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. table publication-also was telling us that con According to plate tectonics, the earth is tinental drift was proved fact. However, in covered by a small number ( 6 to 20) of large, 1968 articles had appeared in the Journal of stable lithospheric plates moving relative to arge plates of lithosphere Geophysical Research referring to the th eory of each other and having boundaries delineated by ind plates are carried into "plate tectonics." Plate tectonics, as depicted in bells of tectonic activity. Signals of activity in Figure 1, incorporates seaftoor spread in g from clude earthquakes and volcanism. The oceanic the oceanic ridges and accounts for continental trenches and ridges are well correlated with . The papers from this drift. Therefore, I begin here with a review of world tectonic activity, and more accurate in y the AAPG (van der plate tectonics and proceed to show how the formation becomes available each year. The re ssence discredited con other concepts are related. markable concentration of earthquakes along rts of the United States narrow belts is apparent from the maps of Bara further attention, ai PLATE TECTONICS zangi and Dorman ( 1969, 1970) showing world d high in many other Figure I shows the general global scheme. seismicity in a recent 8\12-year period (January 1ajor problem was that The outer shell of the earth, about I 00 km 1961 to September 1969). The earthquake belts no mechanism for pro thick, is composed of relatively cool, rigid rock are represented in Figure 3 by the· heavy lines; even though Holmes called the "lithosphere." This overlies the these belts are considered to delineate the bound 1ypothesis in 1928 and "asthenosphere" which, being warmer and less aries of the rigid aseismic plates that are about n as a continental-drift rigid, is capable of slow convective motion in 100 km thick (Fig. 1) and as much as ; model was similar in the solid state. Convecting material rises be thousands of square ki lometers in area. The chemes (see Fig. 1). neath the oceanic ridges and carries the litho bounding earthquake belts can be correlated in ; in Greenland again as sphere away laterally as if on a conveyor belt. Figure 2 with active oceanic ridges, oceanic land Jee Expedition. A The seafloor is thus spread apart, and the ten trenches or subduction zones, and strike-slip Jetermine the thickness sional gap is fi lled with new ocean crust gen faults of the transform variety (Wilson, 1965). a new technique now erated by the injection and eruption of magma Thus, according to the theory of plate tec seismology." Wegener risi ng from the depths. Where convection cells tonics, the apparent, natural division of the 1e ice cap at the age of converge, a slab of lithosphere is carried into earth's surface into continents and ocean basins ot Jive to see general the earth's interior in a subduction zone, where (Fig. 2) is a relatively insignificant feature of hesis, when he died he it is heated, partially melted, and eventually the surfaces of the thick lithospheric slabs. The :iue which later gave it assimilated. These locations are sites of com slabs themselves, however, are remarkably thin c data from oceanic pression characterized by mountain ranges or in contrast to their surface areas. chains, and volcanic volcanic island arcs and oceanic trenches. F ig Distribution of earthquakes locates the bound persuasive evidence for ure 1 shows that the continents form part of the aries of lithospheric plates, and the depths of lithospheric layer; as the lithosphere moves, so earthquake foci provide information about the bout all that could be the continents drift. extension of rigid plates into the earth's interior. ft bad been said, not Plate tectonics is concerned with tbe relative Intermediate- and deep-focus earthquakes are nd either one believed movements and interactions of the plates of considered to delineate subduction zones like lithosphere shown in F igure I, and with the that depicted beneath the Andes in Figure I. 8 Peter J. Wyllie
