History of Plate Tectonics
Plate tectonics is without doubt the single most critical unifying theory in modern geology. Virtually every aspect of the Earth’s crust, every type of rock, and every kind of geology can be related to the plate tectonic conditions existing at the time they formed. Although a number of gainsayers have yet to be totally convinced, especially in terms of the potentially challenging intricate details that “Big Picture Theorists” love to gloss over, the plate tectonics tide has definitely carried most geoscientists with it. Author’s Note: It is no exaggeration to state that today little in geology makes sense except in terms of plate tectonic theory — with the important caveat that certain geological details relating to specific locations and conditions have yet to be resolved to the satisfaction of all involved researchers.
Plate tectonics is a theory of global dynamics in which the lithosphere is believed to be composed of individual plates that move in response to convection currents in the upper mantle. But before we get into additional definition, some preliminary background is required, especially in terms of Continental Drift Theory. After that background is explored, we can delve into the mechanisms of sea-floor spreading/plate tectonics.
Continental Drift Theory: Toward the end of the 16th Century, the Flemish cartographer and geographer Abraham Ortelius (justly credited as one of the founders of modern cartography), who was appointed Royal Geographer to Philip II of Spain and therefore was responsible for compiling and drawing maps that had great geopolitical and economic significance, proposed that the Americas had previously been jointed to Europe and Africa and had been separated from them by a series of catastrophic events. That observation came after Ortelius had long reflected on the maps he had been making for several decades. In the 17th Century the English natural philosopher Francis Bacon and several early cartographers and explorers had noted in various publications the apparent congruence of the coastlines of the African and South American continents. And in the early 1800s, the Scottish theologian and philosopher Thomas Dick and the naturalist-geographer-explorer Alexander Von Humboldt observed that western Africa and eastern South America as well as northeastern North America and westernmost Europe fit nicely together to form a single continent. But no one, scientist or layperson, expressed the revolutionary idea that those continents had been split apart millions of years ago and were continuing to move further apart.
The first scientist to put this radical idea in print was the Italian-American geographer-cartographer Antonio Snider-Pellegrini. In 1858, he proposed that the European and North American continents had been linked during the Pennsylvanian period, about 320 million to 299 mya. He further suggested that all continents had merged together to form a single land mass that had later had been separated by Noah’s Flood, or by some other deluge of global proportion. Snider-Pellegrini’s claim was based not on the shape of the continental coastlines but on the simple and undeniable fact that plant fossils found in Europe and North America from the Pennsylvanian were similar.
Things really jump-started with respect to actual scientific evidence in the mid-1870s, when the Mid-Atlantic Ridge, an undersea mountain range on the Atlantic sea-floor, was first discovered by British explorers. Rising from broad flat plains on either side, the range had peaks that jutted up 10,000 feet from the ocean floor. At that time, geologists had few rational explanations for such a major undersea formation and basically regarded it as a curious but relatively unimportant phenomenon that had no rational explanation.
More scientific hints concerning specific Earth formation mechanisms came in 1880 from Osmond Fisher (1817-1914), a British geologist who not only supported continental drift but also was the first to identify the force that pulled them apart. He believed that the Earth had a liquid core with convection currents that transferred heat-energy from the interior to the crust. Although that idea was far ahead of its time, its numerous wide-ranging implications were completely ignored by geoscientists of the day.
However, the concept of mobile continents, as opposed to solid evidence, was received with considerable sympathy by Europe geologists, who were generally known as mobilists, (in contrast to the stable Earth advocates who were absolutely convinced that everything at the surface of the Earth was fixed in place and moved not one whit in terms of geographic location) and largely conducted their research in quiet isolation amidst the strongly folded, faulted, and overthrust Alps. In 1885, the Austrian geologist Eduard Suess recognized similarities between plant fossils from South America, India, Australia, Africa, and Antarctica and coined the name, Gondwanaland, for the proposed ancient super-continent containing those land masses. However, in 1904 Suess shifted gears 180 degrees and suggested that the Earth’s crust was the result of a gradually cooling planet, with the mountain ranges and ocean basins forming as the crust cooled and shrank. He proposed that, as the shrinking process developed, a very large continental area, Gondwanaland, had been uplifted. He went on to suggest that subsequently sections of that massive continent collapsed, forming ocean basins. Suess’s theory became dominant among professional geologists in the early 20th Century, especially in the U.S., but was greeted with little sympathy and much less agreement by many if not the greater majority of Swiss geologists, whose skepticism was derived from decades of studying the Alps’ daunting complexities and the ever puzzling evidence of sea-floor sediment uplift presented in those torturously folded and thrust-faulted mountains.
