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In 1620, Sir Francis Bacon (English philosopher,1561-1626) noted in his book Novum Organum that thecoasts on either side of the Atlantic Ocean appeared toparallel one another. He may not have been the first tomake this observation, for it was readily apparent evenon the crude maps of that day. During the succeedingthree centuries such observations multiplied. ToBenjamin Franklin the correspondence between coast-lines was so striking that he speculated, far ahead of histime, that the surface of the earth might be a crackedshell whose fragments were driven about by the move-ments of a dense fluid on which they floated. Near theend of the 19th century, Eduard Suess (Austrian geolo-gist, 1831-1914) noted many geologic similaritiesbetween Africa, South America, and India and proposedthe fragmentation of a supercontinent, but envisionedthat the oceans had formed by the foundering of blocksof crust between the present landmasses.

All of this set the stage for the man who became thechampion of an idea that would eventually revolution-ize the way we look at the earth. The man was Germanmeteorologist Alfred Wegener, and the idea was that thecontinents are mobile blocks of crust capable of movingthousands of kilometers around the globe: continentaldrift. While at this point we could simply rehearse themodern version of this theory, there are at least twogood reasons for delaying that part of the story. First,while the observations in favor of continental drift werenot accumulated all at once, they turn out to make arather impressive and fascinating body of evidence, andyou deserve the opportunity to decide for yourself howconvincing you think it is. Second, the entire story pro-vides one of the best examples of how science works—how it presses, often ponderously, toward the truths it iscapable of seeking, sometimes taking wrong turns butslowly correcting its own errors as it goes, always withthe hope that the revised theories will reflect a deeperunderstanding of reality than did the old ones.

Wegener was educated in German schools andreceived a degree in astronomy in 1905. While pursu-ing that degree, however, his interests in meteorologyand geology grew, and he took a position as a lecturer inmeteorology at the University of Marburg. He record-ed that he first began entertaining thoughts about conti-nental drift (a concept that he called “continental dis-

placement”) in 1910 when he, like others before him,considered the parallelism of the coastlines across theAtlantic Ocean. He felt the whole idea of moving con-tinents a bit improbable at first, but then accidentallyran onto an old scientific paper suggesting some corre-spondences between fossils on opposite sides of theAtlantic as evidence of an ancient land bridge betweenSouth America and Africa that had foundered and sunk.The concept of isostasy (see Chapter 30) was wellenough understood by then for Wegener to know thatthe sinking of large blocks of continental crust was notlikely, but the fossil correlations intrigued him. Hebegan in earnest to gather data of all sorts concerninggeologic similarities on different continents, and in1912 he presented a lecture in which he first outlined hisideas about “continental displacement.” He followedthat in 1915 with a book, The Origin of Continents andOceans, in which the comprehensive theory wasdescribed.

During his lifetime, Wegener was highly respectedfor his work in climatology and paleoclimatology(ancient climates), but he is remembered for continentaldrift. Nevertheless, the reception then given his theorywas cool at best (ridicule and scorn were not uncommonreactions), and when he died in an ill-fated climatolog-ical expedition to Greenland in 1930, the theory of con-tinental drift was not faring at all well in the scientificcommunity. Before learning why not, we shall examinethe evidence in favor of the theory.

The Fit of the Continents

The most obvious and compelling evidence forcontinental drift consists of the shapes of the continents.The fit of some continents is readily apparent on casualobservation of a world map, but it is even more strikingwhen they are juxtaposed. Figure 31.1 shows how wellSouth America and Africa match at the edges of theircontinental shelves (which, you will remember fromChapter 28, are the actual edges of the continents). Thefit was optimized by computer in the early 1960s andturned out to be better than even the proponents of con-tinental drift had expected. There are a few regions atwhich the two continents overlap, and this would be ofgreat concern if they were not generally areas in which

31. From Continental Drift to Plate Tectonics: The Evidence

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major rivers empty into the sea and deposit their sedi-ment. Because such sediment would have been deposit-ed during relatively recent times, the “overlaps” wouldnot have existed before the continents separated. Inaddition, it is not unlikely that some deformation wouldhave occurred at, or subsequent to, rifting apart of thesehuge landmasses.

South America and Africa are not the only conti-nents that fit together remarkably well at their shelfboundaries. As later figures will show, India,Antarctica, and Australia join them in the south withNorth America and Eurasia to the north to form a mas-sive supercontinent. Wegener named this large land-mass Pangaea, a word derived from Greek and mean-ing “all earth.” Figure 31.2 shows Pangaea as he envi-sioned it. The ensuing seven decades have producedremarkably few major changes.