F IG. 2-Surface of solid earth. Stable continental platforms (diagonal shading) and stable ocean-basin floor (white) are traversed by active mountain belts (black) and submarine ridges (stipple). Rifted crest of midocean ridge system (thick black lines) is displaced into segments by fault zones (thi n black lines). Heavy dotted lines show deep ocean trenches adjacent to volcanic island arcs or to continental margins. Earthquakes are associated with mountain belts, midocean ridges, oceanic trenches, and island arcs.
FIG. 3-Plate-tectonic model. Distribution of major rigid plates of lithosphere, relatively free of earth quakes, bordered by active earthquake belts. Compare plate boundaries with features in F igure 2. Line between continent and ocean is not significant with respect to lithospheric plates unless it coincides with active earth quake belt. Based on !sacks et al. (1968) and Morgan (1968). Plate Tectonics, Seafloor Spreading, and Contine ntal Drift 9
T he relative motions of adjacent li thospheric 60° plates have been determi ned by the study of A NORMAL 8 REVERSED focal mechanisms of earthquakes. The average orientation of slip vectors for the earthquakes in a given earthquake belt defin es the motion, and the arrows showing the directions of movement of some plates in Figure 3 are consistent with the idea of seafloor spreading from the oceanic ridges. Recent seismic activity defines the present boundaries and relative motions of the litho spheric plates, but this activity does not indicate how long the movements have continued or whether the movements have remained con tinuous in direction and speed. The inferred existence of lithospheric slabs extending 700 km c LAVA FLOWS POLARITY into the earth's interior impl ies that the process MAGNETIZED EPOCHS continued at least long enough to transport the c.n 0 lithosphere laterally through 700 km. Rates of ...... movement must be known before times can be 0 } BRUN HES earth's surface. The lava flows range in age c-d produces a pos1t1ve anomaly above it, and from the present to 4 m.y. (Cox et al., 1964; the older crust, now separated into the two Doell and Dalrymple, 1966; Cox, 1968). The blocks a-c and d-b. produces negative anom magnetized directions of the lavas, which are alies in the earth's reversed magnetic field. In from various parts of the world, show a pattern this way, the sequence of polarity reversals for of alternating polarities termed " polarity the earth (Fig. 4C) is believed to become im epochs" and shorter intervals termed "polarity printed on the spreading oceanic crust. Figure events." The error in the dating method be SC is a cross section of the sequence of mag comes too large to extend this time scale back netized strips resulti ng from the polarity re ward beyond a bout 4 m.y. ago, because the versals of the past 3 m.y. The alternating direc error exceeds the total duration of many polar tions of magnetizati on of the strips produce the ity events. However, using a similar a pproach series of positi ve and negative anomalies, sym an d measuring the age and polarity of a wide metrically disposed about the ridge crest. Each range of rocks, McElhinney ( 1971) presented a anomaly bounda ry in Figure 5C, when cor reversal pattern through the whole of Phanero related with the appropriate reversal time in zoic time. Figure 4C, gives the time taken for the oceanic crust to spread laterally to its present position Magnetic Anomalies of Ocean Basins from the ridge c rest, where it was generated. The hypothesis of seafioor spreading became The magnetic anomalies parallel with the theory when the sequences of weak, linear mag netic anomalies which at the present time parallel the oceanic ridges were measured a nd A. 2·75 million years 8. 