In 1910, American geologist Frank B. Taylor (1860-1939) attempted to explain the formation of mountain belts by applying the concept of continental drift. In 1910, and later in 1928, Taylor published two rather speculative papers suggesting that continental drift would account for what was then called orogenic diastrophism (later his ideas were supported by several American scientists, notably geologist H. H. Baker and petrologist Reginald A. Daly, who wrote the book, Our Mobile Earth, Boston: Charles Scribner’s Sons, 1926). Taylor specifically argued that the Alpine and Himalayan Mountains had been produced the movement of continents away from the poles. Neither those papers nor Baker’s or Daly’s support attracted any attention whatsoever from the geological community. Collectively, their ideas suffered the same fate of being assigned to the trash can of failed theories as had earlier and similar suggestions by Humboldt and Fisher. Taylor’s biggest problem was that his papers provided little specific or even conjectural evidence of the nature of the geophysical motors and mechanisms that would be powerful enough to drive crustal movements. Consequently, his idea proved merely a flash in the pan and quickly faded from serious scientific scrutiny.
Alfred Wegener was one of the first scientists to realize that an understanding of how the Earth works required input and knowledge from many of the geosciences rather than one. Although the majority of Wegener’s scientific research was in meteorology, he is best known today for his theory of continental drift, which he set forth in The Origin of Continents and Oceans, written in 1914 and published originally in 1915 (but not in English until 1924), while he was recovering from a wound suffered as a soldier during WWI. According to Wegener, the present continents originally formed one large landmass called Pangaea. Over millions of years, Pangaea had been subjected to a variety of forces that resulted in it breaking into pieces that separated and drifted apart. His evidence included the matching of certain continental coastlines, including South America and West Africa, as well as stratigraphic and paleontological similarities on either side of the Atlantic Ocean. In particular, the Appalachian mountains of eastern North America matched with the Scottish Highlands and British Isles. And the distinctive and climate sensitive rock strata of the Karoo system of South Africa were identical to those of the Santa Catarina system in Brazil. He also seized on the fact that most paleontologists believed that a land bridge once connected Africa and South America because of the presence of identical fossils on both continents, especially mesosaurus and glossopteris.
In 1927, Wegener’s ideas inspired Alexander L. Du Toit (1878-1948), a respected South African geologist, to write, A Geological Comparison of South America and South Africa. Du Toit was intrigued with the idea that the two continents had been joined in the geologic past. Ten years later, in Our Wandering Continents, he maintained that the southern continents had, in earlier times, formed the supercontinent of Gondwanaland, which was distinct from the northern supercontinent of Laurasia. Well, part of the problem was Du Toit’s highly original writing style, which many at that time compared to that of a political pamphleteer rather than a serious scientist.
Scientific reaction to Wegener’s theory was almost uniformly negative and often became exceptionally harsh and scathing, partly owing to the fact that he was not trained as a geologist. I mean, how dare he even think about things geological? Rollin T. Chamberlin, a well-known and highly influential geologist at the University of Chicago wrote an article in 1928 for a symposium sponsored by the highly regarded American Association of Petroleum Geologists that slammed Wegener in no uncertain terms:
Wegener’s hypothesis in general is of the footloose type [Author’s Note: meaning that it had no roots and therefore no substance], in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories. Its appeal seems to lie in the fact that it plays a game in which there are few restrictive rules and no sharply drawn code of conduct.
Part of the problem was that Wegener proposed no convincing forces that would be sufficiently powerful to account for movement of continental masses. Wegener theorized that the continents moved through the Earth’s crust much like icebreakers plowed through ice sheets and that the Earth’s centrifugal and tidal forces were responsible. His opponents, especially the highly regarded British geophysicist and mathematician Harold Jeffreys, correctly noted that plowing through oceanic crust would distort continents beyond recognition and that centrifugal and tidal forces were far too weak to be the motive force. Jeffreys went so far as to say that Wegener’s theory was “a very dangerous one, and liable to lead to serious error,” which was daunting criticism from a world-renowned scientist. Another critical problem was that flaws in Wegener’s original data resulted in incorrect and unreliable calculations. He suggested that North America and Europe were moving apart at over 250 centimeters per year (about ten times the fastest rates seen today and about a hundred times faster than the measured rate for North America and Europe), which quickly was transformed into a club with which his opponents attacked him.
Although Switzerland’s Alpine geologists never concerned themselves with the motors and mechanisms that would drive crustal movements, their work focused on reading the geological record correctly. That record, including the nature of major orogenic belts and especially the highly complex structure of the folded-faulted Alps, brought them to accept the concept of wandering continents because it explained better than any other theory the origin of the mountains they knew so well. By the early 1920s, after a number of what can only be called brilliant regional tectonic and stratigraphic analyses of the magnificent nappes exposed in the central and western Alps, the Swiss geology schools in Zurich, Berne, Lausanne, and Neuchatel had come to regard Suess’s (and others) stable continent theory as utterly passé and scientifically indefensible with respect to explaining the tremendous horizontal forces that had pushed the Alps ever higher and higher.