Continental Structure

When we reviewed the major structural features ofthe continents (Chapter 28), we found that all containone or more shields, stable platforms, and fold moun-tain belts. The shields, which are the ancient geological(although not necessarily geographical) cores of thecontinents are surrounded by the stable platforms. Yetthere are some shields that end abruptly at coastlines.Figure 31.3 shows the shield areas on South Americaand Africa, with the continents arranged as in Figure31.1. Note how remarkably well shield regions that endat the coastline of one continent appear to continue on

South

America

Africa

Figure 31.1. The close fit of South America and Africa, matched at the edges of their continental shelves.

PennsylvanianTime

TertiaryTime

EarlyQuaternary

Time

Figure 31.2. Pangaea and continental drift, as proposedby Wegener in the third edition of his book. Heavy linesoutline the landmasses as he envisioned them, whilelighter lines show present-day boundaries simply to aidin identification.

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the other, despite the fact that the continents werearranged using the continental shelves, with no regardfor the locations of shields.

The curved lines adjacent to the shields representstructural trends. These are the directions of majorgeologic structures such as zones of deformed rocks andfold mountain belts, and they appear to be continuousacross the continental boundaries, as if they had oncebeen connected. Not only that, but the ages of the rocksalso match, as do localities of economic mineralresources such as tin, iron, and diamond deposits.

We have used South America and Africa to demon-strate geologic continuity across continental boundariesbecause we began with them in discussing the fit of thecontinental shelves. Had we chosen to examine othercontinental boundaries, the results would have beensimilar. For example, the Appalachian Mountains ofeastern North America continue across the British Islesand into the Fennoscandian Peninsula when the NorthAtlantic Ocean is “closed” to bring North America,Greenland, and Europe together.

Could the continuity of structural trends, as well asthe fit of the continental shelves, all be coincidence?

Paleontological Evidence

For Wegener, one of the keys to continental driftwas similarities in fossils, particularly in the fivesouthern landmasses: South America, Africa,Antarctica, India, and Australia. Some of the fossils

involved are the remains of ancient land plants, andothers represent animals of various sorts.

One of the most well-known is a group of plants

Shields

Structural trends

Figure 31.3. Structural features of South America and Africa, with the two continents arranged so that the continentalshelves match best.

(a)(b)

(c)

(d)

Figure 31.4. Fossil leaves from plants that constitutethe Glossopteris flora. Glossopteris itself is on the leftand was about 20 centimeters long.

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called the Glossopteris flora. A “flora,” in biologicalterms, consists of the plants of a particular area, envi-ronment, or time period; so these are fossil plants thatare typically found in association with one another, andthe genus Glossopteris is the characteristic fossil forwhich the flora is named (Fig. 31.4). During Wegener’slifetime this flora was known from rocks of the PermianPeriod in South America, Africa, India, Australia, andMadagascar; it has since been found in Antarctica andthe Falkland Islands, again in Permian rocks. It is diffi-cult to imagine how the seeds of these plants could havebeen spread across thousands of kilometers of openocean, so Wegener reasoned that the oceans must nothave separated these southern landmasses during thePermian Period.

Fossils of animals, too, are common to at leastsome of the southern landmasses. During the PermianPeriod there existed a small freshwater reptile calledMesosaurus whose fossils are found in similar rocks inSouth Africa and in Brazil. A bit later, during the earlyTriassic Period, a medium-sized reptile calledLystrosaurus left its remains in what are now Brazil,South Africa, India, Antarctica, and China. (Just howeastern Asia may have fit into Pangaea is still inade-quately understood.) Neither of these animals werecapable of traversing large bodies of water, yet their fos-sils are now separated by oceans.

Paleontological evidence from North America andEurasia is less spectacular than that from the southerncontinents, but it is there nonetheless. Consider, forexample, the small, bottom-dwelling marine animalsknown as trilobites (Fig. 31.5). During the earlyPaleozoic Era, the trilobites on the North American sideof the heavy line were quite different from those on theEuropean side, as judged from the fossils found in rocksof that age. Note, however, that the line doesn’t quitedivide North America from Europe. There are placeswhere “North American” trilobites are found in earlyPaleozoic rocks of what is now Europe and other placeswhere “European” trilobites are found on the edge ofpresent-day North America. As we look at trilobites ofmore and more recent rocks, however, we find that thedifferences in the two populations decrease and largelydisappear by the middle of the Paleozoic Era. InWegenerian terms, it appears that the closing of theancient Atlantic Ocean to form Pangaea resulted in theinterbreeding of trilobites previously separated by thesea and that the reopening of the Atlantic when thesupercontinent fragmented did not take place exactlyalong the pre-Pangaean boundaries; some of what usedto be one continent ended up with the other.

