential structures: ('buried bumps ' ) in which generated hydrocarbons m a y be trapped. Suitable structures can comprise reefs but frequently take the form of folded or faulted upwarps.

Potential seals: impermeable rocks such as salts and shales that drape over structures and retain, or retard, the upward migration of hydrocarbons.

An appropriate geological evolution so that source rocks m a y mature at a time w h e n sealed, reservoir-bearing, structures already exist, but before reservoir rocks have lost their porosity through cementation.

The last requirement is crucial. In h u m a n terms it is equivalent to being 'in the right place at the right time'. As w e shall see, basin style and geotectonic situation will both strongly influence the probability with which source rocks, reservoir rocks and structures are likely to be present.

Plate tectonics and the formation of basins

M o s t of the oil discovered to date has been found in sedimentary basins that formed on continental crust, so basins formed on oceanic crust will only be briefly addressed here. In concise terms oceanic basins seldom receive the right sorts of sediment in sufficient quantities, are mostly geothermally too cool to allow potential source rocks to mature and are tectonically too inactive to produce viable structures. The foregoing sentence thus eliminates 70% of the earth's surface and future exploration m a y prove it to be unduly (even dangerously) pessimistic. However, after more than 15 years of deep-sea drilling in the ocean basins, there are few indications to inspire optimism of the likelihood of giant oil accumulations in the present oceans.

Mos t of the world's proven hydrocarbon reserves are situated in basins initially generated by tensional forces affecting continental crust (figure 1). The three most important basin types (figure 2) are: interior sags, interior fracture basins (rifts) and marginal sags. Compression m a y subsequently affect basins initiated as sags or rifts while lateral motions form wrench basins. Indeed, wrenching m a y act as a modifier to m o r e simply formed sags. Kingston et al.2 have recently classified basins and assessed the hydrocarbon prospectivity of various basin types, their work forming a major source for the subsequent text.

46

Basins, tectonics and oil fields

Figure 1. M a p showing the distribution of major hydrocarbon basins in relation to the system of plates and plate boundaries on the earth (after North, 1985).

Interior sag basins are more or less circular or oval in shape and, as their n a m e implies, appear to have formed by sagging of the continental crust, usually without major faulting. This type of basin formed in m a n y parts of the world, especially during the Paleozoic (e.g. the Michigan Basin) and arc highly prospective (figure 3).

Interior fracture basins (rifts) are typified by the presence of normal faulting (figure 4). Basin initiation is often preceded by a phase of crustal doming prior to collapse of the linear rift site. Subsidence along the rift m a y be associated with extrusion of alkali basalt and subsequent crustal stretching permits numerous rotated fault blocks to form which are the main structures for hydrocarbon entrapment (e.g. the Gulf of Suez; the North Sea 170 million years ago).

Marginal sags are located on the outer edges of continental crust (figure 5) in areas of divergence (i.e. newly opened oceanic basins). They m a y form as a subsequent phase of crustal evolution in areas initially ruptured as continental rifts (e.g. the Atlantic seaboard of North and South America, Europe and Africa; the R e d Sea shelves).

Wrench or shear basins (figure 6) with hydrocarbon prospectivity are mostly young structures (Tertiary to Recent: < 65 million years). They are initiated by the formation of a fracture system which becomes more sinuous and less linearly continuous as it propagates across a continental region. A s faulting proceeds portions of the belt undergo locking and unlocking. Locked areas experience uplift, unlocked areas become basins. However , such systems are highly mobile and their very activity causes them to have short half-lives as prospective basins. Simply put, prospective structures can be both formed and breached very rapidly. Only if wrenching stops is it possible to 'freeze' processes and preserve hydrocarbon traps.

Active oceanic margins provide complex hydrocarbon plays associated with convergent trench, or trench-arc systems (figure 2).

47

Bruce Seilwood

Figure 2. Hypothetical continent exhibiting m a n y of the prospective basin types mentioned in the text, and some of the less prospective types. 1. Continental interior sag—in this case submerged beneath an epicontinental seaway (shallow sea lying over a continental interior) during a time of high global sea-level. 2. Continental interior fracture (rift). 2-3. Continental interior fracture evolving with continental marginal sag as a new ocean opens. 3. Continental margin sags, some with offshore reefs, some with open marine shelf sediments and another with a major river delta. 4. Continental wrench basins. 5. Oceanic trench and trench-associated basins. 6. Complex basin originating as continental interior fracture which then became an interior sag and is n o w being internally structured by compression.

48

Basins, tectonics and oil fields

Figure 3.

Model of an interior sag basin (simplified after Kingston et al2)

Sandstone

E3 Salt

Limestone

I —| Shale

Figure 4.

Model of an interior fracture basin (rift) (simplified after Kingston et al.2)

1

\ \

Ly T y~—i ^ — i

\ 1 V 1

[ 2 Volcano

K^ r = r TT" W' 1 1 Continental 1 crust

In reality, m o s t of the basins currently producing prolific hydrocarbons have undergone complex histories, having started as rifts or sags that subsequently suffered wrenching and/or compression (e.g. the Gulf area; figure 7).

49

Bruce Seilwood

Figure 5. 2

Figure 6. Model illustrating both the form and simplified m o d e of development of a wrench or shear basin system. Possible oil accumulation site is indicated by the position of drilling rig (after Kingston et al.2).

Basin type and hydrocarbon prospectivity

In the simplest of terms prospectivity involves understanding h o w the evolution of a particular basin-type promotes the conditions suitable for hydrocarbon accumulation mentioned earlier (p.46).

Interior sags were often sites of restricted circulation and such places, in c o m m o n with the present-day Black Sea, become stagnant at depth and accumulate large amounts of unoxidized organic matter in their bottom sediments. Thus they develop regionally extensive source-rocks. Marine basins which receive predominantly marine organic detritus tend to mature to produce oil-prone source-rocks while restricted lacustrine basins containing abundant land-derived detritus are often gas-prone. Interior sag basins m a y accumulate blanket sands and/or perimeter reef belts ideal as reservoirs. Periodic evaporation m a y also produce blanket seals. However , structuring is not intense so the commones t traps are those produced stratigraphically, either by

50

Basins, tectonics and oilfields

(15-0 Ma) Miocene-Recent

(250-200 Ma) Permian-Jurassic

(600-300 Ma) Cambrian-Carboniferous

Figure 7. The evolution of the Gulf area from Cambrian times to the Recent. Structuring has also involved upward migration of salt, such movements themselves often being initiated by wrench or compressive movements (simplified after Kingston et al.2).

porous and permeable sands passing laterally into less permeable ones, or by the biogenic construction of structures (e.g. intra-basinal pinnacle reefs, figure 8).

Interior fractures (rifts) receive non-marine sands within the block-faulted terrain and these sediments provide potential reservoirs. The block faults themselves formed during the initial phases of rifting provide excellent potential structures (figure 9), and these structures m a y be defined linearly by the presence of wrench faults. Subsequent phases of rift evolution m a y involve sustained thermal collapse of the rift site (sagging and sediment-draping over block faults) or the emplacement of new oceanic crust along the axis to the rift trough. In this latter case the interior fracture basin evolves into a newly formed marginal sag.

The marine sediments draping over fault blocks in either of these cases m a y provide both source rocks and potential caprocks. Alternatively, under arid climatic regimes evaporation m a y lead to extensive salt precipitation. W h e r e n e w continental margins evolve, thick continental shelf successions of sands, m u d s or even reefs m a y migrate over the underlying block-faulted basement. Such sediment sequences m a y include localized river delta sequences (e.g. the Niger) or, under appropriate climatic regimes, extensive reefs, carbonate banks and lime-sands (e.g. the Great Barrier Reef or the B a h a m a Banks). Marginal sags m a y thus receive extensive sand or carbonate sheets ideal for potential reservoirs, regional upwelling of nutrient-rich oceanic waters m a y initiate potential source-rock deposition. However , to cause structuring and source-rock maturation an external nudge is often required. Such a state of affairs affected the Gulf area during the Tertiary after the region had lain as an interior sag-marginal sag system since Cambrian times (600 million years ago). O n this belt of continental crust great thicknesses of sands, limestones and evaporites accumulated, the collision of this region with the Asian mainland generating folds and increasing the heat flow so that source rocks could mature (see figure 6).

51

Bruce Seilwood

Figure 8.

Model to illustrate 'pinnacle reefs' which comprise a typical form of stratigraphie trap in interior sag basins. Reef growth here produced the ' b u m p ' while later draping of salts provided the seal. Underlying organic-rich limestones or shales have been the source of the hydrocarbons.

W E

Figure 9. A tilted fault block comprising the typical trap style in interior fracture basins, and in m a n y marginal sag basins too. This example is the July Oilfield from the Gulf of Suez. Rudeis Shale is the source rock, Nubian Sandstone is the main reservoir and the Miocene evaporites comprise the seal (after Sellwood and Netherwood3).

Compressive oceanic margin systems often lack prolific source rocks but contain sands rich in unstable mineral components derived from adjacent volcanic terrains. T h u s , potential reservoirs often lose their porosity before source rocks have matured. Compressive (subduction-related) tectonics generate complicated structural styles that

52

Basins, tectonics and oil fields

are difficult to predict geometrically. Although oil and gas plays are associated with

actively compressive oceanic margins (as in the Gulf of Alaska) their geological

complexity renders them difficult targets for exploration. Despite these difficulties such

areas will doubtless continue to be explored in the future.

Conclusions

Plate tectonics m a y not lead directly to the discovery of individual oilfields, but the theory does provide a predictive framework for exploration. Basins formed o n continental crust are the prime sites for hydrocarbon formation and entrapment. Other factors influencing the hydrocarbon potential of particular basins are the paleo-climatic regime and the availability, at particular times in the earth's history, of particular groups of organisms capable of producing potential reservoirs as reefs, or potential source rocks as accumulating plankton. Future frontier areas for exploration are likely to be rift-sag systems such as the Barents Shelf to the north of N o r w a y , or the less promising and tectonically complex oceanic margins. Providing the political situation is resolved, however, large amounts of oil and gas still await discovery in the sag-collisional system of the Gulf. •

Notes

1. R. S T O N I X Y , Petroleum: the sedimentary basin. In: Economic Geology and Geoleclonics (Ed. by D . H . Tarling) BlackweM Scientific Publications, pp. 51 72, 1981.

2. D . R. K I N G S T O N , C . P . D I S H R O O N and P. A . W I L L I A M S , Global basin classification system. Bulletin of the American Association of Petroleum Geologists, Vol. 67, pp. 2175-2193, 1983.

3. B. W . S E L L W O O O and R . E . N F . T H E R W O O D , Facies evolution in the Gulf of Suez area: sedimentation history as an indicator of rift initiation and development. Modern Geology, Vol. 9, pp. 43-69, 1984.

To delve more deeply

F. K . N O R T H , Petroleum Geology. Allen & Unwin, 607 pp., 1985.

53

Volcanoes and men

Claude Jaupart

Volcanoes often erupt in cataclysmic events which cannot be foreseen. Our understanding and knowledge of volcanic phenomena are extensive, but do not yet allow detailed predictions. Volcanic forecasting is still in its infancy. Man has benefited from volcanoes, and has learnt to live close to them. He now needs to pay them the scientific respect they deserve by watching them over a significant portion of their lifetime. Data must be collected over periods of time spanning many eruptions in order to set the foundations for physical and chemical models. In many parts of the world, vulcanological observatories are being built for this purpose.

Volcanic eruptions are both magnificent and deadly, and have always held a fascination for m a n . They are probably the earliest geological phenomenon described in detail, by Pliny the Younger at Vesuvius in Italy in 79 B . C . Since ancient times they have been thought to express the anger and sorrow of the gods, the most famous example being Fuji-Yama in Japan, and perhaps the most curious is Pelé, the Hawaiian goddess w h o sheds tears and hair. Pelé's tears and hair are lava fragments thrown and spun during flight through the atmosphere.

Eruption styles are varied, but all exhibiting powerful and awesome forces beyond the reach of m a n . Through many years of observation and study, however, scientists have been able to c o m e up with a classification scheme1. A m o n g the most spectacular kinds of eruption is the Hawaiian one, characterized by 'fire fountains' in which what look like walls or curtains of lava are drawn to heights frequently exceeding 100 metres. Another is the Strombolian type, seen for example at Stromboli itself and Etna, both of which have been continuously active throughout historical times. Their trademark is a series of powerful explosions expelling lava fragments at speeds of hundreds of metres per second into the air. Other famous eruptions are those of the Plinian kind, where a huge jet of lava and gas shoots straight up to altitudes of several kilometres, perturbing the whole atmosphere. Perhaps the most frightening are the Pelean eruptions, named after Montagne Pelée in Martinique. These lead to 'nuées ardentes' or 'glowing avalanches', which are deadly surges of hot gas and pumice travelling d o w n volcano slopes at speeds of several hundreds of kilometres per hour and at temperatures above 600°C.

Claude Jaupart is Professor of Geophysics al the University of Paris 7 and at the Institut de Physique du Globe. His research focuses on physical models of magmatic processes, including the structure and dynamics of m a g m a reservoirs, and the nature of the different regimes of volcanic eruptions. Professor Jaupart m a y be contacted at the University of Paris 7, 4 , place Jussieu, 75252 Paris Cedex 05. France.

55 Impact oj science on society, no. 145. 55-61

Claude Jaupart

A complete list of all kinds of eruptions would be m u c h longer and somewhat out of context here. However, the few w e have described m a y serve to explain w h y volcanoes are so fascinating. They were thought to provide windows into hell and give m a n an idea of h o w weak and defenceless he really is. For the physicist, they are a true marvel and, scaled d o w n properly, are natural examples of what happens when m a g m a and gas undergo pressure release as they rise towards the surface. There are, of course, many interesting aspects of vulcanology and it would be impossible to treat them all in such a short article. Instead, after a brief summary of recent advances and our current understanding, w e shall address two questions: W h y have m e n kept on living close to volcanoes which could wipe them out? A n d w h y do w e k n o w so little about the conditions of an eruption and w h y cannot w e predict them with more confidence?

