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Plate Tectonics, Seafloor Spreading, and Continental Drift: an Lntroduction1 Rhe Method of Multiple PETE R J

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Plate Tectonics, Seafloor Spreading, and Continental Drift: an Lntroduction1 Rhe Method of Multiple PETE R J plating the logic and r work." A definitive ::al considerations re­ s of plate tectonics re- Plate Tectonics, Seafloor Spreading, and Continental Drift: an lntroduction1 rhe method of multiple PETE R J . WYlllE' .ce (old ser.), v. 15, p. ir. Geology, v. 5, p. 837- Abstract The present ruling theory of geote ctonics­ that the new global tectonics may indeed be v. 31, p. 155-165; 1965, commonly known os the " new globol tectonics'·-includes what Holl is Hedberg in 1970 called "the answer l. the concepts of plote tecto nics, seofloor spreoding, con­ o a grammar of biology: t inental drift, and polor wondering. Recent seismic oclivily to a maiden's prayer." However, it is time for defines the positions ond relative movements of rigid litho­ geologists to reflect and consider the evidence ite tectonics in geologic sphere plates. The ge o magnetic time scale for polarity for and against this ruling theory, rather than 107- 11 3. reversals seems lo be calibrated lo about 4 m.y. ago, and to assume that the final word has been handed ologists and paleontology extra polated lo about 80 m.y. ago by correlation of oceonic osal : Jour. P aleontology, mag netic anomalies with reversals a nd seafloor spreading. down. Seafloor spreading and the magnetic anomalies thus indi­ This review presents a brief introduction to late tectonics: the gco­ cate the directions and roles of movements of lithosphere 'ace: Science, v. 173, p. the theory, providing a background for the plates during the last 80 m.y. The continents drift with the fo llowing articles, which deal in detail with lithosphere pla tes, and independent paleomagnelic evi­ lf physical geology : New dence permits location of the relative positio ns of the specific topics. It is based on a tape recording o., 1288 p. of an unscripted and over-illustrated talk, and . Sykes, 1968, Seismology co ntine nts and the poles to 500 m.y. ago, or more. The tics : Jour. Gcophys. Re- theory, which explains phenomena previously unexplain­ many illustrations discussed have not been able, is supported by a mass o f persuasive evidence. There reproduced here. The tape was transcribed and 1rth, 5th ed.: Cambridge, is no doubt tha t th e theory is a success, but ii has been so successful that ii has become a ruling theory, a nd s ub­ edited by A. A. Meyerhoff, and I thank him for Press, 525 p. converting my spoken words with an English 1ew global tectonics-an servience to a ruling theory never has served science well. There ore data which do not seem lo fit the the ory. We accent into grammatical American prose. iature of geology: New should strive lo keep o pen minds a nd to search for alter­ No attempt was made to compile a complete old Co., 248 p. nate solutions lo fit all of the data. The record is clear: today's history was yesterday's mode l. Dare we conclude bibliography. Those interested in more de­ . Meyerhoff, 1974, Tests tailed reviews and bibliographies can find them me. that a l last we know the answers? edie medical dictionary, in books by Drake ( 1970), Maxwell ( 1970). and Wyll ie (1971a, b) and in several of the .. Davis Co. INTRODUCTION preading, in I. G. Gass, U.S. Quadrennial Reports to the Fifteenth ,Yilson, eds., Understand­ :, Mass., M.I.T. Press, Theories come and go, but during the last General Assembly of the International Union 12 years one theory has come along very of Geodesy and Geophysics (published in 1971 lOUt the earth: a revolu­ rapidly-so rapidly, in fact, that we now are in issues of Transactions of the American Geo­ nes, v. 13, no. 10, p. 10- the posi tion of having a single rul ing theory­ physical Union). the new global tectonics (Hess, I 960, 1962; Dietz, 196 I ; Morgan, 1968; Heirtzler et al., HISTORICAL BAC KGROUND 1968; !sacks et al., 1968). The global scheme is The earth sciences have been shaped by a illustrated schematicall y in Figure 1. Unfor­ series of great controversies and a host of minor tunately, there is no competing theory. Thus, disputes. It has sometimes seemed that geolo­ we are far from the method of multiple work­ gists enjoy the excitement of debate more than ing hypotheses- the method which we geolo­ they enjoy getting together to define their terms gists (in fact, all scientists) are supposed to fol­ in an effort to resolve a dispute. The great debate low (Chamberli n, 1890). I do not mean to of this century is about continental drift, whi ch imply that it is time to discard the new global is an old idea formul ated originally to explain tectonics- far from it. A large body of data the striking parallelism between coastlines bor­ has been gathered from the ocean basins and dering the Atlantic Ocean. This parallelism