Examples like these show clearly why Wegenerwas so impressed with the paleontological evidence forcontinental drift.

Paleoclimatic Evidence

Rocks contain clues about the climatic or environ-mental conditions under which they formed, or to whichthey have been subjected over the eons of geologictime. For example, fossil plants in sedimentary rockscan often be identified as typical of tropical, subtropi-cal, or temperate climates; coal beds are the remains ofplants that flourished in warm, swampy, coastal envi-ronments; some rocks consist of chemical precipitates(such as rock salt), suggesting the evaporation of poor-ly circulating bodies of water such as lakes or land-locked seas; current ripple marks left in sediment canlater reveal not only the environment of deposition but

EuropeGreenland

AfricaNorth America

South America

Figure 31.5. During the Cambrian and OrdovicianPeriods, trilobites on different sides of the heavy linelooked quite different, as shown. During the Silurianand Devonian Periods, they gradually became identical.Note that the line separating the early trilobites wandersback and forth across the boundaries of what are nowwidely separated continents. Could it be that these ani-mals interbred as the Atlantic Ocean closed to formPangaea, and that the later separation produced slightlydifferent boundaries than the continents previously had?

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also the direction of the water flow.Among the more distinctive indicators of ancient

climates are the deposits left by continental glaciers,great sheets of ice that cover thousands of square kilo-meters. We’ll learn more about those in Chapter 34; fornow, we just need to know that glaciers require a coldclimate and that they leave characteristic deposits ofsediment that can be recognized as glacial in origin.You have probably heard of the Great Ice Age of thenorthern hemisphere—the one we often associate withwoolly mammoths, saber-toothed tigers, and the like.That glacial event was comparatively recent in geolog-ic terms, but there was another ice age that occurredmuch earlier. Evidence of it is found in Permian rocksof the southern hemisphere, distributed as shown inFigure 31.6. Consider the arrangement of these glaciat-ed areas as they appear on a present-day world map. Itseems to make no sense. For instance, if eastern SouthAmerica was cold enough to support glaciers, why notthe Andes Mountains to the west, which are much high-er and therefore colder? If the Indian subcontinent hadglaciers, then why not the Himalayas?

Figure 31.7 shows Pangaea, again with the conti-nents arranged so that their shelves fit best. Note thatthe areas affected by the Permian glaciation are nowcontiguous. Not only that, but the directions in whichthe ice moved (which can be determined by features inthe glaciated rocks) are now consistent. This is no lessremarkable than the continuation of structural featuresand trends that we saw earlier and that could hardly be

the result of coincidence.Figure 31.8 shows the overall climatic picture of

the earth during the late Paleozoic Era, as determinedfrom the rocks of that age. This climatic distribution isunexplainable on the basis of the present arrangementof landmasses in the world.

Figure 31.6. The present distribution of rocks glaciated by the Permian ice age. Arrows show the directions of icemovement.

Figure 31.7. The distribution of rocks affected by thePermian glaciation, with Pangaea reconstructed so thatthe continental shelves fit best. The arrows show thedirections of ice movement.

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Paleomagnetic Evidence

When volcanoes erupt basaltic lava (that is, lavathat will crystallize to form the black volcanic rock,basalt), there are generally some crystals already in itthat have formed below the surface, but most of it is liq-uid and only becomes crystalline as heat is lost to thesurroundings. Eventually (hours to months, dependingon the thickness of the lava), all of the liquid crystallizesto form minerals. One of the minerals present is calledmagnetite, an iron oxide that is magnetic enough torespond strongly to a magnetic field. There is never alarge amount of it in basalt, but what there is providesus with a way of detecting the orientation of the earth’smagnetic field during the time the lava was erupted.