These questions seem timely n o w that there are signs of major disturbances in deep volcanic systems that were once thought extinct, in California and Italy. T o illustrate the fate promised to inhabitants in the advent of an eruption, it is sufficient to look at a geological m a p of the area around the town of Bishop, at the foothills of the Sierra Nevada along the West Coast of North America. The m a p shows a sheet of what geologists call ignimbrites, which is the scientific n a m e for hot ash deposits. The sheet covers an area of about 1000 square kilometres and has a thickness of more than 100 metres. It was deposited in a single event that occurred 700 000 years ago—yesterday on the geological time-scale—and it is conceivable that it could repeat itself. The area is clearly active at present, with seismicity and surface deformation. The last glimpse of m a g m a there was, in fact, only 700 years ago when a d o m e was extruded at M o n o Lake. There is no need to explain further w h y our understanding of this kind of volcanic activity is a priority.

Volcanoes: anywhere and anytime

Volcanoes can be grouped into two categories, using the framework of plate tectonics (see the article by José Achache in this issue). In the first category, they show up as belts parallel to plate boundaries (figure 1). The most spectacular are those of the so-called 'fire-belt' surrounding the Pacific Ocean, which includes Japan. These volcanoes are linked to the effects of subduction, i.e. to the interaction between a cold sinking lithosphère and the overriding one. Other volcanic belts are the oceanic ridges themselves: sea-floor spreading is achieved through repeated injections of m a g m a . The trademarks of volcanic activity can be found on the ocean floor, from the characteristic shape of lava flows emplaced underwater (pillow lavas) to the very existence of volcanic edifices. Yet another kind of volcano can be found along the contact between colliding continents; for example, the Tibetan plateau which is due to the collision of India into Asia is covered by volcanic deposits. In this case, m a g m a genesis (melting) is linked to the thermal evolution of thickened continents. These three kinds of volcanoes show up in elongated belts because they are the result of plate interactions, and hence with effects at their boundaries. Each one has its o w n characteristics, both from the chemical and physical points of view: to each its composition and to each its eruption style. Because these volcanoes are linked to plate tectonics, they depend on large-scale movements that are known with some accuracy. As a result their origin and long-term behaviour are reasonably well understood.

F r o m this brief summary, it would appear that one needs only live outside regions of tectonic activity (mountain belts, subduction zones) to be free of volcanic hazards.

56

Volcanoes and men

I ' . < / ' " ' < ' /• Active volcanoes of the world. Note that volcanoes usually form belts lying along tectonic boundaries. They are due to interactions at the edges of the plates that move on the Earth's surface. Some volcanoes do not lie along plate boundaries. They are called 'intraplate' volcanoes and are due to local upwellings in the mantle.

This is unfortunately not true because there is a second category of volcanoes, defined as intraplate ones: these are not linked to plate interactions and can 'strike' anywhere. For example, metropolitan France appears today to be one of the quietest countries geologically speaking, with only a few small earthquakes disturbing the peace from time to time. However, the Massif Central, located at the very centre of the country, is a region of recent volcanic activity, with explosive eruptions and extensive lava flows dating back only a few thousand years, i.e. a few hours ago on the geological time-scale. Similarly, Ge rmany experienced destructive volcanic eruptions in the Eifel and Kaiserstuhle provinces only 10000 years ago. These events are recent and hence due to deep-seated processes that are not extinct and which could give rise to new catastrophes. Indeed, detailed geophysical investigations have revealed the presence of m a g m a at depths in these regions. Another example of an intraplate volcano is Hawaii, which lies in the middle of the Pacific Ocean far from any boundary.

T h e first category of volcano is linked to plate interactions and movements. Looking deeper into the interior of our planet, they are seen to be the end result of thermal convection in the mantle. Convective motions in the mantle take the form of large 'cells', which are well-known in physics, driven by the earth's internal energy. It is important to realize that m a g m a does not exist everywhere below the earth's surface,

57

Claude 3aupan

contrary to a widely held belief. M a g m a is only generated at places where there are motions, a n d b y t w o m e c h a n i s m s : b y pressure release in rising parts of the mantle, a n d by frictional heating in the d o w n g o i n g lithosphère at subduction zones.

T h e second category of volcano bears n o relationship to the first a n d is attributed to a second form of convection, with sharp local upwellings called 'p lumes ' or 'hot spots'. Their origin is still debated: m a n y scientists see t h e m as being d u e to motions in a d e e p a n d isolated mant le reservoir.

O u r present understanding of the physics of mantle mot ions and convection is sufficient to s h o w that, indeed, there should be t w o different kinds of volcano. It is not sufficient, h o w e v e r , to understand the details. Despite m u c h effort during the last few hundred years, scientists are still unable to predict a n d forecast eruptions. Fortunately, volcanoes d o issue warnings before erupting. A s lava starts its journey towards the surface, it opens u p fractures in the earth's crust that can be recorded b y seismologists. A t M o u n t Saint Helens, in the United States of Amer i ca , such fractures were detected t w o m o n t h s before the eruption of 18 M a y , 1980 (figure 2a). O n Réun ion Island in the

Figure 2.

(a) Seismic activity at M o u n t Saint Helens, U S A , prior to the cataclysmic eruption of 18 May, 1980. (b) Seismic activity at Le Piton de la Fournaise, Réunion Island, prior to the eruption of 3 February, 1981. In both cases, seismic activity is measured by counting the number of earthquakes of given intensity (magnitude in the geological terminology). Note that activity starts a long time before the eruption: about two months before at M o u n t Saint Helens, and two weeks at Le Piton de la Fournaise.

First steam explosion

Cataclysmic eruption

• • • • • *i • • • • ! > • • ¿1 •

Eruption starts

Onset of seismic activity

21 2 3

January

31 1 3 February

58

Volcanoes and men

Indian Ocean, the Piton de la Fournaise volcano issued similar warnings for about two weeks (figure 2 b). However, such warnings are only signs of an impending catastrophe and cannot be used to determine exactly where and when lava comes out. The delay between the first lava motion at depth and eruption at the surface can be very short.

W h y do w e k n o w so little? O n e reason is that the study of eruptions is dangerous and impossible at close range. Vulcanologist D a v e Johnston was watching M o u n t Saint Helens from a distance of ten kilometres and yet was killed almost instan­taneously upon its eruption. Another reason is that a single eruption does not tell m u c h about the whole volcanic system which m a y be active for several tens of thousands of years. Each eruption has its o w n peculiarities and is never repeated, simply because it modifies the structure and geometry of the volcano. A final reason is that the physics and chemistry of such systems are extremely complex, involving as they do gas and m a g m a , interactions with underground water, and crystallization. The consequence is that no model is presently available to reproduce volcanic phenomena.

The situation is far from hopeless, however, and many advances have been m a d e in recent years. It is clear that the different kinds of volcanic eruptions are due to differences in lava viscosity, and theoretical analysis are available3'4. Also, data collected over long periods of time show that volcanoes are remarkably well-behaved, maintaining a rather constant performance. For example, the Kilauea (Hawaii) and Piton de la Fournaise (Réunion) volcanoes have been producing lava at a rate of one cubic kilometre of lava per hundred years for more than 2000 years, which is the time-span of available records. Both volcanoes erupt with a frequency of a few years, and hence the figure corresponds to a large number of eruptions. The output rate is so constant and regular that the time between two eruptions can be used to calculate the amount of lava to be expected each lime5. This behaviour shows that volcanic systems obey simple laws and are amenable to physical modelling.

In order to go further and understand the evolution of volcanoes in detail, the only method is to accumulate data over long periods of time. Because the life-time of a volcano is so long, it is necessary to have an idea of its average behaviour. This is what scientists have done by creating vulcanological observatories. The year 1987 marks the Diamond Jubilee—the 75th anniversary—of the Hawaiian Volcano Observatory, where scientists have been continuously collecting data on ground deformation, the distribution of fractures at depth, and eruptive history. France has operated observatories for about 10 years in the islands of Martinique, Guadeloupe and Réunion. Italy has been monitoring the activity of Etna and Stromboli with modern equipment for a long time. Scientists hope to understand h o w volcanoes work in the long run, as they n o w begin to understand the behaviour of the earth's magnetic field after four centuries of continuous measurements. With all the power of modern technology, they will perhaps reach their goal faster...

Volcanoes and civilization

Given the fact that volcanoes are so dangerous and unpredictable, why is it then that people still choose to live close to them? At Etna, villages are constantly being destroyed by lava flows, yet the inhabitants always return. The reason is not the force of habit, but a straightforward economical choice. Volcanoes offer very fertile ground, and a lot of energy. O n e has only to look at the rich orchards that cover the slopes of Hawaiian volcanoes to get the full measure of the agricultural prosperity in volcanic

59

Claude Jaupart

areas. Fertility is due to the high porosity of volcanic grounds, which are m a d e of either loosely consolidated ash or vesicular lava flows and hence retain high quantities of water. In Iceland, people live off volcanic energy, heating themselves and producing electricity from geothermal plants.

In a more general way , a good case can be made for the impact of volcanoes on the birth of civilization. For example, in Italy, back in the sixth century B . C . , the Etruscan society was the richest and most advanced, producing works of art of everlasting beauty. The Etruscan wealth was due to its agriculture on fertile volcanic tuff. M o r e striking was the outburst of architectural development and town building a m o n g North American Indians in Arizona around the eleventh century A . D . The wonderful cliff dwellings and six-storied castles that have been preserved to this day were all erected in a short period of time, apparently when the Anasazi Indian tribes were rich enough to spend less time wandering for food in the desert. This brief period happened just after the eruption of the Sunset Crater volcano which covered the land with ash, thereby producing fertile ground for all sorts of crops. Sadly, the volcanic activity was short-lived and the tuff underwent compaction, decreasing its porosity and losing its ability to retain water. Thus, the land returned to its arid state and the Indians to their nomadic life.

Unfortunately, if volcanoes can give birth to civilization, so they can also destroy it. The most famous case is that of the Minoan society in Crete, which was wiped from the surface of the Earth in 1400 B . C . on the explosion of the Santorini volcano. O n a smaller scale, Vesuvius was responsible for the deaths of Herculaneum and Pompeii.

Vulcanology and human concern

A final obvious question is w h y volcanoes are not watched more closely, and w h y so m a n y people have died because of them, even though they were aware of the potential danger. O n e reason is that volcanoes either erupt continuously in quiet lava flows, as on Réunion Island, or at large time intervals in destructive and powerful explosions. M a n has therefore tended to take them for granted and to forget about them. O n e Harry Truman w h o lived near M o u n t Saint Helens, refused to leave his h o m e w h e n told of the impending catastrophe in 1980. H e told reporters that he had known the mountain all his life and that it would not 'do this to him'. Unfortunately, it did do it to him and he was killed, buried under tons of ash.

Another reason is the lack of responsibility of administrative authorities, w h o cannot seem to weigh up the danger properly. M o r e than 30000 people were killed in the town of Saint Pierre on Martinique Island on 8 M a y 1902. The M o u n t Pelée volcano had been active since late April, but an election was to be held and it was felt that displacing people would be detrimental to the democratic process. It was, alas, the eruption which turned out to be detrimental to the democratic process. M o r e recently on Réunion Island, local authorities were not in favour of a vulcanological observatory—that was until a lava flow destroyed the police station. Similarly, it was only after the destructive explosion of M o u n t Saint Helens in M a y 1980 that an observatory was built to look after the many volcanoes of Oregon and Washington States.

It seems that h u m a n preoccupation is short-lived and that a catastrophe is soon forgotten. This can be defined precisely, in fact. Figure 3 shows the world 'reporting index', which is the number of reports on volcanic eruptions made each year, and which

60

Volcanoes and men

1860 1880 1900 1920 1940 1960 1980 Year

Figure 3. The last 120 years of vulcanism reporting (adapted from Simkin2). The line shows the number of volcanoes active per year. Note the general tendency to increase, which reflects the development of scientific studies. The curve is characterized by peaks which renew interest in volcanoes. The peaks follow catastrophic events which spur human concern.

is a reflection of h u m a n interest in such phenomena 2 . T h e plot shows two features. T h e first is a general increasing trend, reflecting the general growth of scientific studies. T h e second feature is a series of peaks. In almost all cases, these peaks follow big destructive eruptions, as in 1883 w h e n Krakatoa killed over 36000 people in Indonesia, and 1902, the year of the M o u n t Pelée blast. T h e striking characteristics of these peaks is that they all span the same length of time: 5 6 years. After this relatively short period h u m a n interest seems to decay and it is left to the next eruption to remind the world of the dangers associated with volcanoes. •

Notes

1. H . W I L L I A M S and A . R. M C B I R N E Y , Volcanology, Freeman, Cooper & Co. , San Francisco, 1979.

2. T. SIMKIN, L. SIEBERT, L. M C C L E L L A N D , D . BRIDGE, C . N E W H A L L and J. H . L A T T E R . Volcanoes

of the World, Hutchison Ross Pub. Co.. Pennsylvania, 1981. 3. L . W I L S O N and J. W . H E A D , III, Ascent and eruption of basaltic m a g m a on the earth and m o o n ,

Journal of Geophysical Research, Vol. 86, pp. 2971-3001, 1981. 4. S. V E R G N I O L L E and C . J A U P A R T , Separated two-phases flow and basaltic eruptions, Journal of

Geophysical Research, Vol. 91, pp. 12842-12860, 1986. 5. G . W A D G E , Steady-state volcanism: evidence from eruption histories of polygenetic volcanoes,

Journal of Geophysical Research, Vol. 87, pp. 4035-^049, 1982.

61

Seismicity of Japan: earthquakes and tsunamis

Katsuyuki Abe

Japan and its environs take the typical form of the world's island arcs, in which the oceanic plates subduct on their way back into the mantle. Both shallow and deep earthquakes occur there, and not a few earthquakes set off tsunamis. Japan has one of the longest histories of recorded earthquake activity in the world. If we examine the characteristics of its seismicity more closely, some interesting features emerge.

Earthquakes have been dreaded in Japan through the centuries. People there jokingly rank the four daily dreads, saying "jishin (earthquakeHcami'nan (thunder)-fcq/¡ (fire)-oyaji (my father)". A sensitive person in Tokyo might be able to feel about 30 shocks in one year. There have been more than 600 damaging earthquakes in Japan during the past 1500 years, some of them causing extensive disaster. A great earthquake of 1 September 1923, for example, claimed 142 807 lives and demolished more than 500000 houses in the Kanto area1. Earthquake-generated ocean waves, correctly called tsunamis, have occasionally threatened residents on the coasts. A large tsunami of 15 June 1896, for example, surged to heights of over 20 metres on the Sanriku coast, and killed 21 959 people.