Rudolf Staub, working from the late-1910s through the 1930s, was certainly the leading Swiss Alpine geologist whose research established the theoretical underpinnings of the concept of mobile continents. His work emphasized combining the Earth’s rotational effects with those created by convection that originated within the Earth itself. According to the noted geophysicist Edward Irving, “The principal aspects and main attraction of Staub’s working hypothesis of continental movements are periodicity, very large-scale horizontal movements, and an over all approximate parallelism of the resulting successive orogenetic belts.” In much the same vein, in 1924 Emile Argand argued that the Alpine-Himalayan Mountains were uplifted in the Cenozoic by collisions between northern and southern continental land masses. He also speculated that the Paleozoic Appalachian Mountains had been ocean basin sediments that had been forced up by continental drift and collision.
Author’s Note: Sad to say, among the main reason Staub’s and Argand’s papers attracted so little attention outside of western Europe were that they were written in German and published in European journals that were not the most commonly read by the broader scientific community. Despite the clarity and strength of Staub’s and Argand’s arguments, their radical concepts generated considerable skepticism and very little discussion beyond colleagues and students at their respective universities. Not for the first time in the scientific community, language, nationality, chauvinism, and the choice of geographic-geologic publishing venues turned out to be very influential factors with respect to the dissemination and discussion of any number of hypotheses or theories in geoscience in general, but Staub’s and Argand’s specifically.
Also in 1924, Wegener and his father-in-law, the well-known and highly respected bio-geographer and climatologist Wladimir Köppen, published research that proposed the first paleogeographic synthesis that demonstrated a mobile Earth. They identified climate-sensitive deposits and used their distribution to plot global paleo-latitudes in support of Continental Drift Theory. Sadly, it was all to no avail as the geoscience community pointedly ignored and even denigrated their efforts.
From about the late 1920s through the mid-1930s, Suess’s concept of a shrinking Earth began losing scientific favor owing to growing recognition that radioactive decay was likely the major source of the Earth’s internal heat (see the Historical Background section in my radiometric dating blog on 2-19-12 for additional information). It was only a short step from that observation to the idea that the Earth was not cooling as fast as had been thought only a few years earlier. But in 1928 the U.S. geologic community published a major compendium that constituted a scathing attack on the idea of mobile continents and on that idea’s most visible proponent, Alfred Wegener. With that poisoned atmosphere as a background, in 1929 the highly-respected British geologist Arthur Holmes suggested that radioactive decay as an internal heat source might be sufficient to produce convection currents in the Earth’s mantle, supporting Fisher’s earlier proposal. His idea was based on the fact that as a substance was heated its density decreased and the hot material rose to the surface where it cooled, became denser and sank, only to rise again as it absorbed heat. That repeated heating and cooling cycle would result in a current that Holmes thought would have sufficient power to effect continental movement. Holmes suggested that thermal convection worked like a conveyor belt and that the upwelling pressure could break up continents and convection currents would then carry the broken pieces in opposite directions and eventually downward to be heated again and rise. At the time, the scientific community, especially in America, deep into their derisive and almost personally vindictive rejection of all things Wegenerian, paid very little attention to Holmes’s work, other than to heap professional scorn and ridicule on it.
In 1935 the pioneering Japanese seismologist Kiyoo Wadati published research that proposed that deep earthquakes were located on planes dipping beneath the ocean floor and were concentrated in areas around the edges of oceans close to volcanoes on land. But his ideas were largely forgotten until they were rediscovered and re-invigorated in the mid- to late-1940s by the American seismologist Hugo Benioff. Again, no general theory or context would explain such phenomena so geophysicists regarded them as baffling puzzles that they hoped would be sorted out at a later date. In 1937, the South African geologist, Alexander du Toit published Our Wandering Continents, in which he eliminated many of Wegener’s weaker points and also presented abundant new evidence that supported continental drift. Alas, his efforts fell on intentionally deaf ears.