The magnetism of magnetite originates in the spinsof the electrons in orbitals surrounding the iron nuclei,and above its Curie temperature magnetite is not mag-netic. (Recall that we have encountered the Curie tem-perature before, in Chapter 30.) The temperature of lavaerupted from a volcano is very high, but as the solidifiedlava cools it passes through the Curie temperature andthe magnetite becomes magnetic. In doing so, its elec-trons arrange themselves so that their spin axes line upwith the direction of the earth’s magnetic field at thetime, creating a permanent magnetic field in the rock.Once the rock cools through the Curie temperature, themagnetic field is “frozen” in, and it does not change withrespect to the rock itself, even if the rock is moved or theearth’s field shifts. A device called a magnetometer can

be used to detect the direction of this field in the rock,and if that direction does not coincide with the presentmagnetic field of the earth, then the rock has either beenmoved or the earth’s field has changed directions.

Figure 31.9 depicts, in a schematic way, what isfound when the magnetic fields of late Paleozoic lavaflows are measured in South America, Africa, and India.The directions toward late Paleozoic north are inconsis-tent—the arrows do not point to the same places on theearth. The nature of the earth’s field, originating as itdoes in the core, requires that there be only one northpole at a time, so these results do not make sense. Butlook what happens if these landmasses are placedtogether in our standard Pangaean orientation. Thenorth arrows now all point to a single location. Again,the strong indication is that the continents were oncelocated in different relative positions on the earth thanthey are in now.

Wegener’s Hypothesis: Mechanism, Test, andRejection

Continental fits, structural continuities, paleontol-ogy, paleoclimatology, and paleomagnetism all pointtoward a single answer. With his evidence in place andabsolutely convinced that the continents did drift about,Wegener had only one important conceptual problemleft to solve: There needed to be some mechanism formoving them. He knew that the continents consist ofrock that is less dense than that of the ocean basins, so he

Arid low-latitude climate

Tropics

Arid low-latitude climate

Glaciers

Figure 31.8. Late Paleozoic climates of the world, as determined from the geologic evidence found in rocks of that age.As with several previous figures, the continents have been arranged to match the edges of the continental shelves.

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reasoned that the continents must slide through theunderlying oceanic rocks much as a raft moves throughwater.

Unfortunately for Wegener, he was an empiricistrather than a theoretician—that is, he was best at build-ing theories out of observations but not particularlygood at supporting those theories with the necessarymathematics. It did not take long for Sir Harold Jeffreys(1891-1989), an eminent British physicist with a formi-dable scientific reputation, to carry out some rather ele-mentary calculations and show beyond doubt that theocean floor is much too rigid for continents to be shovedthrough it by any force imaginable. It was clear thatJeffreys was not mistaken, and no one could propose amore reasonable mechanism for separating the conti-nents. To many geologists and scientists of other fields,it seemed almost as though the very validity of the evi-dence itself rested on the need for an acceptable mech-anism, and there didn’t seem to be one.

Jeffreys spent years in the face of mounting evi-dence trying to scuttle the concept of continental drift,and for a long time his efforts were largely successful.He was joined by a host of geologists and other scien-tists who considered the whole idea, to quote one ofthem, “utter damned rot.” Most of the evidence that

came forth early in the debate was found in the southernhemisphere, and some South African geologists(notably Alexander Du Toit, 1878-1948) did much tofoster Wegener’s ideas there. For a time, the theory wasconsidered acceptable south of the equator but heresynorth of it; however, the great majority of geologistswere European and North American, so continental driftfell into disfavor with the bulk of the scientific commu-nity. Despite its general rejection, the theory was farfrom forgotten, and heated debates about the evidencewent on for decades. At an auspicious meeting of 14eminent geologists in 1928, five supported the theoryoutright, two supported it with reservations, and sevenwere unalterably opposed to it. It was not, to say theleast, greeted unanimously as the solution to the grandmysteries of geology!

Seafloor Spreading

To this point in our story, both advocates and criticshad concentrated their attention on the continents.Because little was known about the seafloor except that

(a)

(b)

Figure 31.9. (a) The orientations of the magnetic fieldsof lava flows of late Paleozoic age, with the present ori-entation of continents. (b) The same magnetic fields,with the continents assembled in the Pangaean fashion.

Asthenosphere

Lithosphere

Figure 31.10. Rifting, such as that in the oceanic ridgesystem, is produced by tensional forces.