2·25 million years correlated with the time scale for the earth's GAU SS MATUYAMA polarity reversals. Vine and Matthews ( 1963) Magne tic Fi eld NORMAL Magnetic Field REVERSED lnlensily POLAR ITV lnlensily POLARI TY proposed that the magnetic anomalies were posit ive caused by the magnetization of alternate strips overage of the ocean fl oor in opposite directions, and ne9olive l they explained the existence of such strips by correlating the anomalies with seaflo or spread ing and polarity reversals, as illustrated sche matically in Figure 5; this is the " magnetic tape recorder" a nalogy. If one assumes, quite arbitrarily, that sea .floor spreading began at a ridge 3 m.y. ago, then eruptions of lava d uring successive polarity epochs before that time would have produced no directional m agnetic properties in the oceanic crust, and no systematic magnetic anomaly pattern would be expected . F igure 5A shows the effect produced during part of the Gauss normal epoch (Fig. 4C), between 3 a nd 2.75 m.y. ago, as the new crust a-b was gen erated from magma and spread laterally from the ridge crest. T his rock was magnetized in the direction of the existing ( normal) magnetic fie ld, producing a positive magnetic anomaly along the ridge crest. The Matuyama reversed FIG. 5-Schematic representation of sequence of polari ty epoch began about 2.5 m.y. ago (Fig. magnetization of new ocean flo or generated at mid 4C). Continued spreading for 0.25 m.y. pro ocean ridges as lithosphere is transported laterally duced the situati on in Figure 5B, where the away from ridge. Blocks of crust (with directions of block of new crust c-d is shown to be mag magnetization alternating as reversals of magnetic fi eld take place; see F ig. 4C) produce alternate positive netized in the d irection opposite to that of the and negative anomalies on earth's ambient magnelic original block a-b in Figure 5A. The block field as measured at sea level. Plate Tectonics, Seafloor Spreading, and Continental Drift 11 momaly above it, and Reykjanes Ridge, a part of the Mid-Atlantic parated into the two Geomagnetic Scale Ridge southwest of Iceland, show good sym Geological duces negative anom metry about the ridge, and the ratios of the Magnetic Doted sed magnetic field. In widths of successive anomaly bands correlate Time Scale anomalies polarity f polarity reversals for closely with the successive polarity epochs and (extrapolated) epochs elieved to become im- events. T he publication of these results by 0 0 ocea ni c crust. Figure Heirtzler et al. in 1966 was hailed as proof of the sequence of mag seafloor spreading (Vine, 1966). from the polarity re- Average rates of spreading from various The alternating direc ridges as determined from magnetic-anomaly . the strips produce the bands range from about J cm/ year to more 2 gative anomalies, sym than 6 cm/year. Thus, the time scale in Figure t the ridge crest. Each 4C shows that the distances of the oldest dated w ~ i gu re 5C, when cor rocks from ridge crests range from 40 km to 3 •riate reversal time in more than 240 km. The pattern of recognizable ~ ! taken for the oceanic linear magnetic anomali es extends much farther f- to its present position than this in many areas; therefore, if the as !re it was generated. sumption is made that spreading rates have re lies parallel with the mained constant, the spacing of the linear Earth anomalies can be used to extrapolate the polar formed ity-reversal time scale beyond the 4-m.y.-B.P. x l09 yr. B. 2·25 mill ion years limit imposed by the dating method (Fig. 4C). -- 600 MATUYAMA In this way, the time scale has been extrapolated Magnetic Field REVERSED 6 Intensity POLARITY backward to about 80 m. y., as shown in x 10 yr. Figure 6. 70 Ja Eas;ly identifiable anomalies have been num bered for reference purposes and for correlation xli2!;J- from one profile to another. Each numbered FIG. 6--Radiometric geologic time scale compared anomaly is thus assigned a provisional age, as in with geomagnetic time scale. Sec Figure 4C. Scale for Figure 6, and the distribution of anomalies in each successive column is increased by factor of l 0. the ocean basins corresponds to the distribution of irnchrons for the ocean-basin fl oor. The JOIDES D rilling Results d istribution of magnetic anomalies in many ::s NORMAL POLARITY The Glomar Challenger (Fig. 7) began its parts of the ocean is far less symmetrical and cruises in 1968. A lready, results of the JOIDES more complicated than for the Reykjanes D eep Sea Drilling Project have been hailed Ridge. It is a long extrapolation from the ac widely as confirmation of the theory of seaft oor curately dated period of 4 m.y. ago (Fig. 4C) spreading and plate tectonics. In the South to the Cretaceous (80 m.y.; Fig. 6), and such ~fnegative ::5 Atlantic Ocean, the paleontologic ages