As early as the mid- to late-1930s, Harry Hess at Princeton became involved with Dutch geophysicist and geodesist, Felix A. Vening-Meinesz (1887-1966), who had invented a novel gravimeter that was able to function at sea since it was resistant to external disturbance. With geophysicists Maurice Ewing and Edward Bullard, Vening-Meinesz and Hess began measuring gravity anomalies in the Caribbean and the Gulf of Mexico. Their measurements demonstrated an association between negative gravity anomalies (regions characterized by lower than normal gravity) and regions where the ocean was particularly deep (areas now known as trenches). Familiar with European arguments over continental drift, Vening-Meinesz proposed that convection currents might be dragging the lighter crust downward into the denser mantle below, explaining both the ocean trenches and their associated negative gravity anomalies. Hess, on the other hand, believed that the crust had buckled vertically as expressed on the surface as ocean trenches and in gravity measurements as negative anomalies. Borrowing a term from German geologist Erich Haarmann, he called these downwarpings in the crust, tectogenes. He thought those phenomena were downfolded portions of an orogenic belt caused by horizontal compression that had resulted from the convergence of sub-crustal convection currents. In their discussions, both Hess and his mentor Vening-Meinesz agreed that the gravity readings were signs of crustal disturbance or deformation, indicating that apparently the ocean basins were not static (as was commonly accepted in the scientific community at that time) but were subject to active deformation and movement, at least in certain zones. However, World War II soon roared onto the global scene and shoved geophysical research to the back burners.
Sea-Floor Spreading/Plate Tectonics: During World War II, the Princeton geologist Hess, then commander of the attack transport USS Cape Johnson, used the ship’s echo-sounding instruments to take thousands of miles of depth soundings to produce rough contour maps showing sea-floor topography. In the course of his war service Hess discovered and mapped over a hundred flat-topped underwater mountains. After the war he theorized that those mountains had originated as volcanoes that had been subsequently eroded to their flat-topped state when they extended above the ocean. However, that explanation failed to account for their locations many hundreds of feet underwater. It was a puzzle that occupied Hess’s mind for many years. In 1945 Arthur Holmes published a remarkable textbook, Principles of Physical Geology, that covered all aspects of geology at an introductory level. It also included an explicit statement of his cutting edge ideas regarding convection cells in the Earth’s mantle and continental drift. Holmes’s convictions not withstanding, the controversy over Alfred Wegener’s Continental Drift Theory was effectively over by that time, having suffered a painful death by professional ridicule in the U.S. and by neglect and abandonment elsewhere in the world.
In the early 1950s research scientists from Columbia University’s Lamont Geological Observatory (now known as the Lamont-Doherty Earth Observatory and part of the Earth Institute at Columbia) collected numerous sonar readings taken across the Atlantic Ocean and made the following critical discoveries. The Mid-Atlantic Ridge extended about 9,000 miles. Its crest was virtually bare of sediment. A deep rift valley, ranging from eight to 30 miles wide, ran down its spine. And bottom samples taken from the rift revealed its sea-floor consisted of extremely young, dark volcanic rock. With the discovery of the Great Global Rift in 1953, a volcanic valley running along the mid-ocean ridges, Hess re-examined the geophysical data he and others had collected from the Caribbean prior to WWII and from the ocean floors during the War and tried to assemble the pieces of what appeared to be confusing and unrelated data in a way that made scientific sense.
Another powerful source of new data came in the early and mid-1950s, from the inter-related efforts of University of Manchester (and later the Imperial College) physicist and Nobel Laureate Patrick Blackett and Cambridge University geophysicists Ken Creer, Jan Hospers, and Edward Irving, who under the direction of and in collaboration with Keith Runcorn pioneered the field of quantitative paleogeography by mapping paleolatitudes based on natural remanent magnetization of rocks and comparing those data with paleoclimatic evidence. Hospers became the first geophysicist to present a scientific case for magnetic field reversals and to propose the Geocentric Axial Dipole Hypothesis.
A few years later Ken Creer discovered apparent polar wander. Irving and Runcorn demonstrated that paleolatitude variations for northern Europe calculated from its apparent polar wander path were indeed consistent with the paleoclimatic data. But paleolatitudes calculated for other continents from that same path were inconsistent with the paleoclimatic data for those continents. That inconsistency could only be resolved if the continents were not fixed but had drifted about the Earth’s surface over millions of years. Working with the famous British statistician R. A. Fisher (the mathematician who developed the concepts of randomization and analysis of variance) they came up with the deceptively simple but powerful concept that in order to compare ancient geomagnetic field directions from different locations on the globe, those directions were represented by corresponding paleo-poles. That concept remains the scientific basis for the unambiguous analysis of ancient geomagnetic fields. Those geophysical observations grew almost entirely from the availability of new technologies developed by the military during WWII and during the Cold War between NATO and the Soviet Union and Eastern Block countries. Major technological advances in sonar, bathymetry, magnetometers, and seismic surveying and monitoring methods all permitted enormous amounts of new data to be gathered on ocean basin geology that previously had been virtually unknown. The scientific objections to a mobile Earth slowly wobbled and began to collapse under the weight of hard geophysical evidence that continued to pile higher and higher.