Japan is located in the northwestern part of the circum-Pacific, where the activity of earthquakes and volcanoes is very high. Great earthquakes frequently take place and deep earthquakes occur d o w n to depths of 600 kilometres. This region takes the form of island arc systems, in which oceanic plates are subducting on the way back into the deeper part of the Earth. Where these plates interact, important geological processes are taking place.

Recently our knowledge of seismology has been greatly increased with the advance m a d e in the study of plate tectonics, and an exceptional wealth of data, both macroseismic and instrumental, is n o w available to help us try and understand earthquakes in Japan, and around the world. The examination of the earthquake activity of Japan certainly shows some interesting features.

Large earthquakes

The size of an earthquake is usually represented by its magnitude, M 2 . It can be related empirically to the amount of energy released in seismic waves. A n increase of one unit of

Katsuyuki A b e is an associate professor of seismology at the Earthquake Research Institute, University of Tokyo , Japan, and works on problems of seismicity, source mechanisms and the prediction of earthquakes. His address is: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan.

63 Impact of science on society, no. 145, 63-74

Katsuyuki Abe

magnitude corresponds to an increase of about 30 times the amount of seismic energy. A n earthquake of magnitude 8 has an energy of about 10 2 4 ergs, roughly equivalent to 1250 atomic bombs of the Hiroshima type. Seismologists m a y interpret this earthquake in terms of faulting along a fault having an area of 100 x 100 k m and a surface offset of 4 m.

Table 1 lists the 17 shallow earthquakes of magnitude 7-9 or over between 1886 and 1985 in the Japanese area, for which magnitudes can be determined from instrumental observations. All the shocks were destructive. During the past 100 years, on average, one or two great earthquakes have occurred every 10 years for Japan as a whole. The total energy release in these earthquakes is not m u c h o ver 3 x 102 5 ergs. This amount is roughly equivalent to the energy released in a single earthquake of magnitude 9 1 . A n example of such a super-great earthquake in the world is the Alaskan earthquake of 1964 ( M = 9-2)4. The average annual rate for the past 100 years is 3 x 102 3 ergs/year, which is 7 % of the annual average for shallow earthquakes on the globe. This amount is comparable to the energy release in a single earthquake of magnitude 7-8.

The spatial distribution of the great shallow earthquakes for 1886-1985 is shown in figure 1. It is notable that most of these shocks originated off the Pacific coast along the oceanic trench and trough. The great earthquakes that occurred off the Japan Sea coast are the Niigata earthquake of 1964 and the Central Japan Sea earthquake of 1983. The Nobi earthquake of 1891 is the largest k n o w n shock to have taken place in central Honshu. In m a n y instances, great earthquakes are followed by numerous shocks, called aftershocks, immediately after the main event. In the case of the Central Japan Sea earthquake of 1983, for example, more than 8000 aftershocks were recorded, over a period of two months.

Deep earthquakes with depths exceeding 60 kilometres do occur in the Japanese area5, though the frequency of occurrence is m u c h less than that of shallow shocks. The

Table 1. Largest shallow earthquakes of magnitude 7-9 and over for 1886-1985ui:i-9

Year

1891 1894 1896 1923 1933 1944 1946 1952 1953 1958 1963 1964 1968 1969 1973 1975 1983

Month

10 3 6 9 3 12 12 3 11 11 10 6 5 8 6 6 5

Latitude (°N)

35-6 42-5 39-5 351 39-2 33-8 330 41-8 340 44-4 44-9 38-4 40-7 43-4 430 43-2 40-4

Longitude (°E)

136-6 1460 1440 139-5 144-5 136-6 135-6 1441 141-7 148-6 149-6 139-2 143-6 147-8 1460 147-4 1391

Afs

80 7-9 6-8 8-2 8-5 80 8-2 8-3 7-9 81 8-1 7-5 81 7-8 7-7 6-8 7-7

M„

„

— — 7-9 8-4 8-1 81 81 7-9 8-3 8-5 7-6 8-2 8-2 7-8 — 7-9

M,

8-2 8-2 80 8-3 81 81 8-2 7-8 8-2 8-4 7-9 8-2 8-2 8-1 7-9 81

64

Seismicity of Japan

Figure 1.

Location of largest earthquakes, 1886-1985 Solid circles, shallow; open triangles, intermediate depth; closed triangles, deep5. Data are tabulated in tables 1 and 2.

great deep earthquakes for the period 1904-1980 are listed in table 2, and m a p p e d o n

figure 1 by triangles. T h e earthquake of 15 June 1911, which occurred at depth of 160

kilometres beneath the R y u k y u Islands, w a s the largest k n o w n deep shock in the world.

Despite their great depth, four of the five shocks in table 2 (the exception being the 1950

shock) were strong enough to cause minor d a m a g e .

Small earthquakes

Smaller earthquakes are m u c h m o r e n u m e r o u s than mos t people realize, because the frequency of occurrence rapidly increases with decreasing magnitude. In fact, the n u m b e r of shocks roughly increases b y ten times as magnitude decreases by one unit.

Table 2. Largest deep earthquakes of magnitude 7-5 and over for 1904-19801-2*

Year

1906 1909 1909 1911 1950

Month

1 3

11 6 2

Latitude (CN)

34-0 31-5 32-0 29-0 460

Longitude

fE)

138-0 142-5 1310 1290 144-0

Depth (km)

340 80

190 160 340

mB

7-5 7-6 7-5 8-1 7-5

65

Katsuyuki Abe

Figure 2 shows the location of shallow shocks of magnitude 5 or over for 1926-1985. There are on average 55 such shocks per year in the region. By extrapolation, the total number of earthquakes of magnitude 3 or over is estimated to be 4000 per year. This corresponds to about one shock every two hours.

Certain places in the Japanese islands are visited by earthquake swarms, a long series of relatively small shocks without a principal event. The most famous swarm was the event at Matsushiro in central Japan. The swarm began in August 1965, increased in size and lasted until after 1967. By the end of 1967, the number of all the felt shocks stood at 61000, whilst a total of 670000 shocks had been recorded by sensitive seismographs. The largest shock in the series was of magnitude 5-4.

Earthquake swarms are apt to occur in volcanic areas; they often occur before and after eruptions. In case of the large eruption of the Sakurajima volcano in 1914, m a n y shocks of magnitude 5-2 or less preceded the eruption, and the largest shock of magnitude 7 0 in the series occurred on 12 January about 10 hours after the eruption had started. The recent eruption of the Usu volcano in 1977 was followed by 16000 shocks that were felt in the vicinity for a period of one year.

A unique swarm-like earthquake activity is known at W a k a y a m a city, 30 kilometres southwest of Osaka. Numerous earthquakes of small magnitude have continually occurred there for the past 100 years or probably more. The total number of felt shocks amounted to 7194 for the period between 1880 and 1984. In recent years, about 400 microearthquakes of below magnitude 3 have been located every month by the sensitive seismograph network. The largest shock during the past 40 years was of magnitude 50. The epicentral area was non-volcanic7.

66

Seismkïty of Japan

Tsunamis

The term tsunami, which has come into international use, literally means a large wave in a harbour. It is derived from two Japanese words: tsu meaning a harbour, and nami meaning a wave. The most c o m m o n cause of a tsunami is the vertical movement of the sea floor along a submerged fault, associated with an earthquake. The waves are very low and almost undetectable to ships in the open ocean. A s they enter the shallow water, however, their heights are increased m a n y times by topographic effects. In particular, tsunamis are liable to grow in bays or inlets that are mostly V-shaped and open towards the ocean. Tsumanis have extremely long periods of between 5 and 70 minutes. Not all are large enough to cause damage, but occasional large waves cause great loss of life and extensive damage to property over hundreds of kilometres of the more populated coasts.

Japanese records documenting tsunamis extend back to 684 A . D . though of course the tsunami catalogue for the earlier period is necessarily incomplete, particularly for small tsunamis. In Japan, the overall effects of tsunamis are ranked by the Imamura-Iida scale, m , as given in table 3. Figure 3 shows the distribution of 171 tsunamigenic earthquakes that occurred in the Japan area during the period 684 1985. A glance at the m a p reveals that the Sanriku coast (the Pacific coast of northeastern Japan) and the Tokai-Nankaido coast (the Pacific coast of southwestern Japan) have been subject to more large tsunamis than any other region. This is primarily due to the combined effect of the higher frequency of great earthquakes originating offshore and the very irregular coastlines.

During the past 1300 years, Japan has been struck by 16 large tsunamis of m = 3, and five great tsunamis of m = 4. The great tsunamis of m = 4 devastated the Sanriku coast on 13 July 869 and 15 June 1896, and struck the Tokai-Nankaido area on 28 October 1707 and 24 December 1854. The fifth large (w = 4) tsunami occurred on 24 April 1771 and devastated the small island of Ishigakijima, located beyond the limits of figure 3 in the southwest, resulting in 9313 deaths. During the past 100 years, 120 tsunamis of m = — 1 or over have been recorded for Japan as a whole; there is on average one event per year.

A m o n g the m a n y tsunamigenic earthquakes, there exist unusual events called tsunami earthquakes. This term refers to an earthquake which generates anomalously extensive tsunamis for its magnitude. The most famous event of this type was Sanriku earthquake of 15 June, 1896, with a resultant devastating tsunami. During the earthquake, the ground motion on the coast was very weak. Nevertheless, a large tsunami struck the Sanriku coast, rising to a height of 30 metres or over at Ryori,

Table 3. Imamura-UJa scale of tsunamis

m Tsunami height Damage

— 1 Less that 0-5 m None 0 About 1 m Very small damage 1 About 2 m Damage to houses and boats 2 4-6 m Loss of lives and houses 3 10 m Considerable damage along more than 400 k m of coastline 4 More than 30 m Considerable damage along more than 500 k m of coastline

67

Katsuyuki Abe

Figure 3.

Distribution of tsunamigenic earthquakes during the period 684-19858 . Circles are ranked by the tsunami scale given in table 3.

/ r " ~ m ~

O 4 •' o 3 / O 2

¡ O 1 ; ° o i • -IJ

i -

/l

°\rv >- r-

- • ' " - / ' r

i"Vi V : -V - „ V ü F

f Y . - • ; , - • - < r > ' • - >

\ \

t" \

¿Ü P \

\

\

\r

*,&' F c- - - & " '1 • T f r ^ ^ ' . ^ r''

500 KM

24 metres at Yoshihama, and resulting in 21 953 deaths. The magnitude derived from tsunami waves is as large as 82 , while the magnitude derived from seismic waves m a y be as low as 68 9 . The earthquake of 10 June 1975, off Shikotan island, is a more recent example (see table 1). The cause of tsunami earthquakes is much debated.

Tsunamis respect no international boundaries. Japan is occasionally struck even by tsunamis originating from very remote sources. O n 5 November 1952, the tsunami caused by the Kamchatka earthquake of magnitude 9 0 inundated 1200 houses along the Pacific coast of Japan. Most noteworthy is the tsunami caused by the great Chilean earthquake of magnitude 9-5 on 23 M a y 1960: this travelled across the Pacific in about 22 hours. The waves rose as high as 6 metres, and killed 122 people, injuring 873 others, demolished 5107 houses and wrecked 1137 boats along the Pacific coasts. Japan was also visited by large tsunamis from South America in 1837 ( M 925), 1868 ( M 9-0), 1877 ( M 9-0), 1906 ( M 8-7) and 1922 ( M 87). These tsunamis struck not only Japan but also the Hawaiian islands. For the mitigation of tsunami hazards, regional and inter­national tsunami warning systems have been established, and large-scale tsunami walls and breakwaters have been constructed in tsunami-prone areas (see figure 4 a, b).

Destructive earthquakes

Japanese records documenting damaging earthquakes go back to 416 A . D . ; however, the records for the earlier period are inevitably incomplete. A n extensive catalogue lists over 600 damaging earthquakes during the past 1500 years10. Figure 5 shows the location of damaging shocks recorded for the period 416 1985.

68

Seismicily of Japan

Figure 4.

Tsunami protection measures in Japan, (a) Concrete breakwater units scattered over sand dunes by a tsunami in 1983. Each unit weighed 4 tonnes and the dune was 11 m above sea level. (Photo courtesy of International Tsunami Information Center, Honolulu.) (b) Large sea wall protecting the town of Taro, Iwate Prefecture. It is 2433 m long and 10 m high above sea level. The major tsunamis of 1896 and 1933 killed 1859 and 911 inhabitants respectively in this area alone. (Photo courtesy of Taro Town.)

m ^

(b)

69

Katsuyuki Abe

Figure 5.

Distribution of damaging earthquakes, 416-198510.

í Q M>8 / O 8 > M > 7

/ ° 7>M>6

! o 6>M •'' ( n,r

- S'VÎ t-, : i £ v ° i •.--il-

¿ ¿ . • / ó • "" :,

Í

,'̂ _

Table 4 gives a list of the large destructive earthquakes for 1800 1985, in which more than 500 lives were lost. It is to be noted that earthquakes do not need to be of large magnitude to produce severe damage, because the degree of damage depends not only on the physical size of an earthquake but also on other factors such as where and when an earthquake occurred, the population density in the area concerned, and secondary events such as fire. The Kanto earthquake that occurred at noon on 1 September 1923, was one of the worst k n o w n disasters in Japan. Although buildings and structures were severely damaged by this shock, the greater part of the disaster in Tokyo, Y o k o h a m a and the environs was caused by a great fire which immediately followed the shock. Special features of the Niigata earthquake of 16 June 1964, were the liquefaction effects which occurred within the city due to the underlying water-saturated alluvium. A debris avalanche caused considerable damage at the time of the Nagano earthquake of magnitude 6 8 on 14 September 1984. The local earthquake of Edo on 11 November 1855, caused severe damage to the city of Tokyo. Similarly, the Tottori earthquake of 10 September 1943, the Fukui earthquake of 28 June 1948, and other inland shocks serve to illustrate the destruction caused by earthquakes occurring near congested urban areas. There is much to learn from these earthquakes that is directly relevant to reducing earthquake hazards.

Fortunately, a large earthquake has not occurred within a densely populated Japanese city in recent decades, and the death toll associated with earthquakes has been at a low level since the Fukui earthquake of 1948. The worst death tolls in recent years have been those of 139 persons by the Chilean tsunami of 1960, 103 persons by the Central Japan Sea earthquake of 1983, and 52 persons by the Tokachi-Oki earthquake of 16 M a y 1968.