of the extrapolation involves numerous assumptions. deepest sediments above basaltic basement Unfortunately, the assumptions and the tenuous rocks agree very closely with the age of the nature of the extrapolated geomagnetic time basement according to the magnetic anomalies scale-although stated in earlier papers on this and the geomagnetic time scale back to almost topic-tend to be overlooked in some of the 80 m.y. It has not been established, however, subsequent publications and reviews. that the basalt which was penetrated is definitely Despite the provisional nature of the ages and basement rock. Deeper drilling may reveal more the large amount of magnetic mapping still to sedimentary rock below basaltic layers. :sentation of sequence of be completed, the ability to contour the ocean The need for prudence is indicated in a recent in floor generated at mid basins with isochrons provides the prospect of paper by Macdouga ll ( 1971). He dated basaltic ·e is transported laterally unraveling the history of these basins, and the f crust (with directions of basement in one of the drillholes of the western reversals of magnetic field resultant movement of the continents, with a Atlantic Ocean basin as 16 m.y. old; it lay produce alternate positive precision inconceivable during the debate about beneath fossiliferous Late Cretaceous sedimen earth's ambient magnetic contin ental drift that occupied the first half of 1el. tary beds! Hopefully, the device illustrated in the century. Figure 7 which permits removal of a drill core, 12 Peter J. Wyllie CONTINENTAL DRIFT F igure 1 shows the South Atlantic Ocean in creasing in w idth by seafloor spreading. The lithospheri c plates are about 100 km thick, and the continents range in thickness from 35 to 70 km. They move with the plates like logs frozen into an ice floe; this is the common analogy drawn. Wegener's ( 1 912, 19 I 5) origi nal concept of continents "sailing" through a sima (mafic) sea was discredited in the van der Gracht et al. ( 1928) symposium. Jn the new concept of plate tectonics, however, Wegener's mechanism need not be invoked, and thus one of the greatest obstacles to the con tinental-drift idea no longer exists. Continental F its Therefore, continental drift is simply a result Fw. 7-Sketcb of Glomar Challenger with drilling of seafloor spreading during the course of plate rig, dynamic positioning system, and reentry system on ocean fl oor. tectonic activity. If the spreading process il lustrated in Figure 1 were reversed at the same replacement of a worn bit, and reentry of the half-rate of 2 cm/ year, then 4 cm of ocean drill string into the same borehole on the ocean floor woul d disappear each year, and in Jess floor wil l lead to more detailed studies of the than 150 m.y. the South Atlantic Ocean would nature and age of the basaltic rocks described be closed and the continents of Africa and as basement. South America would be in contact. A glance at the map of the Atlantic Ocean (Figs. 2, 3) Subduction Zones makes it obvious to the drifter that the con If large volumes of lithosph ere are being tinents on either side of the Atlantic, and the generated at the oceanic ridges and the rigid M id-Atlantic Ridge, were once joined. How plates are spreading away from the ridge crests, ever, Dietz ( 1967, p. 73) presented a sketch where is all the materia l going? If one assumes showing how one does have to stretch the evi that the earth is not expanding, lithosphere dence a little to make the apparently obvious must be removed at the same rate it is gen fit work. erated. The Pacific plate moving northwest Bullard et al. ( 1965) were tired of the re from the East Pacific Rise (Figs. 2, 3) ob peated criticism by nondrifters that the con viously must disappear in subduction zones ex tinents did not fit across the Atlantic. Therefore, tending beneath the oceanic trenches that fes they put the computer to work and came up toon the northwest P acific Ocean. with a n objective fit designed to satisfy every The accepted interpretation is that the litho one; a nd even if it does not satisfy everyone, spheric plate is continuous, in a tectonic sense, it certainly seems to have satisfied the majority. and that the