A map of the North Atlantic Ocean showing features of the mid-ocean ridge system was published in 1959 by Bruce Heezen, Marie Tharp, and Maurice Ewing at the Lamont Geological Observatory. Sonar readings made elsewhere had produced similar profiles of the sea-floor throughout the globe, demonstrating that the entire mid-ocean ridge system was about 37,200 miles long. Geoscientists at Lamont and other research centers had also mapped geomagnetic anomalies parallel to the mid-oceanic ridges and a system of deep trenches that nearly ringed the Pacific Ocean. But the then existing scientific data were insufficiently developed for the dots to be connected.
In 1961, Arthur Raff and Ronald Mason of Scripps Institution of Oceanography continued research started in the mid- to late-1930s by the Dutch geophysicist/geodesist, Vening-Meinesz, Princeton’s Harry Hess, and University of California geophysicist David Griggs. Their research noted magnetic anomalies in the pattern of stripes on the ocean floor off the coast of Washington. Beginning with the geophysical research he had done in the Caribbean in the late-1930s, during WWII while as a Navy officer, taken up again in the 1950s and extending into 1962, Hess took the first giant step by proposing that the Earth’s crust was composed of iron-poor rock that had risen to the surface after radioactive decay heated and melted rocks/minerals in the interior of the newly condensed planet. He theorized that once the planet had formed, a convection system of rising and sinking molten material was created by continued heating in the planet’s interior and cooling near the surface. That mantle convection system was subdivided into numerous separate and independent circulating loops extending surface-ward from the core. Where the currents rose to the surface, molten material was extruded, simultaneously building up the mid-ocean ridges and forming new oceanic sea-floor and crust. As the magma continued to flow upward and outward as if carried on a giant conveyor belt, older sea-floor was pushed away in both directions from the ridge by the circulating convection currents. When the currents cooled and older oceanic crust gradually became denser, that crust was plunged back into the mantle at deep ocean trenches. Hess thus reorganized the relationship of the oceans and continents with respect to the motions of a continually spreading, moving sea-floor. Although his theory was compelling and enormously exciting, it seemed untestable, since the movements he predicted occurred at about the rate at which fingernails grow.
However, in 1961 the first large-scale, systematic measurements of the Earth’s electromagnetic field over an ocean basin were published by Mason and Raff of Scripps Institution of Oceanography. They had discovered that the ocean floor had a pattern of alternating strong and weak magnetic fields, aligned in belts parallel to the mid-ocean ridge off the west coast of the U.S. But their explanation for the magnetic anomaly pattern was that the stronger total magnetic fields corresponded with topographic highs and the weaker with topographic lows. In other words, they interpreted sea-floor magnetic patterns as topographic rather than geophysical effects. Also in 1961, a hotbed year in breakthrough research, the marine geologist Robert S. Dietz published an article in Nature that firmly established the foundations of the modern theory of plate tectonics. A similar research paper had been circulated in draft form by Harry Hess in 1960 and was published in 1962. Both Hess and Dietz suggested that the seemingly solid Earth was far less stable than had been believed by nearly all geoscientists. Rather, they described a process of thermal convection that caused the shifting of crustal plates that had been proposed much earlier by Arthur Holmes. Both Hess and Dietz recognized the mid-ocean ridge system as the surface expression of the upwelling limbs of enormous convection cells. Their pioneering concepts pulled together many seemingly unrelated pieces of the tectonic puzzle, including several that had been recognized only a few years before. For the first time, the presence of Wadati-Benioff Zones around the margins of the ocean basins was correctly interpreted as marking the locations of the subducting portions of mantle convection cells, where old and cooled oceanic crust was being dragged under the lighter continental plate, only millions of years later to be returned to the mantle along major fault zones. Dietz named the process sea-floor spreading and the term immediately was adopted by the geoscience community.
In 1962, geophysicist Drummond Matthews collected numerous samples of igneous rock, analyzed their remnant magnetization, and mapped a distinctive pattern of magnetic stripes, consisting of parallel bands of stronger and weaker electromagnetic signals on either side of a mid-ocean ridge crest (Author’s Note: when rock crystallizes in the Earth’s crust from a molten state in an externally imposed electromagnetic field, the tiny magnetic domains within magnetite crystals exhibit a tendency to align parallel to the external field and that field can be measured with respect to its geographic orientation and polarity). And starting in 1963, Allan Cox, Richard Doell, and Brent Dalrymple of the U.S. Geological Survey and Ian McDougall at Australian National University began measuring magnetic directions expressed in lava flows on land and determined their ages by radioactive methods. It was a painstaking process but by 1966 they had charted the magnetic reversal timescale for the past 3.5 million years.