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it was quite different in composition from the continents,it was largely ignored on both sides of the debate.During the 1950s and 1960s, however, it became tech-nologically possible to map the ocean floor in greatdetail, and so the oceanographers set about the task.Among the features they found was a great sinuous ridgethat girdled the globe, poking above sea level only atplaces like Iceland and a few other islands. The ridgeruns down the center of the Atlantic Ocean, having ashape remarkably similar to the boundaries of the conti-nents on either side, and from there proceeds betweenthe landmasses that were supposed to have formedPangaea. Detailed investigation revealed that the ridgehas a central rift valley that runs along its length.Looking elsewhere in the ocean basins, geochronolo-gists found that none of the basalts dredged from theocean floor are older than Mesozoic, and geophysicistsfound large concentrations of deep earthquakes associat-ed with the deep ocean trenches that adjoin the islandarcs.

Enter Harry Hess of Princeton University. He rea-soned that the central rift of the ocean ridge had to be atensional feature (see Fig. 31.10), indicating that theseafloor was being pulled apart there—according to hisscenario, by convection currents in the asthenospherebelow. As rifting proceeded, magma from the asthenos-phere would ascend to fill in the fissures, thus creatingnew oceanic crust. The oceanic lithosphere thus creat-ed would slowly move away from the oceanic ridgetoward the deep ocean trenches, where it would descendinto the asthenosphere to be melted and recycled.Because the seafloor was created at the ridge anddestroyed at the trenches, it was perpetually young.

In 1960 when he presented it, the evidence for allof this was still so tenuous that Hess called it “geopoet-ry”. The confirmation came when the data from conti-nental and seafloor geology were finally merged.

Magnetic Reversals and “Stripes” on the Seafloor

Early in this century, geologists who were measur-ing the “frozen” magnetic fields in basalts in France dis-covered that some were oriented in a direction oppositeto present-day north; that is, the magnetic fields of themagnetite crystals pointed toward the south. As otherold lavas were investigated, it became increasingly clearthat the earth’s magnetic field has experienced rever-sals in the past—there have been times when it haspointed along its present direction (normal polarity)and intervening times when it has been oriented inexactly the opposite direction (reverse polarity). Thereason for reversals was (and still is) a mystery, butthere was no doubt that they had occurred. By studyingsuperimposed basalt flows, the sequence of reversalswas worked out, complete with absolute dates, and itwas discovered that reversals during the Cenozoic Era

have taken place irregularly but on an average of everyhalf-million years.

Independently of all of this work by continentalgeologists, the seafloor geologists were mapping themagnetism in the rocks at the bottom of the ocean bytowing magnetometers across the North Atlantic Ocean.What they were measuring was not just the earth’s mainmagnetic field, but that field plus the magnetic fields ofthe basalts on the floor of the sea. What they foundwere stripes—long, narrow bands running parallel tothe Mid-Atlantic Ridge that alternated symmetrically oneither side of the ridge as bands of high and low totalmagnetism. Not only was this result completely unex-pected, but it seemed inexplicable as well. Not until1963 did the explanation emerge, when F. J. Vine and D.H. Matthews, a graduate student and his supervisor atCambridge University, put Hess’s ideas together withthe magnetic reversals discovered on land and the mag-netic stripes of the seafloor.

Tim

e

Figure 31.11. The development of magnetic stripes ofalternating normal and reverse polarity on the seaflooras basaltic lava is extruded at the oceanic ridge and thenis pulled to either side as the seafloor spreads. The actu-al boundaries between adjacent stripes are more com-plex than shown in this generalization.

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Vine and Matthews proposed that the bands ofstrong magnetism are those for which the earth’s mainfield and that of the seafloor rocks add together (that is,the rocks show normal polarity) and that the others, inwhich the magnetism is weaker, are those in which themagnetic field of the basalts subtracts from the earth’sfield (the rocks show reverse polarity). What had real-ly been revealed by the magnetometer surveys werebands of normal polarity and reverse polarity in theseafloor basalts. As the basalt lavas were extruded atthe oceanic ridge, according to Hess’s proposal, theywould become magnetized by the magnetic field of theearth at the time and then would migrate away from theridge, allowing newer rock to form. At some time thepolarity of the earth would change, and then the subse-quent lavas would result in rocks of the opposite polar-ity. The process is diagramed in Figure 31.11.

Two Canadian scientists, L. Morley and A.Larochelle, must be credited with explaining the mag-netic stripes simultaneously with, but independently of,Vine and Matthews. Their paper was actually submit-ted a little before that of Vine and Matthews, but the edi-tor of the journal to which they submitted it consideredthe concept too speculative and refused publication.Sometimes, in drawing its intellectual gun, scienceshoots itself in the foot! The wound is never fatal, butit can be very painful.