70

Seismicity of Japan

Table 4. Damaging earthquakes with 500 deaths and over for 1S00-19H51' '

Year

1828 1847 1854 1854 1854 1855 1872 1891 1894 1896 1923 1927 1933 1943 1944 1945 1946 1948

Month

12 5 7

12 12 11 3

10 10 6 9 3 3 9

12 1

12 6

Latitude ( :N)

37-6 36-7 34-8 340 33-0 35-7 34-9 35-6 38-9 39-5 35-1 35-5 39-2 35-5 33-8 34-7 33-0 36-2

Longitude CE)

138-9 138-2 1360 137-8 135-0 139-8 132-0 136-6 139-9 144-0 139-5 135-2 144-5 134-1 136-6 137-1 135-6 136-2

M

6-9 7-4 7-6 8-4 8-4 6-9 7-1 8-0 7-0 6-8 8-2 7-6 8-5 7-4 80 6-8 8-2 7-3

m

— —

3 4

— 0

— —

4 2

-1 3

— 3 0 3

—

Region

Niigata Nagano

Kii Tonankai

Nankai Tokyo

Shimane Gifu

Yamagata Sanriku

Kanto Kyoto

Sanriku Tottori

Tonankai Aichi

Nankai Fukui

Deaths

1443 >6000

1800 >1000

3000 10000 >800 7273 726

21959 142807

2925 3064 1083

>871 1961 1330 3769

Houses demolished

9808 >14000 >4300

>10000 > 30000 >17000 >5700 142177 >6000

>11000 576262 21690 6067 7736

>13586 5539

15710 40035

Seismicity and plate tectonics

In the worldwide scheme of plate tectonics, the area around Japan is k n o w n to be a region where the oceanic plate is subducting beneath the continental plate. Three plates meet together (see figure 6) and their motions are rather complex. The Japanese islands are located on the seaward edge of the Eurasian plate. The Pacific plate is subducting

Figure 6.

Arc structure and plates in the area of Japan.

71

Katsuyuki Abe

beneath the northeastern islands along the Kuril Trench and the Japan Trench, and is also subducting beneath the Philippine Sea along the Izu-Bonin Trench. The Philippine Sea plate is subducting beneath the southwestern islands along the Nankai Trough and the Ryukyu Trench.

Figure 7 shows the distribution of shallow and deep earthquakes between 1964-1975. The deep earthquakes are localized within a thin zone, which is representative of the gross structure of the subducting plate.

In the nineteenth century, it was rumoured that earthquakes were caused by namazu, a giant black catfish that lived in the m u d beneath the earth. N o w it is widely accepted that most earthquakes are caused by a sudden release of tectonic energy in the form of faulting. The thrusting motion of the plate gradually accumulates the strain energy around the interface between the oceanic and continental plates. W h e n the energy overcomes the resistance, the slip suddenly occurs along the interface. This mechanism is the most c o m m o n cause of large shallow earthquakes along the Pacific. O n e notable exception is the Sanriku earthquake of 3 March 1933, which occurred through a large-scale faulting within the Pacific plate.

The Japanese area can be grouped into five arc systems: the Kuril arc along the Kuril Trench (Region K in figure 6), the northeastern Honshu arc along the Japan Trench (Region J), the Izu-Bonin arc along the Izu-Bonin Trench (Region I), the southwestern Honshu arc along the Nankai Trough (Region N ) , and the Ryukyu arc along the Ryukyu Trench (Region R). Individual arcs show different features of the seismic activity.

I50°E

72

Seismicity of J upan

Kuril arc and northeastern Honshu arc (Regions K and J)

The activity of large shallow earthquakes is very high near the trenches in the Pacific, as was shown in figure 1. Relatively recently the Tokachi-Oki earthquake of magnitude 8-2 occurred north of the Japan Trench on 16 M a y 1968. It is k n o w n from historical tsunami records that at the same place similar earthquakes of roughly the same magnitude occurred in 1677, 1763 and 1856; the average recurrence time is therefore 97 years. The recurrence time of great earthquakes along the Kuril Trench is considered to be about 100 years. O n the contrary, the recurrence time seems irregular in the region off the Sanriku coast: great earthquakes occurred there in 869, 1611, 1896 and 1933.

N o great earthquakes have occurred near the southern Japan Trench, but a swarm of large earthquakes once took place; in November 1938, 27 shocks of magnitude 6 or more occurred there, and small tsunamis were frequently set off. The activity of shallow earthquakes in the Japan Sea area is low, but large destructive shocks such as the Central Japan Sea earthquake of 1983 often took place off the west coast of the Japanese islands. Occasional destructive earthquakes have occurred in the islands themselves, as shown in figure 5.

Deep earthquakes in this region are situated more or less continuously in a thin zone that dips downward from the oceanic region towards the continent at a dip angle of about 30°. The deepest one is 600 kilometres deep, near the Korean peninsula. This inclined zone of deep earthquakes is known as the Wadati zone. Recently the deep seismic zone has been found to be two-layered. The double seismic zone is observed between 60 and 180 kilometres in depth and the two layers are separated by about 35 kilometres11. This locality is probably the only one in the world where earthquake activity extends to a depth as great as 600 kilometres on a plane with a relatively low dip angle.

Izu-Donin arc (Region I)

In this region the Pacific plate thrusts towards the west beneath the Philippine Sea. The deep earthquake activity is high, and extends to depths of 600 kilometres. The dip angle of the Wadati zone is nearly 45 at the northern part, and increases to the south. Despite its typical island arc feature the activity of large shallow earthquakes is very low; in the area south of 33 N , the magnitude of the largest k n o w n shallow shock during the period 1898 1980 is only 70 .

Southwestern Honshu arc and Ryukyu arc (Regions N and R)

In the southwestern Honshu arc, deep earthquake activity is low, while that of great shallow shocks is very high. In the Ryukyu arc, the deep earthquake activity is limited only to a depth of about 280 kilometres. In the northern Ryukyu arc, shallow shocks of magnitude greater than 7 have often occurred, as shown in figure 2.

Great shallow earthquakes have occurred in the region along the Nankai Trough with remarkable regularity. During the last 1300 years, great shocks in the Nankaido area occurred in 684,887,1099,1361,1605,1707,1854 and 1946; the average recurrence time for the last 400 years is about 110 years.

The Tokai district, including the Suruga Bay, near the northernmost tip of the Nankai Trough, has not experienced a great earthquake since the last event in 1854.

73

Seismicity of Japan

Several lines of evidence indicate that this area is under an impending threat of such an event. T h e Large-Scale Earthquake Countermeasures Act was approved in 1978 to prepare for the anticipated disaster, and careful scientific monitoring to detect premonitory phenomena has been concentrated in this area. T h e impending earth­quake is thought likely to represent a slip along the interface between the Eurasian plate and underthrusting Philippine Sea plate. •

Acknowledgments

The author is grateful to Dr T . Yochii who kindly plotted figures 2, 3, 5 and 7.

Notes

1. Local time is used in the text. For Universal Time, 9 hours should be subtracted from Japan Standard Time.

2. Various scales of magnitude have been developed to accommodate the use of different types of seismic wave or to express the complex process involved in earthquakes: ms is surface-wave magnitude based on the amplitude of surface-wave vibrations recorded by a seismograph. The scale widely used in Japan is close to M s . M „ , is moment magnitude defined by the size of the earthquake faulting. M , is tsunami magnitude measured from the amplitude of tsunami waves. mB is body-wave magnitude based on the amplitude of body-wave vibrations. For details, see H . K A N A M O R I , Magnitude scale and quantification of earthquakes. Tectono-physics. Vol. 93, 185-199, 1983.

3. K . A B E , Physical size of tsunamigenic earthquakes of the northwestern Pacific. Phys. Earth Planet. Inter., Vol. 27, 194-205, 1981. Quantification of major earthquake tsunamis of the Japan Sea. Phys. Earth Planet. Inter., Vol. 38, 214-223, 1985.

4. H . K A N A M O R I , The energy release in great earthquakes. J. Geophys. Res., Vol. 82, 2981- 2987, 1977.

5. More specifically, the general term deep earthquake includes the two classes, intermediate (depth of 60-300 k m ) and deep (300 k m or more).

To delve more deeply

H . K A N A M O R I , M o d e of strain release associated with major earthquakes in Japan. Annual Rev. Earth Planet. Sei., Vol. 1, 213-239, 1973.

K . M O G I , Earthquake Prediction, Academic Press, Tokyo, 355 pp., 1985. C . F. R I C H T E R , Elementary Seismology, Chapt. 30, W . H . Freeman and Co. , San Francisco,

768 pp., 1958. T . R I K I T A K E , Earthquake Forecasting and Warning, Center for Academic Publ. Japan, Tokyo,

402 pp., 1982. A . S U G I M U R A , and S. U Y E D A , Island Arcs-- Japan and Its Environs, Elsevier, Amsterdam, 247 pp.,

1973. T . U T S U , Seismology 2nd edn., Kyoritsu Shuppan Publ. Co . , Tokyo, (written in Japanese),

310pp., 1984." S. U Y E D A , The View of the Earth, W . H . Freeman and Co. , San Francisco, 217 pp., 1978.

74

Earthquake-proof construction and architecture

Anand S. Arya

Earthquakes have claimed the lives of millions of people over the centuries, and no doubt most of those perished amid the rubble of their homes, their schools and their places of work. Whilst short-range earthquake prediction has a somewhat limited role to play in disaster mitigation, much can be done to protect those living in seismically active areas of the world by the adoption of earthquake-resistant construction methods—whether they be for the high-rise block of the city or the more modest rural dwelling.

Earthquakes constitute one of the greatest natural hazards to life and property on earth. They have caused the destruction of villages, towns and cities throughout recorded time on nearly every continent. D u e to the suddenness of their occurrence, they are the least understood and most dreaded of phenomena. The instantaneous and total devastation caused by a major earthquake, like the Tangshan, China, earthquake of 1976 in which 655 000 lives were reported lost, leaves an unparalleled psychological impact on m a n and calls for his tackling the problem of his o w n protection by using all the scientific and technological means at his disposal.

The disastrous effects of earthquakes primarily concern the destruction of various m a n - m a d e structures like buildings, bridges, d a m s and power plants. The collapse of buildings such as houses, apartment blocks, schools, hospitals and shops is the most significant direct effect of earthquake disaster, and most of the loss of life during earthquakes can be attributed to their destruction. However, in some cases, earth­quakes can also lead to chain effects that greatly multiply the loss of h u m a n life and scale of economic damage. For example, the large-scale ground settlement and liquefaction in the Niigata, Japan earthquake of 1964, the fires in the San Francisco, U S A earthquake of 1906 and the Kanto, Japan earthquake of 1923, and the sea wave and ground liquefaction in the Bengal, India earthquake of 1737, all claimed huge loss of life.

Taken together, direct and secondary effects of earthquakes, have accounted for a large number of h u m a n lives in the course of recorded history. Table 1 gives the death tolls of just some of the major earthquakes that have occurred in the world.

Anand Arya is Professor and Head of the Earthquake Engineering Department of the University of Roorkee. H e is the author/editor of 5 books and some 200 papers, and is currently engaged in research and development work related to the earthquake resistance of non-engineered constructions. Fellow of the Indian National Science Academy, Professor Arya has served abroad as a Unesco expert many times, and has lectured widely on his subject. His address is Department of Earthquake Engineering, University of Roorkee, Roorkee, 267667, India.

75 Impact of science on society, no. 145, 75-87

Ananá S. Arya

Earthquake prediction and the engineering approach

Prediction of seismic events is sometimes emphasized as one of the most important approaches for mitigating the disastrous effects of earthquakes. Certainly, earthquake prediction was used effectively in saving thousands of lives during the Haicheng, China earthquake of February 1975, by the issuing of warnings about the impending hazard a few hours before the event, although buildings were destroyed. However, this approach did not work in the Tangshan, China, 1976 earthquake and the loss of life was colossal due to the almost instantaneous destruction of houses. It would seem that accurate earthquake prediction or warning will help save lives by bringing people out from their dwellings and into the open, but cannot do anything to reduce the destruction of houses or other m a n - m a d e structures. Such an aspect of earthquake disaster can only be countered by building appropriate earthquake-resistant structures using reliable design and construction methods. Therefore, whereas even a completely successful earthquake prediction programme can neither eliminate nor minimize the need of safer construction, the effective application of earthquake engineering knowledge to structures will ensure their safety from collapse, save lives and property, and ensure the continued functioning of essential services such as water supply, sewerage, highways, railways, power lines, communication networks and so on. This approach greatly reduces the need for short-range earthquake prediction programmes and instead emphasizes engineering design and construction programmes for effective earthquake disaster mitigation. Earthquake prediction studies are, of course, valuable for understanding the immediate causes of earthquakes in a region and their associated precursory phenomena; long- and medium-range forecasting enabling suitable action to be taken for disaster preparedness, and for fixing priorities in the implementation of earthquake disaster mitigation.

Earthquake hazards pose to an engineer a unique problem that does not occur with either the other forces of nature or normal operating loads. A n earthquake can produce the most severe of all loadings on a structure, yet the probability of such a severe loading actually occurring is very low. If cost were not a consideration one would, of course, be tempted to design the structure to withstand the most severe probable earthquake forces. But the fact is that, in most countries of the world, other more basic needs of the population preclude such a totally safe approach, and so a fail-safe system has to be considered.

The most appropriate engineering approach in the circumstances is to design and build a structure so as to avoid partial or complete collapse, even in the m a x i m u m probable earthquake, but to accept some distortion or damage which m a y be repaired after the event. T h e basis of this approach is the assumption that to build all structures to a damage-proof standard is m u c h more expensive than to repair or replace those few structures that will actually be subjected to the most severe earthquake loading. Thus, structures need to be designed to withstand only the more frequent low-intensity earthquakes.

Since the seismic force generated on a structure depends on the properties of the structure itself—besides of course the intensity of the earthquake—the design engineer has two choices: (a) to m a k e the structure strong, or (b) to reduce the imposed seismic force by adjusting the structural stiffness. In other words, by choosing alternative structural systems, very different seismic force levels can be achieved, as for example, by adopting seismic isolation techniques, the earthquake loads on the isolated structure

76

Earthquake-proof construction and architecture

can be reduced to any desired level. The costs involved in these two approaches, as well as the state of technological development in a given country, will govern the final solution to be adopted for seismically safe designs.

In the overall scheme of safety from earthquake disasters, buildings m a y be seen to play a most crucial role, and the remainder of this article will be devoted to their planning, design and construction for earthquake resistance.