lithospheric slab bends at the However, geometric fits are not really good trenches and moves clown beneath the island evidence for or against drift, as Voisey (1958) arcs (or marginal moun tain range, as in Fig. 1). and Lyustikh ( 1967) showed only too well. T his interpretation presents mechanical prob Therefore, it is logical to seek an independent lems. Lliboutry ( 1969) p roposed one of sev line of evidence to determine whether con eral solutions that have been published . He ti nental d ri ft has taken place. l s there an in postulated that the continuous oceanic plates dep endent line of evidence? Fortunately, there are sheared into vertical fragments beneath the is. oceanic trenches, and that the contiguous frag ments move down into the asthenosphere in Paleomagnetism such a way that the overall p icture is one of an T he earth has a magnetic fi eld (Fig. 4A) inclined plate clipping in to the inte rior. and, if it can be assumed that the approximate Plate Tectonics, Seafloor Spreading, and Continental Drift 13 axial dipole field of today has predominated at different times, extending back to more uth Atlantic Ocean in during the whole of earth history, then one can than 500 m.y. ago (Fig. 6). :afloor spreading. The use some of the principles of paleomagnetism Reconstruction of Pangaea by 1out 100 km thick, and to see whether continents have drifted. If they Dietz and Holden thickness from 35 to have, it is possible to determine their "wander h the plates like logs ing" paths. The theory is based on the fact that Wegener had outlined the breakup and dis ; this is the common a rock becomes magnetized in the direction of persal of the original supercontinent called ·'s ( 1912, 1915) origi the magnetic field prevailing at the time the "Pangaea." In l 970, D ietz a nd Holden posi ts "sailing" through a magnetic material in the rock was formed. tioned Pa ngaea for the first time in absolute jiscredited in the van From oriented rock specimens collected in the coordinates on the earth's globe. Their guiding 8) symposium. I n the fi eld, one may then determine the ancient paleo rationale for the reconstruction was the drift : tectonics, however, latitude and the distance from the magnetic mecha nism associa ted with plate tectonics and :ed not be invoked, a nd pole of the rock when it was magnetized. If seaftoor spreading (Figs. J, 3, 5) . Using these t obstacles to the con- the magneti c axis of the past coincided with the same guidelin es, they prepared four maps il 1ger exists. rotational axis, then one has, from paleomag lustrating the breakup and dispersion of the netic measurements, a record of the locations continents during the past J 80 m.y. Absolute tal Fits in the past of the rotational axis. The whole geographic coordinates were assigned for the l drift is simply a result collection a nd measurement process is one conti nents, as well as for the active oceanic ·ing the course of plate which requires great delicacy, care, and pre rift zones and the oceanic trenches, as the con . spreading process ii cision. tinents migrated to their present positions. re reversed at the same Studies of oriented rock specime ns ra nging They a lso extrapolated present plate move , then 4 cm of ocean in age from the present to 20 m.y. old show ments to predict the appearance of the world ~ac h year, and in less that the paleomagnetic pole has not moved 50 m.y. from now. Among the anticipated 1 Atlantic Ocean would muc h during these 20 m.y. relative to the rock cha nges are northwa rd movement of Australia tinents of Africa and locations. Statistically, in fact, the pole has not into contact with the Asian plate, easterly shift : in contact. A glance at moved a t all. There is a wide spread in plotted of India, the virtual closure of the Mediter ic Ocean (Figs. 2, 3) pole positions; that is to be expected in paleo ranean Sea, and some significant changes in the e drifter that the con magnetic studies. The spreads are so wide that geography of California. They estimated that in f the Atlantic, and the elaborate sta tistical procedures must be applied about 10 m.y. Los Angeles, on the Pacific :re once joined. How- to the results in order to determine an average