Further Tests and Triumph

If the hypothesis of Vine and Matthews is correct,then it should lead to some predictable results that canbe tested, and this is the case. Consider, for instance,where the youngest and oldest ocean floor should be.The youngest should occur at the ocean ridges, and theseafloor should get progressively older away fromthem. Our ability to dredge basalts from the seafloorand determine their radiometric ages should facilitatethe necessary test. In addition, the thickness of marinesediments deposited on the seafloor should increaseaway from the ridges, because older seafloor has hadmore time to accumulate it. Finally, the oldest marinefossils in the present oceans should be found in the bot-tom-most layers of sediment away from the ridges. Thedata have been examined, and seafloor spreading haspassed all of these tests. Had it failed any one of them,seafloor spreading could not have been accepted as real-ity. Because it passed all of them, it is virtually incon-ceivable that it is not reality.

In its broad outlines, Wegener’s continental driftwas vindicated. The continents do move about the sur-face of the earth, but they do not plow through the crustof the ocean. Rather, they ride as passive passengers ongreat plates of lithosphere that originate at ocean ridgesand are destroyed at the deep ocean trenches. Theselithospheric plates are outlined by the earthquakes that

occur where plates meet and bump against one another.(We shall get into the mechanics of such interactions inthe next chapter.) The worldwide distribution of earth-quakes over a six-year period and the outlines of thelithospheric plates implied by it are shown in Figure31.12.

The theory we have just explained is called theTheory of Plate Tectonics, and nearly all geologistsnow accept it as a viable model for how the earth works.(“Tectonics” is a word that suggests the broad structur-al aspects of the earth.) Many observations that used tobe geologic mysteries are now nicely explained by thetheory. Interestingly, one of the verifications of platemotion comes from observations of objects literally bil-lions of light-years away—quasars (see Chapter 27).By observing the light of these strange objects from dif-ferent points on the earth and combining the observa-tions using a technique called interferometry, themotion of the plates can, in principle, be detected. Suchexperimental measurements are still in their earlystages, but so far they confirm quite well the directionsand speeds of plate motions as determined from earth-bound evidence.

The Breakup of Pangaea

We return now to Wegener’s Pangaea—not “thebeginning” of the earth nor even the “original” conti-nent, but a landmass that came together by the randommotions of plate tectonics as a discrete event in the longhistory of the earth. The same forces that broughtPangaea together during the late Paleozoic Era shifteddirections and ripped it apart during the Mesozoic.Figure 31.13 shows a modern version of Figure 31.2.

Like nearly all things scientific, the final chapter onthe reconstruction of ancient plate motions has not yetbeen written. Much work is still being done on pre-Pangaea plate motions, and even the timing of thebreakup of the supercontinent is not entirely sure.There are unanswered questions that do not invalidatethe broad aspects of the theory but which may requireserious reconsideration of the absolute dates of someevents. For example, the fossil remains of a dinosaurcalled Titanosaurus are found in South America, Africa,and India. No problem, right? Wrong. The age of therocks in which the fossils are found is late Cretaceous,and by that time the conventional scenario has theselandmasses too far apart to allow migration. Could thefossils be very similar but not actually identical?Possibly. Or perhaps there is another answer that willbring more surprises when we discover it.

Postscript: A Comment on the Way Science Works

You will probably agree that all of the evidence thateventually accumulated in favor of continental drift dur-

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(a)

South Americanplate

Africanplate

Arabianplate

Eurasian plateNorth American

plate

Pacific plate

Australianplate

Antarctic plate

Nazcaplate

Philip

pine

plat

e

Cocosplate

Caribbeanplate

(b)

Figure 31.12. (a) The distribution of earthquakes for a period of six years. The narrow lines of seismic activity delin-eate the boundaries of the lithospheric plates. (b) The outlines and names of the lithospheric plates.

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ing the first half of the 20th century (much of whichWegener, himself, had) is pretty impressive. Any indi-vidual observation might be accepted as merely coinci-dence, but considered as a body it is indeed imposing.Yet for half a century after Wegener proposed Pangaeaand its breakup, the theory met with open oppositionfrom most scientists. Because of the hindsight that isalways available to those of a later generation, it is easyfor us to be critical of the scientists who rejected conti-nental drift; but that judgment may overlook some of theimportant strengths inherent in the methods of science.