Engineered and non-engineered buildings

Buildings can be classified in various ways, using criteria such as occupancy, height, construction materials used, location in urban or rural area, and so on. For the purpose of this discussion, however, w e shall classify them as (a) engineered, and (b) non-enginccred buildings.

Engineered buildings are those designed and constructed by professionally qualified architects and engineers. They use modern building materials such as concrete, steel and other high-technology materials, and are normally situated in urban centres, and governed by the country's standard regulations and byelaws. They are often high-rise multi-storey buildings.

Non-engineered buildings are those which are spontaneously and informally construc­ted in the traditional manner without the intervention of qualified persons in their design or construction. They m a y nevertheless follow recommendations derived from the observed behaviour of such buildings during previous earthquakes in the region. Non-engineered buildings include load-bearing masonry wall buildings, constructions in w o o d , adobe (unfired clay brick or block) and earth: that is, using local and traditional materials and skills. Such buildings are found in all rural and semi-urban areas of the world, and even in the urban areas of developing countries. They are the ones that suffer the most damage, including collapse, even during moderate earth­quakes of magnitudes 6 to 7-5 (see figure 1) and have accounted for m u c h of the loss of life given in table 1. Unfortunately, a very large proportion of humanity lives in such buildings, even though their location m a y lie in moderate and severe seismic zones of the world. T h e situation is worsened by the fact that in the published literature very little attention has so far been paid to the seismic safety of non-engineered buildings; instead most studies have been related to engineered ones serving limited urban populations.

Only the basic principles governing earthquake-resistant design and construction of the two types of building are dealt with below, but some details of non-engineered buildings are given for illustrative purposes.

The architectural planning of multistorey buildings

Studies of damage caused by past earthquakes in Chile, Mexico, N e w Zealand, Yugoslavia, the U S A and Venezuela have shown that the architectural configuration of a building, i.e. its size and shape in plan and elevation, constitutes one of the major factors in its seismic success or failure. This is true for both engineered and non-engineered buildings, and therefore deserves the utmost attention at the initial planning stage. The important points in the architectural planning and design of multistorey buildings are described below.

77

Ananá S. Arya

Figure 1. D a m a g e caused to an adobe non-engineered house by an earthquake of modest intensity.

Table 1. Loss of lives in past earthquakes

Year

342 565 856

1201 1268 1290

1456 1556 1622

1641 1653 1667

1688 1715 1737

1755 1755 1759

1783

Location

Antakya, Turkey Antakya, Turkey Corinth, Greece

Aegean Sea, Greece Seyhan, Turkey Jehol, China

Naples, Italy Shansi, China Kansu, China

Tabriz, Iran Izmir, Turkey Shemakha, Iran

Izmir, Turkey Algiers, Algeria Bengal, India

Lisbon, Portugal Kashan, Iran Sfat, Jordan

Calabria, Italy

Deaths

40000 30000 45 000

100000 60000

100000

30000 830000

12000

30000 15000 80000

15 000 20000

300000

60000 40000 20000

50000

Year

1847 1853

1853 1861 1869

1905 1908 1915

1920 1923 1934

1935 1939 1960

1968 1970 1972

1972 1976

Location

Zenjosi, Japan Shiraz, Iran

Isfahan, Iran Mendoza, Argentina Riku-Ugo, Japan

Kangra, India Messina, Italy Avezzano, Italy

Kansu, China Kwanto, Japan Bihar, India

Quetta, Pakistan Chilian, Chile Agadir, Morocco

Khorasan, Iran Ancash, Peru Ghir, Africa

Managua, Nicaragua Tangshan, China

Deaths

12000 12 000

10000 18000 27 000

19 000 83 000 30000

180000 140000

11000

30000 30000 15 000

13 000 40000 17 000

23 000 655000

From J. H . Tatsch. Earthquakes, Talsch Associates. Sudbury, Massachusetts, U.S.A. p. 1-3. 1977.

78

Earthquake-proof construction and architecture

Simplicity and symmetry

T o avoid unexpected concentrations of stress, it is an advantage to have the structure as simple as possible. Symmetry is desirable about both axes of the building, in plan as well as in elevation, in order to avoid torsional effects. Internal details like stairs, lifts, lobbies, etc. should also be located so as to maintain near-symmetry if not full symmetry. These points are best illustrated by the sketches in figure 2.

Simplicity and symmetry in extended or complicated plans can be achieved by splitting the building into smaller blocks, through the use of separation and crumple sections, as s h o w n in figure 3.

Adequate gap between buildings or blocks

In order to avoid damage to two adjoining buildings or blocks by the pounding or

hammering action between them caused by their vibration during an earthquake, there

Figure 2. Buildings with irregular configurations

Figure 3. Improving the building configuration by the use of separation/crumple sections.

79

Anand S. Arya

should be an adequate gap between them from plinth upwards. The size of this gap will primarily depend upon earthquake intensity, and the height and flexibility of the buildings themselves1.

Lateral force-resisting elements along both axes

There is one important difference between wind or blast effect and earthquake action on a building: while the former forces act externally on the exposed areas of the building, the latter acts internally on its mass. Therefore, whereas for wind load, the small dimension of the building is usually critical for design, for earthquake load both are equally important. T o achieve adequate earthquake resistance, the column arrangement shown in figure 4 is appropriate. Similarly, shear or bracing panel walls must be provided along both axes of the building.

Core-type building

T o o great a concentration of shear walls in the form of rigid cores with very flexible columns has led to severe damage or collapse, due to inadequate means of horizontal shear transfer from floors to the cores. It is good planning for the shear walls to be well distributed over the plan along both principal axes, resulting in a symmetrical distribution of stiffness (figure 5). If the functional requirements of the building dictate the adoption of geometrical assymetry in the plan, it is necessary to adjust the stiffness of shear walls so that the centre of mass of the building on each floor coincides with the centre of stiffness.

Infill masonry panels

Infill masonry panels within a reinforced concrete or rigid steel framework have two effects on the structure. The lateral stiffness of the structure is increased several times as compared to the bare frame. Its strength is also increased m a n y fold. But the behaviour

Figure 4.

Column configuration for equal seismic strength along X X , Y Y .

H X -—\-

I—I—I—I—I—I—I

- H - H - H - M -

H - M — M

80

Earthquake-proof construction and architecture

Figure 5.

Distribution of shear walls, (a) Core only-scismically weak, (b) Core and disturbed shear walls-seismically strong.

• • •

• • •

Core • • •

• • I

(a)

S . W .

1 • • •

• • •

1 • •

Is.w. 1 — • • •

Core

• •

1 • • •

• • •

m m 1 (b)

of the structure in the initial range of loading changes from ductile to brittle. Because of

increased rigidity larger seismic force will be attracted and m a y cause d a m a g e to the

brittle infill. O n the other hand, if the panel wall is left free from the frame, the wall m a y

fall over due to inertia force on its o w n mass. Therefore, some means should be

adopted of connecting the frame to the wall so that considerable deflection of the frame

is possible whilst the wall is prevented from falling by the top beam. T h e gap between

the infill and frame should be filled with some flexible sound-proofing material.

Stiffness continuity

Sudden changes of stiffness and resistance lead to dynamic irregularity and introduce

the danger of d a m a g e . Such changes m a y be due to several factors such as the arbitrary

positioning of infill walls, the omission of bracing elements in otherwise rigid frames, or

indeed due to stepped elevations (see figure 6).

Leaving the first storey without walls in order to provide a parking area but at the

same time filling in walls in the upper floors leads to a severe change of stiffness. Studies

have shown that large ductility d e m a n d s are imposed on the first storey due to

increased stiffness and strength in upper storeys, and this m a y lead to its collapse, in

such cases secondary elements will also have to be provided in the first storey to act as

dampers in order to reduce the total seismic motion of the building.

Appendages

Sometimes a water tank or an elevator housing m a y be constructed on the roof of a

building, and the mass and stiffness of such appendages are usually quite different from

that of the building itself. The result is that the amplified motion at the top of building

causes a seismic acceleration on the appendage several times larger than that on the

Anand S. Arya

Abrupt change Omission of Stepped elevation; Soft first storey; in column size b e a m s interruption of shear arbitrary infills

walls

Figure 6. Discontinuity in stiffness of buildings.

building itself. Such elements will therefore be liable to severe damage and should be designed for larger seismic coefficients. T h e same argument holds for large pieces of equipment or machinery fixed to floors.

Structural design principles for multi-storey buildings

In order to achieve a collapse-proof building, the main structural requirements are (a) the overall stability of the structural frames, (b) the stability of the columns, and (c) such details of the members and their connections that will permit ductile deformations without fracture or instability, even under the most severe earthquakes expected. Clearly the m a n y details needed to achieve this safety cannot be covered in this article, and the reader is referred elsewhere2,3-4. Attention is invited here only to the example of the reinforcement detailing for the desired ductility in reinforced concrete frames, as shown in figure 7, and to the need for rigid or monolithic connections between beams and columns, whether of steel, prefabricated concrete or cast-in-situ concrete.

The architectural planning of low-rise and small buildings

Simplicity and symmetry

The qualities of simplicity and symmetry apply equally to small buildings as they do to multi-storey buildings. It is also desirable to build separate blocks for different functions according to their post-earthquake importance to the community. In this way different structural strength criteria m a y be used for them.

Enclosed space

Within a building, smaller rooms with properly bonded long and short walls forming a box-like enclosure are seismically stronger than those with long, uninterrupted walls. This is a very important consideration in load-bearing masonry wall buildings. The spacing of cross walls will depend on the mortar used. For strong cement mortars, the wall length to thickness ratio m a y be up to 40, but for adobe walls it should not be more than 10.

82

Earthquake-proof construction and architecture

Figure 7.

Sample detail of reinforcement for ductility, with closely spaced stirrups and ties near beam-column joints.

Exte column

riorN,

Floor b e a m

Tie beam

Individual footing

Openings in walls

W i n d o w , ventilator and door openings reduce the shear and bending strength of walls, and their size as well as location are significant. Openings should be as small and as centrally located as is functionally feasible.

Height

Restriction in the height of load-bearing wall buildings is necessary for better seismic safety. T h e guidelines suggested are s h o w n in table 2 , it being assumed, of course, that reinforcing methods appropriate to the seismic intensities likely in the area would be used in the construction.

83

Anand S. Ar ya

Roofs

T h e type of roof plays an important part in the seismic behaviour of a house. Lighter roofs are preferable to heavy roofs; sheeted roofs are better than tiled ones. All elements of a roof should be integrated so that it m a y have the capability of acting as one stiff unit in holding the walls together. In this respect four-slope hipped roofs are better than trussed roofs, trussed roofs are better than lean-to roofs, and complete trusses are preferable to rafters with collar-ties.

Floors

A s with roofs, those floors that are rigid horizontally, such as concrete slabs, are m u c h superior to wood-joist floors and jack-arch or flat-arch floors. For holding the walls together, the floor elements should have full bearing o n the walls. This will help prevent the floors from falling d o w n during severe shaking of the walls.

Gables

Gables, whether external or internal, constitute the most unstable part of walls and should either be avoided altogether or m a d e of lighter material, such as sheeting, boarding, and so on. External gables can be avoided by using hipped roofs, and internal gables can be left open if a false ceiling is used in the building.

T h e strengthening of low-rise and small buildings

T h e adoption of the architectural principles described above will, in itself, improve the seismic behaviour of buildings, particularly against collapse in seismic zones where earthquakes of magnitudes of less than 6 0 are expected to occur. But severe d a m a g e is still likely in areas of higher intensity, and antiseismic strengthening will be necessary. F r o m economical reasons, the strengthening elements m a y be installed only in critical parts of the building, as explained below, since full reinforcing of walls m a y be rather expensive.

Table 2. Suggested height restrictions on buildings in moderate and severe seismic zones

Building type Suggested height

Adobe house O n e storey, or one storey plus attic Field stone (random rubble masonry) A s above

in clay/mud mortar Dressed stone masonry in cement T w o storeys, or two storeys plus attic

mortar Brick masonry in m u d with critical T w o storeys, or two storeys

sections in cement mortar plus attic Brick or cement block masonry in Three storeys, or three storeys

good cement mortar plus attic Reinforced masonry A s per design by a qualified engineer W o o d frame T w o storeys, or two storeys plus attic

84

Earthquake-proof construction and architecture

Mortar

Use of stronger mortar, say 1:6 cement-sand or richer, should be m a d e where economically feasible, in order to achieve stronger and better-quality construction. In stone masonry walls the use of 'through' or 'bond' stones is a must to avoid delamination of walls and buckling of the stone wythes (see figure 8).

Seismic bands

The most important concept in the strengthening of masonry buildings is the provision of horizontal seismic bands (variously called collar beams, ring beams, etc.) at (a) the plinth level, (b) the lintel level of openings, (c) the floor level, (d) the roof level or the eaves level of trussed roofs, and (e) around the gabled ends. A seismic band is a continuous runner of reinforced concrete or w o o d going into all external and internal walls with proper connections at the corners and the T-junctions of walls (see figure 9). S o m e bands can be omitted at certain levels, as indicated below.

A plinth band should be provided in those cases where the soil is soft or uneven in its properties, as usually occurs on hill land. It will also serve as a damp-proof course. This band is not too critical.

The lintel band is the most important band and should incorporate all door and window lintels, the reinforcement of which should be extra to the lintel band steel. It must be provided in all storeys of the building.

A roof band will be required at the eaves level of trussed roofs or where a flexible w o o d joist roof or precast roofing units are used.

A floor band is needed below or at the level of such floors as consist of joists and covering elements. These bands m a y be omitted where a concrete slab having full bearing on all four walls is used for roof or floor.

A gable band is used to enclose the triangular part of masonry walls, the horizontal part being continuous with the eaves level band on longitudinal walls.

The use of steel mesh or wooden dowels is sometimes recommended at corners and T-junctions of walls, for bonding and integrating the perpendicular walls. This is.

450 mm

Hooked link

S shape ' • Wood bar c~

ess:

Q§8 £ Coursed rubble stone wall

( Fir

UUÜ

CXD

QQl

1600 mm

600 mm

Floor level

Wall plan Wall section

The use of'through' or 'bond' stones. If stones are not available they are replaced by wooden bars or hooked steel links.

85

Ananá S. Ar ya

\ r^

Long

\

s* -\ 1 Cross links

or stirrups

\

) . y i

Figure 9. Details of seismic band reinforcement at corners and T-junctions. Note continuity of longitudinal bars at wall junctions.

frankly, a poor alternative to the seismic bands described above, but w h e n used the

damage-resisting capability of the house is enhanced.