plate, will be abreast of San Francisco, on the 13 ) presented a sketch pole position. American plate (Fig. 3), and in about 60 m.y. have to stretch the evi Roc ks older tha n 20 m.y. show that the Los Angeles will start sliding into the Aleutian the apparently obvious paleomagnetic pole-and by inference the ro Trench south of A laska. The nice thing about tational pole-has moved relati ve to the con such predictions is that it will take millions of ) were tired of the re tinents, or vice versa. In fact, the farther back years to prove or disprove them! ndrifters that the con in time one goes, the more divergent are the the Atlantic. Therefore, pole positions. Rocks measured from a single CAUSES OF PLATE MOTIONS to work and came up continent give a pa th of polar wandering Those who still feel somewhat dissatisfied signed to satisfy every through time, showing the position of the paleo about the evide nce for lateral movements at :s not satisfy everyone, magnetic pole relative to the position of the the earth's surface might be happier if the 1e satisfied the majority. continent. causes of these movements were known- but fits are not really good If continents have held the same relative posi the causes are not known. With this fact under drift, as Voisey (1958) ti ons with respect to one another through time, stood, we can speculate. One basis for specula showed only too well. the pola r-wandering paths of a ll continents tion on the dynamics of the earth's interior is to seek an independent should coincide. They do not. The polar a study of the effects of surface movements. ~ t e rmin e whether con wandering paths for North America, Europe, Studies of effects such as earthquakes may pro . place. Is there an in Asia, Africa, India, South America, and Aus vide clues to the causes. ncc? Fortunately, there tralia diverge from each other. Therefore, on According to the theory, earthquakes result the basis of these paths, one may conclude that when plates collide at the margi ns. Most of the 1gnetism the continents h ave moved with respect to each major earthquakes around the Pacific are at tributed to the subduction of oceanic plates gnetic fi eld (Fig. 4A) other. Some reasonably consistent pictures now seem to be emerging from attempts to recon beneath island a rcs or mounta in ranges. For :d that the approximate struct the relative positions of the continents example, a disastrous earthquake in the Peru- 14 Peter J. Wyllie vian Andes in 1970 killed 50,000 people. It better than any previous hypotheses on the caused half a mountainside to move downward matter. a distance of 4 km and laterally a distance of I think that we should work the theory of 15 km. Within a few minutes, this mass of the new global tectonics for all it is worth-just earth, rocks, and melting ice, rushing at veloci as we should do with any theory or model, ties on the order of 300 km/ hour, buried the for it is through such intensive studies that we village of Yungay in the valley below, in places obtain large quantities of valuable informa to a thickness of 15 m (Browning, 1973). tion. We should reexamine the geologic data I have illustrated elsewhere (Wyllie, 197 la, within this new conceptual framework, but we p. 354) four mechanisms that have been con must not assume that we have arrived at the sidered. The first is a simple convection cell in fin al solution- because geology just is not that the asthenosphere- the conveyer-belt model simple. Wegmann ( 1963) wrote: "Commonly (Fig. 1). The second shows a passive plate the motions, concepts, and hypotheses control being underridden by a cold downgoing plate; the selection of facts recorded by the observers. gabbro is converted by polymorphic transition They are nets retaining some features as useful, to denser eclogite, and the slab sinks, pulling letting others pass as of no immediate in terest. the plate behind it. The third shows a slight The history of geology shows that a conceptual slope from the flanks of the ridges, an.ct the development in one sector is generally followed oceanic lithosphere slides toward the contments by a harvest of observations, since many geolo under the action of gravity. In the fourth model gists can only