was from studies of paleomagnetism to demonstrate noted first by Alexander von Humboldt ( 1801 ). A book by Snider (1 858) seems to have been • Manuscript received, August 7, 1972. (This manu­ script was submitted originally to The Geological So­ the first of several works in the 1800s in which ciety of America on December 16, 1971.) continental drift is explicit or implicit. Con­ z Department of the Geophysical Sciences, University tinental drift certai nly is implicit in the classic of Chicago, Chicago, Illinois 60637. works by Suess ( 1908, 1909) and Taylor 5 6 Peter J. Wyllie Mid-Atlantic -(Ridge Fm. I-Schematic representation of seafl odr spreading and continental drift. Large plates of lithosphere containing continents migrate away from midoceanic ridges as if on a conveyor belt, and plates are carried in to earth's interior along boundaries where plates collide. (1910). However, Wegener (1912) generally New York City in 1926. The papers from this gets the credit for origillating modern concepts symposium, published by the AAPG (van der of drift. What Wegener really did was to· pro­ Gracht et al., 1928), in essence discredited con­ mote the idea more fully and forcefully than tinental drift. Jn most parts of the United States anyone before him. the ideas received little further attention, al­ Wegener's 1912 paper apparently was not though interest remained high in many other well received, and he went off to the Green­ parts of the world. A major problem was that land ice cap on an expedition led by J. P J the physicists could find no mechanism for pro­ Koch. After spending the winter at a base ·in· pelling the continents, even though Holmes northeast Greenland, they trekked 700 mi published a convection hypothesis in 1928 and (l,120 km) across the ice cap with a team of then proposed convection as a continental-drift ponies. During the long winter night at· lat. mechanism in 1931. His model was similar in 77°N there was little to do but complete the many respects to recent schemes (see F ig. 1) . chores necessary for subsistence, admire the In 1930, Wegener was in Greenland again as stark beauty of the surroundings by moonlight, leader of the German Inland Ice Expedition. A and cogitate. Wegener's cogitations bolstered major objective was to determine the thickness his faith 1n the idea of continental drift, and of the ice cap by using a new technique now he renewed his efforts after returning home and known as "explosion seismology." Wegener published a book (Wegener, 1915) . The great perished tragicall y on the ice cap at the age of 50. Although he did not live to see general debate was under way. The topic was sufficiently novel and challeng­ acceptance of his hypothesis, when be died he ing that it became the subject of varied discus­ was pioneering a technique which later gave it sions. Advocates of Suess's (1908, 1909) strong support. Seismic data from oceanic Gondwanaland found the hypothesis particu­ ridges, active mountain chains, and volcanic larly appealing, and Wegener's views quickly island arcs now provide persuasive evidence for gained wide acceptance in the Southern Hemi­ the new global tectonics. sphere. The arguments pro and con raged until By the late 1930s, about all that could be the 1930s, but they waned after the first sym­ said for continental drift had been said, not posium on continental drift, held by The Amer­ once but many times, and either one believed ican Association of Petroleum Geologists in it or did not. Plate Tectonics, Seafloor Spreading, and Continental Drift 7 In the 1950s, interest was revived by ex­ consequences of these movements and interac­ ploration of the ocean floor' and the develop­ tions through time. It is thus concerned largely ment of paleomagnetic studies. In 1960, the with the surface and crust of the earth, although late Harry H. Hess (1960, 1962) revived the causes of plate movements are within the Holmes's (1931) model of mantle convection earth. and introduced the hypothesis of seafloor The surface features of the solid earth are spreading. Hess considered the idea so fanciful shown in Figure 2. There is a primary distinc­ that he called it "geopoetry." In the mid-1960s, tion between two levels, the continental plat­ an increasing variety of evidence lending sup­ forms and the noors of the oceanic basins. In­ port to the hypothesis was reported, and a cluded with the continents are the epeiric seas bandwagon atmosphere developed. and the submerged continental shelves. The Io January 1970, a fu ll page in Time mag­ primary tectonic features are the oceanic ridges, azine explained bow Hess's geopoetry had be­ the geologically young mountain ranges and come geofact, and th e heading declared "It's a volcanic island arcs, the oceanic trenches, and revolution." At about the same time, the Jour­ the major fracture zones that transect the nal of Geophysical R esearch- another respec­ oceanic ridges.
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