Vertical reinforcement

T h e other important strengthening provision is the installation of vertical reinforcing

elements such as steel bars, b a m b o o or canes. T h e provision of such elements in the

walls is complicated but suitable details have been worked out for their convenient

use 1 5 . The critical places for vertical reinforcing are the corners of walls and the jambs

of window and door openings.

Figures 10 and 11 show the total reinforcing pattern that would be adequate in the

most severe seismic zones for preventing the collapse of a building and reducing the

extent of its damage .

Wooden houses

T h e basic requirement of w o o d e n buildings concerns their durability against weathering and insect attack by the use of seasoning and preservative treatments. T h e joints between the m e m b e r s should be strengthened with framing, nails, bolts or disc­dowels, and kept tight by using steel straps.

In wooden buildings of stud-wall or brick-nogged construction, the most important strengthening provision is that of diagonal bracing elements, both in the horizontal and vertical planes, so that the house is restrained from twisting deformation in its plan and shearing deformation in the walls.

D u e attention must be paid to the fire resistance of w o o d e n buildings in order to avoid hazards caused by earthquake damage to gas pipes, electrical conduits or by objects falling onto open fires.

Conclusion

T h e main points that emerge from this brief account are the following: For earthquake protection, m o r e emphasis should be placed o n earthquake

engineering programmes. Whereas it m a y be technologically possible to arrive at earthquake-proo/construc­

tions, such an approach will neither be cost-effective nor economically feasible. Hence a

86

Earthquake-proof construction and architecture

Figure 10.

Reinforcing pattern for building with a sloping roof.

Gable band

Lintel band

Roof band

Figure 11.

Reinforcing pattern for building with a flexible flat roof.

Roof band (necessary for sloping roofs and under flexible

floors and roofs) Roof band

fail-safe design approach for achieving earthquake-resi.sfaHce is the mos t logical and

adequate. T h e architectural planning of buildings along these lines should achieve a

m u c h better seismic performance, at practically no additional cost. •

Notes

1. 'IS: 4326 1976, Code of Practice for Earthquake-Resistant Design and Construction of Buildings, N e w Delhi, Indian Standards Institution, March 1977.

2. A . S. A R Y A . Lessons from Behaviour ofMultistoreyed Buildings during Past Earthquakes. Proc. Symp. on Modern Trends in Civil Engineering, Roorkee. November 1972.

3. Earthquake Resistant Régulations, a World List, International Association for Earthquake Engineering, Japan, 1980.

4. D . J. D O W R I C K , Earthquake Resistant Design—A Manual for Engineers and Architects, N e w York, John Wiley and Sons, 1977.

5. Basic Concepts of Seismic Codes Vol. I, Part (ii) Non-Engineered Construction, The International Association for Earthquake Engineering, Japan, (available as A Manual of Earthquake-Resistant Non-Engineered Construction, Indian Society of Earthquake Tech­nology, Roorkee, 1981).

87

Deep continental drilling on the Kola Peninsula and the structure of the earth's crust

Oleg L. Kuznetsov

In the Soviet Union, study of the complex processes that take place within the earth's hard crust and the upper layers of the mantle, is being undertaken within the framework of a vast integrated programme involving geographical, geophysical and geochemical methods, as well as the development of new techniques in deep drilling. The first drilling, in the Kola Peninsula, on the fringe of the Baltic Shield, has thrown new light on the evolution and structure of the early continental crust of the earth as a whole.

The earth helps us to understand ourselves in a w a y that

books are unable to do. For the earth resists us. M a n

discovers himself through a struggle against obstacles. But

for this struggle he requires tools.

Antoine de Saint-Exupéry

Wind, Sand and Stars

Earth scientists today face the very same problems that were described with such poetic simplicity by the French author, Saint-Exupéry. Super-deep drilling, which is based on the methods of deep geophysics and geochemistry, has become one of the most important tools for investigating the earth's interior. The discoveries m a d e in the course of a large-scale experiment carried out in the Soviet Union—the super-deep drilling project on the Kola Peninsula—confirm that Planet Earth is 'alive' even in its oldest continents and crystalline shields; in other words, it possesses m a y features characteristic of a living substance.

Firstly, there is the record of past physical and chemical events preserved by geological bodies including crystallites. This record is preserved in the spatial distribution of magnetic, gravitational, electrical and elastic deformation character­istics of the earth's crust in the form of trails left in minerals by ionizing radiation.

Oleg Leonidovich Kuznetsov is Professor and Director of the All-Union Scientific Research Institute for Nuclear Geophysics and Geochemistry of the Ministry of Geology of the U S S R . His scientific interests embrace various areas of exploratory geophysics, oil prospecting, exploration and development, and he is presently engaged in the development of a new area—non-linear geophysics. Professor Kuznetsov is the author of m a n y scientific papers and joint author of several specialized monographs in the earth sciences. H e is holder of the U S S R Slate Prize. His address is as follows: c/o U S S R Commission for Unesco, Ministry of Foreign Affairs of the U S S R , 9, Prospekt Kalinina, M o s c o w G - 1 9 , U S S R .

89 Impact of science on society, no. 145, 89-96

Oleg L. Kuznetsov

Secondly, in the crystalline shields, there are active thermodynamic and hydrody-namic processes at work whose presence is indicated by intense thermal radiation and the spatial and temporal redistribution of the components of the stress-tensor; the migration of volatile components through rock masses by micro- and macro-filtration processes and by diffusion.

Thirdly, there is the interaction of the processes taking place in the crystalline masses and in the earth's lower and upper layers. The thermodynamic life of crystallites is determined to a great extent by physiochemical processes in the layers beneath the earth's crust and also in the atmosphere.

Finally, the important role played by the biosphere in the formation of the present crystalline shields also suggests that we are dealing with a living, dynamic substance.

Research into the evolution of geological material and the laws governing the organization of geological, geophysical and geochemical space in world science is increasingly becoming an area for measurement rather than visual description. The bulk of the information on the structure of the environment of plutonic processes and on the thermodynamic state of rock masses is obtained by remote geophysical and geochemical methods. These methods produce a unique and invaluable picture of the structure and properties of the geological environment but one which is, as a rule, very varied in terms of the solution to inverse problems and problems of geological interpretation. Hence the constant and unfailing interest of scientists in methods for direct penetration into the plutonic layers (and specifically using drilling equipment). Over the decades, this desire has spawned the most surprising and at times fantastic projects for penetrating the earth's crust, even including the use of special nuclear reactors that would sink by themselves into the earth by melting the rock beneath. A number of practical technological projects were carried out in the 1970s and 1980s, and here methods based on the mechanical crushing of rock combined with hydrodynamic action on the critical zone proved to be in a class of their o w n .

In the search for hydrocarbon deposits in the 1960s, prospectors in m a n y regions of the world had to penetrate to depths of three kilometres and, in some cases, more than five kilometres. Soviet and United States oil engineers were obliged to drill single boreholes seven to nine kilometres deep in order to reach the sedimentary cover of the earth's crust at the m a x i m u m possible depth. But the investigation of a comparatively limited complex of sedimentary rocks did not produce any significant results. At the same time, it was discovered that gaseous hydrocarbons could occur at such depths.

it was precisely at this time that a group of geophysicists, geochemists, geologists and technologists in the U S S R laid d o w n the scientific pre-conditions for super-deep drilling to a depth of 15 kilometres in conjunction with deep geophysical research. A n Interdepartmental Scientific Council for 'The Study of the Earth and Super-deep Drilling' was set up to organize and manage this extremely large venture. Under the supervision of the Council, which included leading Soviet scientists, a programme for the study of the earth's deep structure, unprecedented in scale in the history of the earth sciences, is being developed during the 1980s. The programme was based on the integration of geological, geophysical and geochemical data obtained from super-deep boreholes and extended geotraverses, employing the methods of deep seismic surveying. In the years that followed, the study of the plutonic structure was supplemented by information from earth satellites and aircraft.

O n e of the first projects in this programme was the Kola super-deep borehole which was sunk in 1970 and reached a record depth of 12 500 metres.

90

Deep continental drilling

Thus the foundations for a purposeful assault on inner space were laid in the U S S R . B y then, of course, mankind had achieved its first successes in the exploration of outer space, and it was already clear that the two problems were comparable in their scientific and technological complexity and their ultimate practical value.

O f course, the distance travelled into the outer atmosphere was m u c h greater than the depth of penetration into the earth.

O n e of the reasons for this difference was connected with the need to develop unique drilling equipment capable of drilling rock under extreme conditions.

The construction of even a single super-deep borehole constitutes in itself a most difficult undertaking involving the development of unique specialized drilling equip­ment, a system of automation and the development of technology for the construction and reinforcement of the shaft. The technology used for constructing boreholes in crystalline rock is different from that used for boreholes in sedimentary rock. Experience in drilling deep holes in sedimentary basins cannot therefore be directly transferred to these new conditions.

The basic problems connected with the construction of a shaft in crystallites showed up clearly after drilling only the first few kilometres. M a n y technical problems were created by the hole's non-standard trajectory, its highly abrasive and porous walls and the ellipsoid section of the shaft formed under the action of non-equiaxial mechanical stresses.

Moreover, in crystallites, unlike highly porous sedimentary rock, there is no clay crust on the wall of the hole, and this ultimately increases the resistance when the drill rotates.

These and m a n y other problems were successfully overcome by the highly qualified team of the Kola Geological-Prospecting Expedition in collaboration with scientists from the A U - U n i o n Scientific Research Institute for Drilling Technology.

In addition to the recovery of samples for analytical work in laboratories it was equally important from the scientific and practical viewpoint to carry out in-situ geophysical, geochemical and hydrological measurements. The need for in-situ analysis was dictated by several fundamental factors:

1. The impossibility of 100"o core recovery and its preservation intact, particularly in rocks that are highly fissured or present anomalous stresses.

2. The properties of rocks are difficult to model in a complex environment. 3. W h e n material is brought to the surface the thermodynamic stress on its surface

changes significantly and some key petrophysical data obtained from the core, such as the elastic wave velocity and electrical resistivity m a y differ substantially from the same features measured in situ.

4. The geophysical exploration of deep and super-deep boreholes is a most important methodological link in the chain of measurements involving the core, the hole and the earth's surface and provides the petrophysical basis for the interpretation of methods of surface, air and space geophysics and geochemistry.

5. Finally, the geophysical exploration of boreholes is the only direct method of studying the thermodynamic state of the rock mass around the shaft at such depths.

Certain types of well-logging have naturally had an auxiliary role in relation to the boring of the shafts themselves (determination of the current position of the drill and shaft bottom, caliper logging and assessment of the stability of the shaft).

91

Oleg L. Kuznetsov

Soviet geophysicists tackled the problem of developing unique technology for the high-precision measurement of seismo-acoustic, electric, nuclear-geophysical, m a g ­netic, thermal and other fields at extreme depths (over ten kilometres), temperatures (over 250°C) and pressures (above 1500 kg/cm 2 ) . These measuring systems were designed by scientists and instrument-makers of the U S S R Ministry of Geology and were appropriately referred to as 'underground sputniks'.

Over 40 types of geophysical investigation were carried out in the Kola super-deep hole and have produced invaluable data. A series of petrophysical, pétrographie and mineralogical investigations were carried out to establish a petrophysical basis for the interpretation of geophysical borehole data. The management and comprehensive interpretation of the geophysical and geochemical data is the responsibility of the All-Union Scientific Research Institute for Nuclear Geophysics and Geochemistry, while the All-Union Geological Institute and a number of other institutes of the U S S R Ministry of Geology and of the U S S R Academy of Sciences are similarly responsible for the geological data.

Geophysical and geochemical borehole methods were required to solve the following problems:

1. The detailed lithological-petrographic analysis of the section, the classification of geological environments in terms of their geophysical and geochemical characteristics, including environments in the geological section not represented in the core.

2. The geometrization of geological heterogeneities, including evaluation of their spatial distribution, thickness and angle of gradient.

3. The differentiation of geological bodies on the basis of porosity, the presence of fractures and the thickness of the layers.

4. The direct determination of the fundamental features of the rock in situ: the velocity of longitudinal and transverse seismic waves (for a wide frequency range), the attenuation of seismo-acoustic energy, electrical resistivity, magnetic susceptibility, and so on.

Finally, the geophysical and geochemical borehole data were to contribute to answering a number of fundamental geological questions: W h a t is the physical nature of the flat seismic boundaries located by surface seismic surveying in the crystalline shield? W o u l d the Conrad discontinuity be encountered at depths lower than seven kilometres? W h a t gradients, wave velocities and temperatures would be encountered and h o w would they change with increasing depth? Could open fissures and hence, filtration channels for liquid and gaseous components, exist at depths below nine kilometres?

Answers have now been obtained for most of these questions. But perhaps an analysis of the geophysical 'behaviour' of the earth's interior should

be preceded by at least a brief account of the geological and geochemical structure of the earth's crust in the section revealed by the Kola borehole.

By and large, the profile of the Baltic shield is composed of two basic types of rock: meta-sedimentary and meta-effusive structures. There were two main stages in the geological history of the shield (in the Archean complex): (a) sedimentation and volcanism; (b) metamorphism and ultrametamorphism.

The geological history of the development of the crystalline shield revealed by the Kola borehole covers an enormous period from a m i n i m u m of 3 5 to a m a x i m u m of 4

92

Deep continental drilling

billion years. The profile is mainly composed of Prccambrian crystalline rock which is spread over the territory of N o r w a y , Finland and Sweden.

T w o basically different tectonic regimes shaped the two major evolutionary stages in the history of the shield. In the first protosynclinal and protogeosynclinal stage, the plastic upper layers of the lithosphère, deformed by the action of geodynamic forces, formed fantastic folds which were imprinted in the hardened earth's crust. It was followed by platform stage.

The question of the ore mineralization of sedimentary-vulcanogenic sequences was of particular interest. Several basic types of ore mineralization could be identifed on the basis of the data from integrated geological and geochemical research. The ore bodies were very different in kind. Firstly, there was sulphide copper-nickel mineralization formed by basic and ultrabasic intrusions. Secondly, there was ferro-titanic mineraliz­ation in the metabasites of the Kola series. Thirdly, there were ferruginous quartzites in the granite gneisses of the Kola series. Fourthly, there was hydrothermal sulphide mineralization in areas of retrograde dislocation metamorphism.

