Chapter 2 Plate Tectonics. A combobulation of much of what we’ve trudged through up to now If you...

Chapter 2Plate Tectonics

• A combobulation of much of what we’ve trudged through up to now

• If you even looked at this chapter, you (should) know why it has been held back until now– You needed at least a little background info

• We’ve gone through volcanoes, weathering, deposition of sediments, formation of rocks

• Well and good……with what you now know (at least, I hope)….

• How does one go about forming, for example, ……

Figure 2.1 Mountains…..

• Many explanations, most did not explain things well

• Plate tectonics explains many

• Your instructor took a similar course as this at a time when there were arguments about this theory

• We had to know which professor believed which theory/hypothesis to “get the grade”– Consider yourselves lucky!

• Who ever did a jigsaw puzzle?

• Who has looked at a map of the world, and noticed at least one jigsaw fit?

• And that fit is?..........

To begin:

(ignore the details for now)

• It appears certain continents were joined in the past

• Evidence?– The last slide shows a fossil critter (a

Mesosaur), found in both parts of South America and South Africa

– Also, a fossil plant called Glossopteris– Not to mention, rock types and structures

(mountain ranges, etc.) match when the two continents are put together

– Its not a perfect fit

Figure 2.3

Fit using the continental shelves

Figure 2.6 – rock type/structure evidence

Figure 2.7 – paleoclimate evidence

White = glacial coverage at end of Paleozoic

Blue arrows = direction of ice movement from glacial grooves in bedrock

• It seems obvious now• Not so when I was coming through “the system”• The original hypothesis was called “continental

drift”. – Conceived by one Alfred Wegener, in 1915

– Looking at South America and Africa, it appears that the two continents broke apart and drifted away from each other

– Well and good….but, how does solid rock drift about?

– The explanation took a while

• The basic data on which the hypothesis was formed were good, valid data

• The data also were used to support other hypotheses

• Later data could be combined to test each hypothesis (that would be, the scientific method)

• The basic hypothesis (continental drift) was a bit flawed

Objections to continental drift

– The mechanism – tidal forces (too small)– How the continents moved: kinda like icebergs

in the ocean (would imply a very weak material forming the ocean floor)

– Tried to measure actual movement of Greenland over a number of years (did not have accurate enough measurements back then)

– Wegener died in 1930; his ideas did not

• Most scientists ridiculed Wegener’s ideas; others thought some of them to be valid

• 1928 – first plausible explanation of the driving mechanism

• 1937 – book published which threw out the weaker parts of the hypothesis, and added new evidence in support of the stronger parts

• 1950’s – other evidence gathered; one was paleomagnetism

Earth’s magnetic field and paleomagnetism

• Earth has a magnetic field, with a north & south magnetic pole

• These poles align roughly with the geographic (rotational) pole

• The field is like that produced by a bar magnet• We do not feel the magnetic force, but it is

revealed by use of a compass• Lastly, it’s a vector field (has a strength and a

direction)

Figure 2.8

• Iron-rich minerals (specifically magnetite) become magnetized as they crystallize– This happens mainly in igneous rocks (esp. basalts),

although some sediments may contain this mineral

• As the rock cools below a certain temperature, they become magnetized in the direction of the existing magnetic lines of force– That is, the grains “point” toward the magnetic poles at

the time of formation

– We actually measure vector components of the grains’ magnetization

• The magnetic grains not only indicate the direction to the pole– They also indicate the latitude where they formed (i.e.

distance from the pole)

• How does they do that?– Compass needle – constructed to freely rotate in a

horizontal plane– Can also be constructed so that it rotates in a vertical

plane (measures magnetic inclination)• Inclination – the angle between horizontal and the

line pointed by the needle– Measurement of the inclination of magnetization in a

rock indicates the latitude at the time the rock became magnetized

Figure 2.9

Polar wandering

• Investigations (in the 1950s) of magnetic alignment of magnetic minerals in lava flows of different ages indicated many positions for the ancient magnetic poles

• Data from rocks in Greenland and in Europe • These data showed interesting patterns

Figure 2.11A

• Interpretation of these data– Based on the assumption that the magnetic

poles correspond with the geographic poles– If North America moved relative to Europe,

there is a good match in the direction to the poles – another kudo for the old idea of continental drift