see what they are asked to record the plates are being pushed; it has been suggested by their conceptual outfit." What we have to that a sort of magmatic head is built up beneath do now is use the conceptual outfit but not the ridges, and that the resulting stress pushes be hampered by conceptual blinkers. the plates to either side. The ardent advocates of plate tectonics as a None of these models really works. Not one panacea for all problems in earth science are of the processes proposed can work to the ex wont to say something like this. They say that clusion of the others and, if a mechanism ever certainly there are minor details which we can is worked out, it probably will involve all of not yet fit into the model; however, we can attri bute these details which do not fit to a these processes. At the U niversity of Chicago, where I teach present lack of understanding rather than to and do research, mantle motions have attracted any flaw in the new global scheme. They say the attention of several experts in fluid dy to themselves that all will become clear in due namics. Experimentation and theoretical re course. Perhaps this is true. Nevertheless, we search with their famili ar convective systems surely owe it to ourselves and to the ghostly au in water or air have not been directly applica thors of theories past to examine critically all ble to material with the properties of the mantle, evidence which appears to support the hy but the experts faced the challenge. Their con pothesis, to consider carefully any evidence clusions to date are that there is no complete which does not fit the hypothesis, and to con mathematical or dynamic theory of seafl oor tinue to seek alternate explanations for all of spreading, and they see little prospect of for the evidence. mulating one until more data are available. Of the following articles in this volume, some There seems to be some doubt as to whether are strongly for the new global tectonics, some the critical data ever can be obtained. are strongly critical and present data which do not appear to fit, and others are neutral. One DISCUSSION article presents a new interpretation of the This, then, is today's ruling theory and the linear magnetic anomalies of the ocean basins; status of its model. I think it is important to I have noted that these anomalies have pro remember that many other models and many vided one of the strongest arguments in favor other hypotheses have been proposed. Yester of the new global tectonics. Hopefully, the day's model is of historic interest only-al reader will find these papers stimulating and though within it one may find the clue for the thought-provoking, and will strive to combat model of today. One must also remember, how the " ... natural coldness toward those [facts] ever, that today's model is tomorrow's history. that seem refractory" to his preferred theory Today's model of global tectonics looks far (Chamberlin, 1890) . Plate Tectonics, Seafloor Spreading, and Continental Drift 15 s hypotheses on the REFERENCES CITED geologique de l'Amerique meridionale: Paris, Jour. Barazangi, M., and J. Dorman, 1969, World seismicity Phys. Chimie, Hist. Nat. et Arts, v. 53, p. 30-60. j work the theory of maps compiled from ESSA, Coast and Geodetic Isacks, B., J . Oliver, and L. R. Sykes, 1968, Seismology Survey, epicenter data, 1961-J 967: Seismal. Soc. and the new global tectonics: Jour. Geophys. Re or all it is worth- just America Bull., v. 59, no. 1, p. 369-380. search, v. 73, no. 18, p. 5855-5900: my theory or model, --- and --- 1970, Seismicity map of the Arctic Lliboutry, L., 1969, Sea-floor spreading, continental ensive studies that we compiled from ESSA, Coast and Geodetic Survey, drift and lithosphere sinking with a n asthenosphcre of valuable informa epicenter data, January 1961 through September at melting point: Jour. Geophys. Research, v. 74, 1969: Seisma l. Soc. America Bull., v. 60, no. 5, no. 27, p . 6525-6540. ine the geologic data p. 1741-1743. Lyustikh, Ye. N., 1967, Criticism of hypotheses of .al framework, but we Browning, J. M ., 1973, Catastrophic rock slide, Mount convection and continental drift : Royal Astron. Soc . e have arrived at the Huascaran, north-central Peru, May 31, 1970: Am. Geophys. Jour., v. 14, nos. 1-4, p. 347-352. eology just is not that Assoc. 