The distribution of mineralized fractures in the section revealed by the borehole was investigated for the first time ever by Soviet geologists on the basis of a series of petrophysical, geophysical and petrological data. It was discovered that mineralized crush zones, with faulting, cataclasis and low-temperature hydrothermal changes connected with sulphide mineralization occur at m u c h greater depths (3 to 4 times as deep) than was previously supposed.

The spatial distribution of gases from diffuse organic material was carefully studied, along with the behaviour of ore components. The organic matter referred to here consisted of carbon compounds disseminated in the rock, carbonaceous matter and graphite. The gases studied included free gases in fractured, uncompacted areas and gases associated with the surface of crystallite bodies, particularly fluid inclusions in minerals.

Thus, the behaviour of volatile components was examined in parallel, as it were, on the macro- and micro-levels of gas dynamics.

The behaviour of helium was extremely interesting. It occurred in greatest concentration only in the bands of the uncompacted, gas-permeable areas which can be very accurately located by geophysical borehole methods.

The generally high gas readings in the section were associated with the action of various factors, including the lithological properties of the rock, its micro- and macro-structural interstitial space, stratification and permeability. The established increase in the relative content of heavy hydrocarbons, particularly below 8800 metres, is worthy of note. The enormous role played by the biosphere in the evolution of the earth's crust was assessed through an analysis of the carbon isotopes in the uncovered sequences. T h e appearance of life had an enormous effect on the formation and subsequent development of the atmosphere, the hydrosphere and the lithosphère. A study of super-deep boreholes, revealing a complete cross-section of the very earliest deposits, is of fundamental significance for an understanding of the regenerative role of the biosphere and makes it possible for us to investigate the effect of the biosphere on the formation of the deposits. It was found that the carbon in vein and diffuse carbonates was a unique indicator of the rate of development of the biosphere.

The isotope composition of the carbon in carbonates at depths of seven to ten kilometres is constant in the Archean rock and is similar to that of endogenous deep carbon, so that it is possible to speak of the insignificant effect of the biosphere on the

93

Oleg L. Kuznetsov

formation of the lithosphère at that period, i.e. of the negligible development of the biosphere in the Archean period.

In the Proterozoic rock, at depths of less than seven kilometres, light-isotope carbonates appear and the range of fluctuation in the isotope composition of carbon increases; this is clearly connected with the 'capture' of the carbon in the biosphere. Furthermore, products of the life forms of the biosphere are found widely disseminated in the Proterozoic rock. In terms of its isotopic characteristics, the carbon in the carbonaceous phyllites of the Proterozoic rock (0 to 5000 metres) corresponds to the organic matter in younger sedimentary deposits and enables us to speak of the formation of carbonaceous phyllites during the metamorphism of primary sedimentary rock from products of the biosphere.

In terms of hydrocarbon composition and isotopic features, the gas in the area of tectonic ruptures at depths of 900-1400 metres resembles gases in the sedimentary deposits of oil- and gas-bearing basins, which indicates that it was formed during the transformation of organic matter from the biosphere.

The results of isotope research that confirmed the slow development of the biosphere during the Archean and, hence, the lack of significant influence on the formation of the lithosphère, at the same time attested to the fact that the developed biosphere of the Proterozoic was already able to exert a fundamental influence on the formation and development of the earth's crust. The data collected allow us to consider that the subsequent evolution of all of the earth's geospheres was perceptibly influenced by the biosphere.

Fundamentally new information about the structure of the crystalline shield was obtained from the geophysical research on the Kola borehole, and its findings have turned a n e w page in the study of the plutonic areas of the earth's crust. The most important of these results are as follows.

In the area of study of seismo-acoustic fields and the elastic-deformation characteristics of rock: T h e absence of a positive vertical velocity gradient for longitudinal (Vp) and transverse (Vs) waves throughout the research range from 0 to 12 500 metres was established. This behaviour of Vp and Vs as a function of depth indicates that the distribution of the vertical component of the stress tensor in the crystalline shield is significantly different from the traditional (hydrostatic) relationship for sedimentary basins az = pgh (where <JZ is the value of the vertical component of stress, p is the average density of the overlying rock, g is the acceleration of the force of gravity and h is the depth).

At the same time, a zone of reduced velocity for longitudinal and transverse waves (and also of reduced values for clastic-deformation moduli) was discovered at a depth of 4500 metres in the Pechenga complex of the Proterozoic.

By calculating the value of the components of the stress tensor in the mass surrounding the shaft, it was possible to identify zones which were partly relieved of vertical stresses. This means that rocks in an open fractured state m a y exist at great depths.

O n the basis of a comprehensive interpretation of the material obtained from the geophysical exploration of boreholes, vertical seismic profiling and surface seismic surveying, it has proved possible:

(a) to establish the thermodynamic, but not the lithological, nature of a number of flat seismic boundaries in the earth's crust;

94

Deep continental drilling

(b) to demonstrate that it is not the hypothetical Conrad discontinuity which occurs below 6800 metres but a lithological boundary coinciding with the change from the rock of the Pechenga complex to the rock of the Kola complex represented by granite gneisses;

(c) to establish such important characteristics of w a v e fields in the borehole and in the rock mass surrounding the hole as the high intensity of transverse waves, the w e a k frequency separation of longitudinal and transverse waves and the extremely low values of the seismic w a v e attenuation coefficient.

Study of the radioactivity of the rock established that the radioactive content of elements depended to a large extent o n their level of granitization. W h e n the contribution of radioactive decay to the overall thermal balance of the rock mass w a s determined, it w a s s h o w n that the highest level of heat generation w a s associated with tuffaceous sedimentary rocks (1 -0-2-5 / i W / m 3 ) and gabbro-diabases (0-7-1-7 / i W / m 3 ) which are found at depths of u p to 25 kilometres. T h e remaining rocks were characterized by a comparatively low level of heat generation, although narrow zones were encountered in which anomalously high heat-flow readings were registered, which were apparently linked to intensive heat-mass transfer and the possible introduction of radioactive elements. T h e contribution of the radiogenic componen t w a s estimated to be almost 4 5 % of the overall heat flow to the earth's surface.

T h e profile of the Kola borehole is generally characterized by irregular variations in the vertical geothermal gradient. Its value varies from 1 to 2°C per 100 metres and is governed by several ma in factors, the mos t important of which are the lithological, microstructural and filtration properties of the rock and the presence of radioactive elements. These factors have a determining effect o n the thermal diffusivity of rocks and their capacity to generate heat.

It seems that the contribution of individual components of heat-mass transfer also alters in relation to the lithophysical properties of rock.

T h e results of precision magnetometric measurements carried out in the shaft of the borehole and o n rock samples are generating a great deal of interest.

T h e magnetic susceptibility H , the natural remnant magnetization and the so-called Königsberg factor Q (the ratio of natural remnant magnetization to induced magnetization) were measured. F r o m these magnetic characteristics of the rock it w a s possible to obtain information about the spatial and temporal variations of the magnetic field.

M a x i m u m values of magnetic susceptibility (0-2-0-3 SI) were linked to the presence of magnetite, hematite and pyrrhotite. T h e sharp variation in the values for H and In, even for small volumes of rock, w a s linked to the superimposed nature of the mineralization. In terms of its magnetic characteristics, the section can be clearly divided into three major zones. T h e upper zone of sulphide mineralization contains rock with low levels of magnetic susceptibility ( H < 4 1 0 ~ 3 S I ) . T h e middle zone of oxide mineralization has a higher degree of magnetic susceptibility ( H % 0-2-0-3 SI). T h e lower zone has a low level of magnetic susceptibility lower than H % 2 - 1 0 " 3 SI.

Traces of repeated changes in the intensity of the magnetic field were clearly registered in the crystallite mass . For example, roughly 20 zones with differing geomagnetic polarity were identified in the section under study.

Finally, w e turn to the n e w facts produced by analysis of the stress fields which was carried out by combining the methods of mathematical modelling and special tectonophysical research. B y m e a n s of tectonophysical analysis it w a s possible to

95

Deep continental drilling

reconstruct the tectonic stress field in geological time and space in the region of the Kola borehole and predict its depth.

Analysis of the tectonophysical model of the geoblock thus obtained showed, in particular, the clear horizontal stratification of the earth's crust that is not linked to the extensive occurrence of horizontal displacement but to changes in the local conditions of deformation of separate parts of the geoblock. This last point raises a number of new questions in relation to the concept of plate tectonics.

It should be emphasized at the end of this brief s u m m a r y of the results of the unique geological and geophysical investigation on the Kola Peninsula that geology and geophysics have penetrated the earth to previously inaccessible depths. A s a result, one of the oldest sciences, geology, has been able to penetrate directly the secrets of the earliest stage in the earth's geological development.

T h e active participation of geologists from m a n y countries, particularly the United States, the Federal Republic of Germany , France and Canada , in the implementation of the International Lithosphère Programme and its 'Continental Drilling' section must be noted with satisfaction. The integrated study of the earth as a planet increasingly demands the combined efforts of the international community of research scientists.

96

Readers' forum An invitation Reasoned letters that comment, pro or con, on any of the articles to readers in Impact or which present the writer's view on any subject

discussed in Impact are welcomed. They should be addressed to the Editor, Impact of science on society, 7 place de Fontenoy, 75700 Paris, France.

This letter, sent to us by Mr Jean-Bernard Condat, a musicology student (scientific option) at the Université Lumière, Lyon, France, concerns the 1985 double issue on Science and aesthetics in sound and hearing. His address is: B.P. 8005, 69351, Lyon Cedex 08, France.

M a n y thanks for issue N o . 138/139, which has recently come m y way. I was fascinated both by the subject-matter (sound and noise as acoustic phenomena) and by the fields covered (musicology, architecture, education and physics).

As a musician with a wholly scientific outlook, m y dream of obtaining m y baccalauréat C was to study to become an 'acoustician' or at least to engage in research in musical acoustics like H . V . M o d a k (author of the article 'Musical curiosities in the temples of South India'). But musical acoustics was taught only as an option as part of the first-degree music course ( D E U G musique) at the Université de Lyon 2.1 did not hesitate in m y choice, mistakenly as it turned out. The reason why G . L . Fuchs did not mention this course in his article (Education and acoustics), although it still exists, was perhaps that it is given in the Department of Nuclear Physics of the Université de Lyon 1 and is supervised by a non-musician!

But why did M r Fuchs not refer to the Laboratoire d'acoustique musicale at the Faculté des Sciences de Paris? Its founder and director, M r Emile Leipp, died on 5 January 1986. This highly gifted researcher—whose work L'acoustique et la musique has just been reprinted (Paris, Masson, April 1984)—was particularly interested in the organ/auditorium complex. H e would certainly have been very glad, as I was, to read T a m a s Tarnóczy's article on the acoustic problems posed by multi-purpose halls. I take this opportunity to pay tribute to this distinguished figure in the field of French musical acoustics.

M y congratulations once again on this double issue, which occupies a prominent place in m y library.

Jean-Bernard Condat

Lager Heuberg, Stetten a.k. Markt

97 Impact of science on society, no. 145, 97

Did you know that educators, research workers and students can use Unesco coupons to purchase

books, periodicals, films art work, sheet music

radio and television sets

typewriters scientific equipment

tape-recorders machine tools musical instruments measuring instruments

U N E S C O C O U P O N S can also be used to pay for subscriptions to educational, scientific or cultural publications, for university registration fees and copyright dues

U N E S C O C O U P O N S are S 1000 Unesco coupon S H I U O issued in the following denominations:

US Dollars ^ l.ooo /tfuv/u, r% Ç O \ V N 1 *"" y^u^

aL-oo, [-<|Hca<ionsl. Seanulii. »ml í ullural Orean

100 30 10

1

t:x",^Cy fiic'riiousand dollar^

(Blank coupons which can be made out for the sum of 1 to 99 cents also available)

H o w do they work?

In every user country there is a body—often the National Commission for Unesco—which is responsible for the sale of the coupons.

Write to Unesco Coupons Office, Unesco, 7 place de Fontenoy, 75700 Paris, France, for list of main suppliers participating in the programme, as well as for the name and address of your country's Unesco Coupon Sales Office.

You pay for the coupons in your own currency and enclose the coupons with your order to the supplying company of the material you want.

Erratum

During the final preparatory stages of issue No. 142 an important section of the text of the article by A. D . Coleman entitled Lentil soup—a meditation on lens culture was unfortunately omitted.

The publishers of Impact apologize to the author and readers for this error and reproduce below the missing text, which should be taken in between paragraphs 4 and 5 of page 216 of issue No. 142.

At roughly the same time—circa 1550—the compound lens was invented, possibly by the British mathematicians Leonard and T h o m a s Digges, though there is endless dispute over its actual originator.

A vital period

T would propose that it is within this three-year period, from 1550-1553, that Europe became a lens culture. Though Cardano, Maurolycus, and (for convenience's sake) Digges were working independently of each other, their separate ideas combine, when viewed in retrospect, to form the necessary infrastructure of a lens culture.

Cardano's lensing of the camera obscura allowed one for the first time to study the lens image without one's o w n eye being, in Simon Henry Gage's terms, 'an integral part of the optical train'6—as it is, for example, in its relation to eyeglasses, magnifying glasses, and telescopes. This crucial displacement permits us to see the imaging process itself—to contemplate that process, abstract ideas from it, and metacommunicate about it (metacommunication being communication about communication). As a device, Cardano's tool is the prototype of the contemporary photographic camera; I would go so far as to posit that the photograph- i.e., the permanent version ofthat lens image— is implicit in Cardano's invention, an inevitable consequence of it, since the reproduction and dissemination of visual images was already a century and a half old.

99 impact of science on society, no. 145. 99

Third World Academy awards its first prize

O n Sunday 26 October 1986, the Third World Academy of Sciences gave its first Awards to four scientists from developing countries w h o m a d e singular contributions to basic sciences.

The Awards went to Professor L. D e Meis (Brazil) for his fundamental studies on the function of the C a 2 + ATPase of the sarcoplasmic reticulum with particular regard to the mechanisms of energy transfer in biological membranes; to Professor S. Siddiqui (Pakistan) for his fundamental contributions in the chemistry of Rauwolfia alkaloids; to Professor Liao Shan Tao (China) for his fundamental contributions in two different areas of mathematics: periodic transformation of spheres, and the qualitative theory of dynamics; and to Professor E . C . G . Sudarshan (India) for his fundamental contributions to the understanding of the weak nuclear force, in particular for his part in the formulation of the Universal V - A Theory of Sudarshan and Marshak. The first two Awards were for Chemistry, the third for Mathematics and the fourth for Physics.