Figure 2.11B

• In addition to the direction to the poles, the inclination data suggested climatic data for the past– 300 mya: about the Pennsylvanian Period– Inclination data indicate Europe & N. America

were near the equator– The Pennsylvanian (or Carboniferous) was a

time of coal formation for several areas – suggestive of tropical climates

• And a bit before all of this magnetic stuff was noted, we had WW II

• Has to do with beasts called U-boats (submarines)– In the 1950s, US Navy funded much

oceanographic exploration– Wanted to know depths to the bottom, so they

could differentiate subs from the bottom– Sonar – Discovered oceanic ridge systems winding

through the major oceans (like the seams on a baseball)

• Investigations of the Mid-Atlantic Ridge revealed a central rift valley– Suggested tensional forces – the crust is being

pulled apart here– High heat flow and volcanism also noted

• Earthquake studies in the western Pacific indicated activity at depth below deep-ocean trenches (not well explained by previous theories)

• Samples of oceanic crust were to dated to be less than 200 my

• Sediment accumulations in the deep ocean were found to be thin, not the thousands of meters (or feet) predicted

• These discoveries did not fit the existing models– Existing models stated that cooling and

contraction of the earth produced compressional forces that deform the surface

– The thin sediment layers in the oceans indicated either that the rate of sedimentation was much less than thought, or that the ocean floor was actually young

A new hypothesis

• Early 1960s, Harry Hess presented a paper called “an essay in geopoetry”– Oceanic ridges located above zones of

convective upwelling in the mantle– Rising material spreads laterally, carrying

seafloor away from the ridge crest– The convective currents descend at the location

of deep-sea trenches

Figure 2.12

• This hypothesis was open to ridicule, yet provided testable ideas

• One test came a few years later– The data came from magnetic measurements of rock

samples

– Over a period of time, the Earth’s magnetic field reverses polarity (that is, sometimes the magnetite compass needle points to the north, and sometimes to the south)

– Rocks showing the same magnetism as the present field are normal; those opposite are reverse

Figure 2.13

Rock magnetization in northern hemisphere

• Investigation of numerous lava flows (magnetically and radiometrically) led to a time scale based on magnetic reversals– Major divisions of about 1 my, called chrons– Also found several, shorter-lived reversals (less

than 200,000 yrs) during a given chron

Figure 2.14

• At the same time, oceanographers were conducting magnetic surveys of the ocean floor as part of investigations related to seafloor topography

• These surveys showed alternating stripes of high- and low-intensity magnetism

Figure 2.15

High intensity

Low intensity

• An explanation came in 1963, by a grad student & his advisor– The high-intensity strips are areas where the

paleomagnetism shows normal polarity (the rocks reinforce the present field strength)

– The low-intensity strips are areas showing reverse polarity (the rocks weaken the observed field strength)

– Has to do with vector fields– Also a quick jump into plate tectonics

Figure 2.16

Figure 2.17

• For us geologists, things were still in turmoil• Some believed seafloor spreading and “continental

drift” could account for observed data• Others preferred an expanding Earth to explain

how continents drifted apart, with new seafloor filling in between– Not a good explanation, since most materials shrink as

they cool

– Also did not explain how mountains were formed

Figure 2.18

The expanding Earth

The basis of plate tectonics• The year…1965• The man….one J. Tuzo Wilson (don’t worry about

the name, just a bit of science history)• Originally a physicist, he went even more wacko

and became a geologist• His idea:

– large faults (the ridges) connect global mobile belts into a continuous network

– Earth’s outer shell divided into several “rigid” plates– Also described how the plates moved relative to one

another

• The plate movements are:– Divergent – two plates move apart, with upwelling of

material from the mantle to create new crust

– Convergent – two plates move together• One plate moves beneath an overriding plate

• Collision creates mountains

– Transform fault – two plates move past each other without production or destruction of crust

• In his published paper, he also referred to the idea as “geopoetry” – meaning, he knew the ideas would draw criticism

“Geopoetry becomes geofact”

• The title of the main science story for a Time issue in January 1970 (it was deemed that important)