Although Lawrence Morley, a Canadian geophysicist, saw those data and submitted a paper to Nature in 1963 arguing that the pattern of geomagnetic anomalies could be better explained as a pattern of parallel bands of normal and reverse magnetization of the sea-floor lava deposits, his work was rejected as far-fetched and scientifically untenable. Author’s Rant: Shades of the treatment that was accorded Alfred Wegener, only this time Morley was a geophysicist and supposedly a member of the club. That only illustrates the power of entrenched attitudes and what the philosopher Thomas S. Kuhn, in his seminal work, The Structure of Scientific Revolutions, called received beliefs that exerted an almost strangle hold on the collective scientific mind. IT was as if most geo-scientists were unable to think outside the tidy box they had created and into which they threw all their hopes and dreams for academic fame.
In 1963, the marine geologist and geophysicist Fred Vine (who at the time was a young English graduate student) of Cambridge University and Drummond Matthews (Vine’s graduate supervisor) hypothesized that the sea-floor had recorded Earth’s geomagnetic field orientation at the time the new molten rock extruded from the mantle. If spreading of the ocean floor occurred as Dietz and Hess suggested, those blocks of alternately normal and reversed magnetized materials would be carried away parallel to either side of the ridge and thus should form a symmetrical system. More than any previous research their paper paved the way to acceptance of the emerging theory of plate tectonics. The greatness of their contribution to geoscience was that it connected two concepts that had previously been thought to be totally unrelated: geomagnetic reversals and Hess’s sea-floor spreading ideas. Boom! All of a sudden the pieces were there for all with eyes (and functioning brains) to see.
But once again the geophysical community at large failed to support their work, partly because the geomagnetic reversal data that had been analyzed by Vine and Matthews were incomplete. Consequently, when their hypothesis was initially published, their data poorly matched the sea-floor anomaly data. The pattern of sea-floor magnetization was first predicted by Robert Dietz as a corollary of the sea-floor spreading hypothesis but Vine and Matthews were the first to take the suggestion and demonstrate in convincing fashion that it was true.
Further evidence for a surprisingly young age for the oceanic crust was offered by Canadian geophysicist J. Tuzo Wilson at the University of Toronto in 1963 when he published an article showing that the ages of ocean islands were only a small fraction of the average age of the continental crust. He demonstrated also that the ages of ocean islands increased with distance from mid-ocean ridges, suggesting that the islands had been formed along with the oceanic crust at the ridge crests and had moved to successively greater distances over time.
What permanently changed the view of the scientific community was work by Columbia-Lamont’s Jack Oliver, Bryan Isacks, and Lynn Sykes. Beginning in 1964, they collected seismic data in an effort to identify the subterranean foci of earthquakes in a trench near the South Pacific island of Tonga. They observed, as had Wadati and Benioff, that the foci outlined a plane tilting down from the ocean floor at an angle of about 45 degrees. But the Lamont team was the first to recognize that this plane was a slab of descending material that had cooled and was hard enough to sustain earthquakes and, moreover, that the slab, which contained the sea-floor itself, virtually created the earthquake zone as it was pulled down into the trench. They determined that the descending slab of sea-floor was about 60 miles thick. What was moving was not merely the surface of the sea-floor, or the crust alone, but a much thicker block: the oceanic plate itself.
In October 1965, Fred Vine and J. Tuzo Wilson published a paper proposing a model for sea-floor spreading in the northeastern Pacific, using as evidence bands of reversed geomagnetism that marched out from either side of a ridge. Shortly thereafter, a slight discrepancy between the sea-floor geomagnetic reversal bands and the timing of known field reversals on land was corrected by a new land-based field reversal discovered (in 1963 and 1966) by Allan Cox, Richard R. Doell and G. Brent Dalrymple of the U.S. Geological Survey. With that new information, the two data sets matched astonishingly well.
The confirmation of sea-floor spreading was supported by other research projects in 1965 and 1966. Critical to those observations were ocean sediment samples analyzed by Columbia-Lamont’s Neil Opdyke. The samples were from vertical cores taken from the ocean floor in the South Pacific. The timing and pattern of magnetic reversals in Opdyke’s core samples matched those determined from lava flows on land and from sea-floor magnetic stripes.