The Awards ceremony took place in the Main Lecture Hall of the International Centre for Theoretical Physics in Trieste in the presence of the President Professor Abdus Salam, Vice-Presidents Professors T . R . Odhiambo (Kenya) and M . G . K . M e n o n (India), and Members of the Council Professors L u Jiaxi (China), A . R . Ratsimamanga (Madagascar) and E . Rosenblueth (Mexico).

The citations were read by the Executive Secretary Professor M . H . A . Hassan (Sudan) while the prizes, consisting of a medal and a cheque for U S $ 1 0 0 0 0 were handed over by Dr F. Salleo, Director of the Italian Department for Co-operation to Development of the Ministry for Foreign Affairs, the major financial contributor to the Third World Academy of Sciences.

100 Impact of science on society, no. 145. 100

Presentation of D i m e Medals in Physics

O n 15 November 1986, one of the 1985 and one of the 1986 Dirac Medals of the International Centre for Theoretical Physics (ICTP) in Trieste were officially given to Professor Yakov Zeldovich (Space Research Institute, M o s c o w , U S S R ) and Professor Alexander Polyakov (Landau Institute of Theoretical Physics, M o s c o w , U S S R ) respectively, by Professor Abdus Salam, Director of the I C T P , and Professor Stig Lundqvist, Chairman of the I C T P Scientific Council. The other 1985 Medal had been presented to Professor Edward Witten (Princeton University, U S A ) on 7 February 1986, while Professor Yoichiro N a m b u (Enrico Fermi Institute for Nuclear Studies,

Professor Stig Lundqvist (left) and Professor Abdus Salam (right) presenting the Dirac Medal to Professor Yakov Zeldovich.

(From left to right) Professor Stig Lundqvist, Professor Abdus Salam and Professor Alexander Polyakov.

101 Impact of science on society, no. 145, 101-102

Chicago University, U S A ) will receive the other 1986 Award in Spring 1987. M o r e than three hundred scientists and officials attended the award ceremony which took place in the Main Lecture Hall of the ICTP.

After the Medals had been presented, Professor Zeldovich and Professor Polyakov lectured on recent developments in cosmology and directions in string theory respectively.

The Eklund Prize

At a special ceremony which took place on 19 November 1986 at the International Centre for Theoretical Physics, Professor Emeritus Chike Obi, of the University of Lagos (Nigeria) received the 1985 I C T P Eklund Prize awarded this time for outstanding contributions in the field of mathematics. The Prize is named after D r Sigvard Eklund, Director General of the International Atomic Energy Agency (IAEA) in Vienna (Austria) from 1961 to 1981 and a staunch friend of ICTP.

D r Eklund himself handed over the prize, a cheque for U S $ 1 0 0 0 and a certificate, while Professor Abdus Salam, Nobel Laureate for Physics 1979 and Director of the ICTP, read the citation in front of an audience of some 300 scientist from all over the world.

Professor Obi has made significant contributions in the study of nonlinear ordinary differential equations with several parameters for which he established numerous results on the existence, number and some analytical expressions of harmonic, subharmonic or uniformly almost periodic solutions. H e has also been engaged in the development of mathematics in Africa.

102

Looking ahead...

The next issue of Impact of science on society (No. 146) will deal with

THE THIRD INDUSTRIAL REVOLUTION

Authors include: E . Filèmon (Budapest, Hungary), on robots: their present-day use and prospects for the future; D . Blackburn (Open University, United Kingdom), on storage, recovery and utilization of industrial and engineering data; B. Rebaglia and S. Sartori (Istituto di Metrología 'Colonnettf, Turin, Italy), on mechanics, electronics and computer science: their integration in manufacturing; Academician B . Sendov (Sofia, Bulgaria), on teaching and preparing our children to face the new world; Roger B . Smith (President, General Motors, Detroit, USA), on the twenty-first century corporation.

N o . 147 The social influence of inventions

N o . 148 N e w and renewable energy sources

103

Chemistry International The News Magazine of the International Union of Pure and Applied Chemistry (IUPAC) IUP A C Secretariat: Bank Court Chambers, 2-3 Pound Way, Cowley Centre, Oxford OX43YF, England

I U P A C is the international organization dedicated to establishing and maintaining standards in pure and applied chemistry and promoting international cooperation among chemists. Chemistry International will keep you up to date with regular progress reports on all I U P A C ' s activities, including those of its policy committees, and tell you h o w you m a y comment on tentative recommendations on nomenclature and symbols at the stages where opinions are being sought by Commission experts. Chemistry International will show you h o w you may have some influence in the way the world's chemists communicate with each other and with scientists in other disciplines. A n extensive conference calendar indicates I U P A C -sponsored conferences in all areas of chemistry, while C/also keeps the scientist up to date about technical reports published by I U P A C and includes frequent subject indexes to reports.

I U P A C represents chemistry internationally and thus Chemistry International is the magazine for all seriously concerned chemists worldwide. Subscription Information Chemistry International is published bi-monthly. Subscription rates for 1987 are for institutions £29.50 ( U K ) , $43.00 ( U S A , Canada & Japan), £35.00 (elsewhere), and for individuals $21.50 post free.

Order Form Please tick the appropriate box and return to Blackwell Scientific Publications Ltd, P . O . Box 88, Oxford, England. • I woyld like to subscribe to Chemistry International I | I wish to pay by cheque/money order (¡Mete as «nnan-i and enclose the sum of • I wish to pay by Access/Barclaycard/VISA/Mastercard (delete as necessary)

Please debit my'credit card no.

Expiry date with the sum of

Signature Date

• Please send m e a specimen copy of Chemistry International

N a m e

Address

Blackwell Scientific Publications P.O. Box88, Oxiord, England

H o w CI can help you. . . * D o you as a chemist know of the decisions being taken on your behalf by I U P A C in relation to nomenclature and all matters chemical that require international and interdisciplinary codification or regulation?

* D o you as a chemist know h o w you m a y become involved in some way in your International Scientific Union?

* D o you know h o w to exert some influence on the International Scientific Union that represents you?

* D o you know what I U P A C ' s current programmes are?

* D o you know w h o the responsible people in I U P A C are?

* D o you know h o w to comment directly on aspects of I U P A C ' s work to some of the 1500 chemists currently engaged in I U P A C ?

Chemistry International will provide the answer to these and many other questions... Can you afford not to subscribe?

RAZVOJ/DEVELOPMENT-INTERNATIONAL

Publish«] by: INSTITUTE FOR DEVELOPING COUNTRIES, Zagreb, Yugoslavia

Managing Editor: Dr Zoran R O C A

RazvoJ/D$Ytlopmtnt-lnt»rnational, is a new, biannual journal devoted to the mul-tidltciplinary scientific study of socio-eco­nomic development, particulary, but not exclusively, from the viewpoint of social, economic, political, cultural, technological, environmental and other aspects of develop­mental experience and the goals of the develo-Ring countries at national and international jvels.

Razvol / Divtlopment-lnttrnationêl, is intended to provide a forum for contributions by scholars, experts end prominent public figures from developing countries and the international academic and political com­munity working in different fields of deve­lopment linked by a c o m m o n respect for the overall developmental •mancipation of peop­les end countries, as well as for equitable international relations and co-operation for development.

R A2VOJ/DEVE LOPME NT - INTERNATIONAL Vol. I, No. 1.1986

Content*

ARTICLES

ZarkoPUHOVSKI The Phllosophlco-Historical Concept of Development Paul S T R E E T N in tar-Generational Responsibilities RikerdSTAJNER O n the Study of Development, Economic Underdevelopment and Internetional Econo­mic Relations Urlich H I E M E N Z Development Economics at the Crossroads? Zoran T R P U T E C The Crisis, its Roots and Present Aspects Oinesh S INGH The Non-Aligned Movement end Disarmament Blagoje BABIC Crisis of the "European" Type of Develop­ment Li Z O N G The Obstacles to the Development of the Developing Countries and Their Way Out

Susan G E O R G E The Right to Pood and the Polities of Hunger Surendra P A T E L A n Educational "Miracle" in the Third World 1950 to 1981 Memandou N D O Y E The Impact of Transnational Corporations on Education, Culture end Science in Africa Nede S V O B - O O K I C The Culturel Dimension of Technological Development of Developing Countries Zoren R O C A Socio-Economie and Ecological Aspects of the Demographic Explosion in Developing Countries VelimirSRICA Compromise Planning and the Development of Yugoslavia

B O O K R E V I E W S

I N F O R M A T I O N

Price per copy U S 3 8 (US 3 16 by air mail) Annual Subscription U S 3 16 (US 3 20 by air mail)

Send manuscripts, comments on articles and subscription correspondence to: Institute for Developing Countries, 8 . maja 82,41000 Zagreb, P. O . Box 303, Yugoslavia

NICCOLÔ STENONE NELLA FIRENZE E NELL'EUROPA

DEL SUO TEMPO MOSTRA DI D O C U M E N T I , MANOSCRITTI, OPERE

NEL TERZO CENTENARIO DELLA M O R T E

23 setiembre - 6 dicembre 1986

Firenze

Sala Donatello - Basilica di S. Lorenzo

Catalogo a cura di

S T E F A N O D E R O S A

Nel quadro delle manifestazioni per Firenze capitale europea della cul­tura, l'Accademia toscana di scienze e lettere « la Colombaria », l'Arcidio-cesi di Firenze, l'Istituto Incontri culturali N . Stensen, in occasione del terzo centenario della morte, hanno promosso una mostra di manoscritti, docu-menti, opere, nella Sala Donatello della Basilica di San Lorenzo: Niccolö Stenone nella Firenze e nell'Europa del suo tempo (23 settembre - 6 dicem­bre 1986).

II catalogo, curato da Stefano D e Rosa, ricalca fedelmente il percorso espositivo e siarticola in tre sezioni: Stenone a Firenze. I Medici e V am­biente scientifico; L'Europa cultúrale/religiosa e la conversione; Stenone Vescovo, nell'intento di delineare la figura di Stenone nei complessi aspetti della sua personalità, fiorita in un tempo e in una situazione storica precisi, in una particolare cultura, toscana ed europea del secondo '600, senza rozze césure dell'uomo in compartimenti separati, in ambiti disciplinan e travagli umani troppo rígidamente definí ti.

In appendice al catalogo è pubblicato il testo dell'inventario naturalístico dello Studio pisano compilato nel 1671-1672 da Niccolö Stenone.

Il testo è presentato da Stefano D e Rosa con un comment© di Giusep­pe Mazzetti.

1986, cm. 17 X 24, 92 pp. con 7 ill. - Lire 19.000 [ISBN 88 222 3440 5]

CASA EDITRICE L E O S. OLSCHKI - CASELLA P O S T A L E 66 - 50100 FIRENZE

QUESTIONING EXCHANCE

&

&

EDITOR

Jim T. Dillon

School of Education, University of California,

Riverside, California 92521, U S A

QUESTIONING EXCHANGE

'¿W SÄiSfS.-.

Questioning Exchange serves a multi-disciplinary and international network of interests encompassing all aspects of questioning in all fields of endeavour. It is a n e w kind of journal that is:

(1) organized by topic rather than by discipline or field (one topic, all fields versus one field, all topics);

(2) designed such that all articles in an issue serve the reader's interest, rather than one or two articles of interest to this and that subgroup of readers;

(3) devoted to direct contact and exchanges within a network of active readers, rather than to a one-way outlet for authorial publication;

4) directed to rapid communication and publication of interesting developments regarding questioning, rather than to archival deposit of inert products.

Regular features include field or topical reviews; national reviews; speculative essays; featured scholars; book reviews; current abstracts and special series.

1987 Annual Subscription (Vol. 1) Published three times a year £40 $72 D M 1 6 0

SEND FOR A FREE SPECIMEN COPY

TAYLOR & FRANCIS U K : Rankine Road, Basingstoke, Hants R G 2 4 O P R U S A : 2 4 2 Cherry Street, Philadelphia P A 1 9 1 0 6 - 1 9 0 6

A selection of forthcoming contents Questions and reading. Thomas Andre, Iowa State University Questioning and intelligence. Robert Sternberg, Yale Univ rs/ty Harrah's theory of questions. Ruth Manor, Tel Aviv University Reply to Manor. David Harrah, University of California A question-answer game. Ceroid Stahl, Paris International reviews Questioning in the United Kingdom, edited by David McNamara, University of Lancaster, including:

Discussion and questioning. David Bridges, University or Cambridge Questioning to find out about people. Brendan Burchell, University of Warwick Classroom questions in England. Trevor Kerry, Doncaster Institute

Reviews of the South Pacific: edited by C//ve JVfcGee, Hamilton Teachers College, New Zealand; FR Germany; Canada; and Bulgaria A feature on question generation edited by Arthur Graesser, Memphis State University Also to be published are book reviews; abstracts; and features on questioning in various fields: counselling, m a n a g e m e n t and information science.

SOCIAL EPISTEMOLOGY ajournai of knowledge, culture and policy

&

^

E D I T O R Steve Fuller Department of Philosophy, University of Colorado, Boulder, C O 80309-0232, U S A

SOCIAL EPISTEMOLOGY

A journal of knowledge, culture and policy

J » Taylor & Francis

Social Epistemology will be devoted to

research in the sociology of knowledge. It

intends to provide a forum for philosophical

and sociological inquiry, and will incorporate

the work of scholars drawn from a variety

of humanist and social science disciplines

based on the assumption that a key

feature of a claim's epistemological status

is its need to be certified by an appropriate

social group before passing as knowledge.

The journal will cover not only empirical

research into the production, distribution

and transmission of knowledge, but also

normative considerations which arise in

the course of using this research as a

guide for directing contemporary

knowledge enterprises.

Topics will include

• H o w knowledge is a source of political power

• H o w basic research filters through the educational system

• Legal aspects of knowledge production and use

• Economic considerations in knowledge production

• The ethnography of h u m a n - c o m p u t e r interaction

• Status, credibility and knowledge legitimation

• Error and knowledge production

1987 Annual Subscription

(Vol. 1) Published quarterly

£50 $90 D M 2 0 0

Reduced subscription rates for individuals

£25 $45 D M 1 0 0

SEND FOR A FREE SPECIMEN COPY

TAYLOR & FRANCIS UK: Rankine Road. Basingstoke, Hants RG24 OPR USA: 242 Cherry Street, Philadelphia PA 19106-1906