Wegener’s treatment at the hands of the scientificestablishment is reminiscent of Galileo’s treatment at thehands of the religious establishment. It is clear that someof the opposition was not to Wegener’s theory per se butto a scientist who had wandered outside the limits of hisown field and proposed radical ideas in a discipline forwhich his formal educational credentials did not qualifyhim. On the other hand, Jeffreys was, in fact, absolute-ly correct when he announced that continental move-ment through the oceanic rocks could not possibly haveoccurred. Had he or someone else not challenged atleast this proposed mechanism and it had been widelyaccepted, it would still have been incorrect; truth is, afterall, not determined by majority vote. Thus, sending theproponents of continental drift “back to the drawingboard” to come up with a more convincing mechanism

was not only reasonable but also required. Despite occasional statements to the contrary by sci-

entists who wish to appear completely objective, scienceis an intensely personal endeavor to those engaged in it.Each scientific issue of any consequence is represented bya number of differing viewpoints, each with its own groupof proponents. New developments or proposals put forthby one group are scrutinized and tested by the othergroups, and weaknesses are exposed while strengths areadmitted. Instead of tearing science down, this processbuilds it by preserving only the theories and ideas mostconsistent with observation, and eventually those theoriesthat can withstand the most rigorous testing come to beaccepted even by those who initially disagreed.

Even then, it is recognized that good theories are nomore than close approximations to the truth and can berefined. (Consider, for example, that Newtonianmechanics and gravitation were considered accuratedescriptions of nature for nearly three centuries untilquantum mechanics and relativity revealed them to beonly very good approximations.) Thus, if the methodsof science are often painstakingly slow, they are at leastself-correcting, and serious errors are eliminated as longas old ideas are honestly questioned and those foundlacking are replaced by better ones.

Figure 31.13. A currently accepted arrangement of the present continents in the supercontinent, Pangaea.

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Summary

The synthesis of evidence from continental andseafloor geology have revealed that the lithosphere ofthe earth is fragmented into several large plates that jos-tle against one another and move about the surface ofthe planet, essentially floating on the partially moltenasthenosphere below. The theory that deals with thisphenomenon is known as plate tectonics. Over the spanof geologic time, movement of continental massesacross thousands of kilometers has formed superconti-nents that have subsequently been broken apart. Oneresult is that the oceanic crust has been continuallyrenewed and has remained geologically young. In thenext chapter, we shall examine the details of plate tec-tonic theory.

STUDY GUIDEChapter 31: From Continental Drift to PlateTectonics: The Evidence

A. FUNDAMENTAL PRINCIPLES: No new funda-mental principles.

B. MODELS, IDEAS, QUESTIONS, OR APPLICA-TIONS

1. The Theory (Model) of Plate Tectonics: Themodel of the earth in which the rigid outer layer ofthe earth (lithosphere) is fractured into separatepieces (plates) that move relative to one anothercarried by convection currents originating in themantle and driven by heat released by radioactivi-ty. The plates move on a partially molten layer(asthenosphere) underneath the plates and maypush against one another (convergent boundary),move away from one another (divergent boundary),or slide past one another (transform boundary).The model unifies the ideas of continental drift andseafloor spreading.

2. What is meant by “continental drift”?3. What is the evidence for continental drift and what

problem was unresolved by the continental driftmodel?

4. What is mean by “seafloor spreading”?5. What is the evidence for seafloor spreading?

C. GLOSSARY1. Continental Drift: The idea that the continents are

mobile blocks of crust capable of moving thou-sands of kilometers around the globe.

2. Continental Fit: The edges of continents can beplaced together in a manner similar to a jigsaw puz-zle to form a massive supercontinent calledPangaea.

3. Continental Glacier Evidence: Continental glac-iers are great sheets of ice that cover thousands of

square kilometers of land surface and flow outwardfrom the center of the glacier due to the extremeweight of the ice at the center. The positions andapparent motions of Paleozoic continental glaciersare taken as evidence for continental drift. This isa type of paleoclimatic evidence.

4. Continental Shelf: See Chapter 28.5. Continental Structure Evidence: The locations

of major structural features of the continents, i.e.,shield, stable platforms, and fold mountain belts,suggest that the continents were once part of asupercontinent.

6. Curie Temperature: See Chapter 30.7. Glossopteris Flora: A collection of species of

plants (flora) of the Permian Period of thePaleozoic Era that is common to South America,Africa, India, Madagascar, Antarctica, and theFalkland Islands. The existence of this particularflora on widely separated continents is taken as evi-dence for continental drift.