Scientists then held the key to a whole new way of understanding Earth dynamics. Tuzo Wilson, in a 1965 effort to explain sea-floor fault lines, was the first to tackle the far-reaching implications of sea-floor spreading. Around the globe, researchers had noted a series of transverse (strike-slip) faults, fractures perpendicular to the mid-ocean spreading ridges that cross whole oceans and break the ridges up into offset segments. When Wilson took up the question, the favored interpretation was that the faults were evidence of the tearing of the ocean crust from edge to edge. The ridges were assumed to have started out as continuous features that were later fragmented and offset by the faults. Wilson strongly disagreed. Yes, the transform faults were evidence of crustal tearing at points of accumulated stress, but only between the spreading ridge segments, segments that had always been offset. Unlike ridges and trenches, Wilson believed that the crust was only being offset horizontally by the transform faults, without creating or destroying crust.
That new view suggested that active deformation was concentrated at the ridges and along their connecting faults and that the rest of the ocean crust simply drifted along, unbroken. Wilson was the first to give the name “plate” to those large masses of moving rock. He further proposed that Earth’s surface was divided into about seven large crustal plates and several smaller ones. The plates were growing at sea-floor spreading centers, sliding past each other along transform faults, and were subducted into the mantle along oceanic trenches or piled up in places like the Himalayas where continents collide. The lighter continental materials were carried along, embedded within the plates, but were not returned to the mantle along with the oceanic crust surrounding them because of their buoyancy.
All told, those marvelous ideas added up to modern plate tectonic theory. At that time scientists held the key to a whole new way of understanding Earth dynamics.
Wilson’s ideas about oceanic faults and plates could be easily tested by the earthquake location data set gathered by the Lamont scientists who had been working in the Tonga Trench. Lynn Sykes at Lamont immediately tested Wilson’s theories and discovered that they passed with flying colors. Sykes determined that oceanic earthquakes were concentrated along the mid-ocean ridges and their connecting faults and that the interiors of the oceanic “plates” were nearly aseismic, or earthquake-free, exactly as Wilson had predicted. The result was unequivocal: Lamont’s field data confirmed Wilson’s theory. In a speech before a convention of his colleagues in 1967, Tuzo Wilson declared that sea-floor spreading and plate tectonics “could be as important to geology as Harvey’s discovery of the circulation of the blood was to physiology or evolution to biology.” Also in 1967 Dan McKenzie and Robert Parker of Scripps Institution of Oceanography published the first paper to define the quantitative principles of plate tectonics.
As the 1960s drew to a close, Xavier Le Pichon at Lamont, Dan McKenzie at Scripps, and W. Jason Morgan at Princeton University, all working independently, put the icing on the cake by defining the shapes of the contiguous plates and demonstrated that their movement and location on the globe could be described by elementary spherical geometry, not only for the present but for the past and future. McKenzie and Morgan later worked together to identify the stability of triple junctions, the point where three plate boundaries meet.
Finally, the 1968 paper, “Seismology and the new Global Tectonics,” published in Volume 73 of the Journal of Geophysical Research by Lynn Sykes, Bryan Isacks, and Jack E. Oliver of Columbia-Lamont was one of the classics of plate tectonics that demonstrated that earthquake foci became progressively deeper from a trench to beneath an island arc, proving the geometry of a subduction zone, where rigid slabs of oceanic crust are pulled into the mantle, creating earthquake zones. Since that time, sea-floor spreading and plate tectonics have become primary tools in the ongoing explanation of the dynamic Earth.
By the late 1960s, plate tectonics and the revised and rejuvenated Continental Drift Theory had become well received by most geologists and geophysicists. Scientists accepted that Wegener’s theory was correct in general form but wrong in detail. Continents do not and could not plow through the ocean floor. Instead, both continents and ocean floors form solid plates that float on the underlying rock, known as the asthenosphere, that experiences such tremendous heat and pressure it behaves as an extremely viscous plastic or fluid. Wegener also failed to understand that both continents and oceanic crust move together in a coordinated dance. But, in so many other ways, his ideas were right on track.
Since the mid-1930s, scientists have mapped and explored the great system of oceanic crust, mid-oceanic ridges where molten rock rises from below the crust and hardens into new crust, and trenches (the sites of frequent earthquakes). That research conclusively demonstrated that the farther you travel from a ridge, the older the crust is and the older the sediments on top of the crust are. The clear implication is that the plates are moving apart at the ridges. Where plates collide, mountain ranges may be pushed up or if one plate sinks below another, oceanic trenches and chains of volcanoes may form. Earthquakes are by far most common along plate boundaries and rift zones. Plotting the location of earthquakes allows seismologists to map plate boundaries and depths. Paleomagnetic data allow scientists to map past plate movements much more precisely than before. It is even possible, using satellite technology, to accurately measure the speed and direction of continental plate movement.
In conclusion, Wegener’s basic insights remain sound and the points of evidence that he used to support his theory are still actively being examined. It was on the critical issue of driving force that he badly missed the target.
The principal elements of continental drift, sea-floor spreading, and plate tectonics are summarized below.