• Plate tectonics accepted by many• The theory provided a unified explanation for

numerous seemingly unrelated observations among the various geoscience fields

• Opposition to the theory continued at least through the early 1980s (I got my BS in 1978, so I know that well)

The new “geogospel” (a dangerous term)

• Theory of plate tectonics• A composite of a number of ideas• It explains many things that previously were

difficult to explain• Some of the basis provided in Chapter 12; more

details in Chaps. 13 & 14

• The lithosphere (crust & uppermost mantle) is broken into fairly rigid pieces called plates

• These overlie a weaker region called the asthenosphere (and, the two are mechanically disconnected)

• The plates are in motion relative to each other, and are continually changing in shape and size

• Seven major plates, with a few smaller• Average plate movement is 5 cm (2 in) per year

(about how fast your fingernails grow)• And remember, all this motion is on a sphere, not

a 2-D map

Figure 2.19Left

Figure 2.19Right

A quick overview of the boundaries

• Divergent boundaries (Ch. 13)– Where two crustal plates move apart– Two types: spreading centers, and rifts

• Spreading centers– Usually, along the crests of oceanic ridges– New crust is generated here– Source of the new crust is a rift valley

Figure 2.20

• Continental rifting– Splitting that occurs within a continent– Modern example – East African Rift

• Appears to be the initial stage of a breakup

• Vigorous volcanic activity along the edges

• We don’t know whether this process will continue

Figure 2.21

• Convergent boundaries (Ch. 14)– If some things are moving apart on a

constrained volume such as a sphere, some things must be moving together

– Generally, two bodies meet, one moves up (relatively), the other moves down (relatively)

– Thus, two plates collide, one tends to slide beneath the other

• The surface expression is a deep-sea trench• Part of a subduction zone, since something is

descending into the earth

• Two types of crust, three possible encounters– Oceanic crust (average density 3.0 g/cm3)– Continental crust (average density 2.7 g/cm3)– The densities are important

• Water is most dense at 4º C – 1.000 g/cm3

• “Normal” water’s density is 0.9999 g/cm3

• Density of ice is 0.917 g/cm3

• Oceanic-continental convergence– Continental crust, being less dense, overrides– Oceanic crust dives down

• Can take with it water-saturated sediment, as well as water in the crust itself

• Leads to partial melting at a depth of about 100 km

• Resulting magma MAY reach the surface; often does not

Figure 2.22A

• Oceanic-oceanic convergence– Similar to above, but the two plates are oceanic

crust– Volcanism may occur; tends to build chains of

volcanic islands– Ultimately builds arc-shaped chains of volcanic

islands called (simply) island arcs• Young examples: Tonga, Aleutian Islands, Lesser

Antilles

• Older examples: Japan & Indonesia arcs

Figure 2.22B

• Continental-continental convergence– Can you guess? Two continental masses

collide– Happened in the past, the best modern example

is India butting heads with Asia (creating the Himalayas)

– Generally, involves closing of an ocean basin

Figure 2.23BC

• Transform fault boundaries (no Chap.)– Here, parts of plate slide past one another

without production or destruction of crust– Most associated with midocean ridge systems– May form part of a plate boundary (an example

is the San Andreas fault)

Figure 2.24

Figure 2.25

• Tests of the theory– Outlined in the text– Data from oceanic drilling– Hot spots– Paleomagnetism data– Recent measurements from spacecraft

So how does this all happen?

• Overall, the concept of convection

• Most models cannot account for all the details we observe

• What is convection?

• Convection – basically, warm, buoyant rock rises, forcing cooler, less dense material downward

• Occurs in the mantle– Three main ideas

• Ideas regarding the mantle– Three main ideas– First – convection layers– Two layers in which convection occurs;

material between the two layers does not mix

Figure 2.31A

• Whole-mantle convection– Overall convection by hot plumes deep in the

mantle driving shallower processes– Hot up, cold down– The core being hot, releases heat to the bottom

of the mantle

Figure 2.31B

• Deep-layer model (the lava-lamp model)– Similar to the last model, but more complicated

as to how processes occur– More along the lines that the real world does

not follow our perfect circles that we draw– Convection within a spherical, constrained

body is not as simple as in a 2-dimensional model

• More details on boundary types are given at the end of Chapter 2

• Incorporated in chapters 13 and 14

End of chapter 2