8. Interferometry: A method used to measure themovement of the continents. Very distant astro-nomical objects (quasars) are observed from posi-tions on two different continents and the light sig-nals are artificially combined into an interferencepattern as if the two observation sites representedtwo slits through which the light passed to form aninterference pattern. As the continents move, thepattern changes in a way that allows the measure-ment of the relative motion of two continents.

9. Lystrosaurus: A medium-sized, early Mesozoicreptile whose fossil remains are found in Brazil,South Africa, India, Antarctica, and China. Thepresence of the fossils on the widely separated con-tinents is taken as evidence of continental drift afterthe beginning of the Mesozoic.

10. Magnetic Reversal: The magnetic field of theearth sometimes reverses so that its direction isopposite to that of an earlier time. The magneticfield may either exhibit normal polarity (the pre-sent orientation) or reversed polarity (in which thepresent south pole becomes magnetic north).

11. Magnetite: An iron oxide mineral found in basalt.The magnetic field of the magnetite aligns with theearth’s magnetic field as molten basalt cools tosolid form and makes a permanent record of thedirection of the earth’s magnetic field when therock formed.

12. Mesosaur: A small, freshwater reptile whose fos-sils are found in similar rocks in South Africa andBrazil. These fossils on the widely separated con-tinents is taken as evidence for continental drift.

13. Paleoclimatic Evidence: Evidence from theancient climates, as determined by rocks and fos-sils, that the continents were joined together mil-lions of years ago and were in different positions

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than at present.14. Paleomagnetic Evidence: Evidence obtained from

the ancient record of the earth’s magnetic fielddirection obtained from lavas erupted on the sur-face of the earth. This evidence suggests that thecontinents’ past positions differ from their presentpositions.

15. Paleontological Evidence: Fossils of plants suchas those from the Glossopteris flora and of animalssuch as Mesosaurus, Lystrosaurus, and trilobitessuggest that the continents were joined togethermillions of years ago.

16. Pangaea: The large landmass (supercontinent)reconstructed by joining all of the continent togeth-er using their continental shelves as boundaries. Itis believed to have formed during the Paleozoic Eraover 200 million years ago.

17. Seafloor Spreading: The idea that the seafloor isbeing pulled apart at oceanic ridges by convectioncurrents in the upper mantle and asthenospherecausing the ocean floor to slowly move away fromthe oceanic ridge. New ocean floor is created asmagma from the asthenosphere fills the fissurecaused by spreading, and old ocean floor isdestroyed at deep ocean trenches.

18. Trilobites: Small, bottom-dwelling marine ani-mals which lived during the Paleozoic era and pro-vide evidence for continental drift.

D. FOCUS QUESTIONS1. Consider the idea of continental drift:

a. What is meant by continental drift?b. Describe evidence from five different kinds ofobservations that is consistent with the idea of con-tinental drift.c. Describe the major problem unexplained by theidea of continental drift.

2. Consider the idea of seafloor spreading:a. What is meant by seafloor spreading?b. Describe evidence from two different observa-tions that is consistent with the idea of seafloorspreading.c. Explain how it is possible for the seafloor tospread (expand) on an earth of constant size.

E. EXERCISES31.1. When attempting to fit continents together

geometrically, we use the edges of their continentalshelves rather than the coastlines. Why?

31.2. The occurrence of the Glossopteris flora inPermian rocks of South America, Africa, Antarctica,India, and Australia is evidence that

(a) these plants have lived in the southern hemi-sphere since the Permian Period.(b) these plants can survive in a wide variety of

climates.(c) the southern landmasses have separated sincethe Permian Period.(d) the southern landmasses have become closersince the Permian Period.

31.3. Which of the following is not evidence forcontinental drift?

(a) Changes in Paleozoic trilobites in NorthAmerica and Europe.(b) The presence of Glossopteris on several conti-nents.(c) Glaciated Permian rocks on several continents.(d) The geometrical fit of North America andAustralia.

31.4. How does paleomagnetism support the theo-ry of continental drift?

31.5. The alternating stripes of strong and weakmagnetism found in rocks of the seafloor are

(a) parallel to the oceanic ridges.(b) perpendicular to the oceanic ridges.(c) oblique to the oceanic ridges.(d) too far from the oceanic ridges to judge theirrelative orientations.

31.6. What testable predictions arise from thehypothesis that the seafloor is spreading at the oceanicridges?

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