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J., 1968, Rises, trenches, great faults, ons, since many geolo- Jour. Geology, 1931, v. 31, p. 155-165; Science, and crustal blocks: Jour. G eophys. Research, v. 1965, V. 148, p. 754-759). 73, no. 6, p. 1959-1982. 1ey are asked to record Cox, A., 1968, Lengths of geomagnetic polarity in Morley, .L. W., and A. Larochelle, 1964, Paleo it." What we have to tervals: Jour. Geophys. Research, v. 73, no. 10, p. i:nagnetJsm as a means of dating geological events, ceptual outfit but not 3247-3260. zn F. F. Osborne, ed., Geochronology in Canada: ual blinkers. -- R. R. Doell, and G. B. Dalrymple, 1964, Re ~oya l Soc. Canada Spec. Pub., no. 8, p. 39-51. versals of the earth's magnetic field: Science, v. Srnder, A., 1858, La creation et ses mysteres devoiles of plate tectonics as a 144, no. 3626, p. 1537- 1543. . .. : Paris, France, A. Franck et E. Dentu, 487 p. is in earth science are Dietz, R. S., 1961, Continent and ocean basin evolution Sues.s, E., 1908, Das Antlitz der E rde, erster Band: ike this. They say that by spreading of the sea floor: Nature, v. 190, no. Vienna, A ustria, F. Tempsky, 779 p. 4779, p. 854-857. --- 1909, Das Antlitz der Erde, dritter Band zweite · details which we can --- 1967, More about continental d rift: Sea Fron H alfte: Vienna, F. Tempsky, 789 p. ' del; however, we can tiers, v. 13, no. l, p. 66-82. Taylor, F. B., 1910, Bearing of the Tertiary mountain vhich do not fit to a - -- and J. C. Holden, 1970, Reconstruction of belt on the origin of the earth's plan: Geo!. Soc. anding rather than to Pangaea: breakup and dispersion of continents, America Bull., v. 2 1, no. I, p. 179-226. ibal scheme. They say Permian to present: Jour. Geophys. Research, v. van der Gracht, W. A. J. M. van W., et al., 1928, 75, no. 26, p. 4939-4956. Theory of continental drift : Am. Assoc. Petroleum ill become clear in due Doell, R. R., and G. B. Dalrymple, 1966, Geoma'gnetic Geologists, 240 p. true. Nevertheless, we polarity epochs: a new polarity event and the age Vine, F. J., 1966, Spreading of the ocean floor: new > and to the ghostly au of the Brunhes-Matuyama boundary: Science, v. evidence: Science, v. 154, no. 3755, p. 1405-1415. 152, no. 3725, p. 1060-1061. > examine critically all --- and D. I-I. Matthews, 1963, Magnetic anomalies Drake, C. L., l 970, The geological revolution: Eugene, over ocean ridges: Nature, v. 199 no. 4897 p. 947- s to support the hy Oregon, Oregon State System of Higher Education, 949. ' ' arefully any evidence 55 p. Voisey, A. H., 1958, Some comments on the hypothesis iypothesis, and to con Hedberg, H. D., 1970, Continental margins from view of continental drift, in S. W. Carey convener Con point of the petroleum geologist: Am. Assoc. Petro tinental drift, a symposium: Hobart, 'u niv. Tas:nania explanations for all of leum Geologists Bull., v. 54, no. 1, p. 3-43. p. 162-17 1. ' Heirtzler, J. 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South Africa, anriex. to v: 66, e anomalies have pro servations," 38 p. p. 1-78. :est arguments in favor --- 1962, H istory of ocean basins i11 Petrographic Wilson, J . T., 1965, A new class of fa ults and their tonics. Hopefully, the studies-a volume to honor A. F. Buddington: Geo!. bearing on continental drift : Nature, v. 207, no. 4995, papers stimulating and Soc. America, p. 599-620. p. 343-347. Holmes, A., 1928, R adioactivity and continental drift : Wyllie, P. J ., 197 la, The dynamic earth: textbook in will strive to combat Geo!. Mag., v. 65. geosciences: New York, John Wiley, 41 6 p. !SS toward those [facts] - -- 193 1, R adioactivity and earth movements: --- 1971b, Revolution in the earth sciences m o his preferred theory Geol. Soc. Glasgow T rans., v. 18, pt. 3, p. 559-606. The great ideas today 1971: Encyclopaedia Brltan H umboldt. A. von, 1801 , Esquisse d'un tableau nica, p. 168-237. Memoir 23 -r "'/ Plate Tectonics- fb~) /If IV/~(--~ -· --, Assessments and Reassessments IAJ. __~, Edited bJ' CHARLES F. KAHLE :.--.\ t- -· 'L Ll ty Fund Published by The American Association of Petroleum Geologists Tulsa, Oklahoma, U.S.A., 1974