- The Earth’s surface consists of numerous crustal plates, continental and oceanic.
- The ocean floors are in constant motion, spreading outward from mid-ocean ridges and sinking (subducting) at the edges; over a period of significant geologic time that material is regenerated at depth and rises again at the mid-ocean ridges.
- Convection currents within the mantle move crustal plates in different directions and at different speeds.
- Radioactivity deep in the Earth’s mantle forms the energy source generating the convection currents and the various plate movements.
For additional related information, see Walter Sullivan, Continents in Motion, 2nd ed., New York: McGraw-Hill, 1991. Ursula Marvin, Continental Drift: The Evolution of a Concept, Washington, D.C.: Smithsonian Institution Press, 1973. William Glen, The Road to Jaramillo: Critical Years of the Revolution in Geoscience. Stanford, CA: Stanford University Press, 1982. Naomi Oreskes, The Rejection of Continental Drift: Theory and Method in American Geoscience. Oxford: Oxford University Press, 1999. Although they are not the most up-to-date sources, they are well worth the time.
Driving Mechanisms of Plate Tectonics
Today, geophysicists are somewhat divided as to which of two theories more correctly describe the mechanism that drives plate movements. The oldest concept, Mantle Convection Theory, is discussed first and then the more recent Slab-Pull Theory. The problem that geophysicists are anxious to solve is the differentiation of causal forces from products of those forces.
Mantle Convection Theory: Process theoretically driving the movement of lithospheric plates over the Earth’s surface that was proposed in the 1930s by the famous British geologist, Arthur Holmes, to account for continental drift. Although mantle studies are in their infancy, at this time many geophysicists believe that plate movement results from varying aspects of mantle convection and ridge-push (for an opposing point of view, see Slab-Pull Theory below). The heat deep within the Earth is a result of two sources: the magma remnants of the Great Bombardment of the early Earth and radiation from radioactive elements. In accordance with the 2nd law of thermodynamics, heat must flow from a warmer place to a cooler place. Subsequently, hot currents rise toward the Earth’s surface at constructive margins as cooler currents carrying dense slabs descend in subduction zones. Since the Earth is relatively large, the heat flows by convection rather than simply by conduction. The resulting convection currents around the Earth’s surface are thought to be the driving force behind plate movement. In addition to the convective forces, some geologists argue that the intrusion of magma into spreading ridges provides an additional force (known as ridge push) to propel and maintain plate movement.
As a result, subduction processes are considered to be secondary forces that are logical but largely passive consequences of sea-floor spreading. The time scale involved for enormous masses of hot materials to rise from the lower mantle to the surface (following the single layer whole mantle convection model), become cooled, and return to the interior is estimated to be around 200 million years. Various models have been proposed to account for convection cells. 1) The mantle is generally considered to convect as a single layer, known as whole mantle convection. 2) There may be, at most, two layers, which is the standard geochemical model. 3) However, it may be more likely that the mantle convects in multiple layers as a result of gravitational sorting during accretion-compaction and the density difference between the mantle products of differentiation.
Real World Problems: The thermal convection that occurs in the mantle is critically different from the typical cartoon metaphors provided in almost all geoscience textbooks, like the usual pot of boiling water on the stove or even somewhat more sophisticated diagrams purporting to show mantle dynamics and geochemical reservoirs with colored motion arrows. What nearly all of the simple illustrations leave out are the enormous complexities characteristic of the mantle, like pressure, secular cooling (decreases in cooling over time), the non-uniform distribution of radioactive elements within the Earth, variances in viscosity (with depth and temperature), and solid-solid phase changes (some endothermic and some exothermic) that occur at various depths. To quote the well-known geophysicist Don Anderson from his valuable mantle plumes web site (http://www.mantleplumes.org/), “Rheology [flow characteristics] changes with stress.” Studies of conditions within the mantle are in their infancy and research opportunities are enormous. Geophysicists have not been able to generate computer simulations that include a self-consistent thermodynamic treatment of the effects of temperature, pressure, and volume on the mantle’s physical and thermal properties. And even research into the “exterior” problem (unstable surface boundary conditions including non-isothermal, non-stress-free, heterogeneous, and non-uniform conditions) has only just begun. Numerous other challenges remain to be addressed. For example, melting is an important but incompletely understood aspect of real mantle convection. In addition, sphericity, pressure, and the distribution of radioactivity ensure that the problem is asymmetrical and that surface and bottom boundary conditions play quite different roles than are shown in the simple calculations and cartoons of mantle dynamics and geochemical reservoirs. As a result, conventional mantle convection theory as it stands in 2011 may turn out to have relatively little to do with plate tectonics as is presently understood. Author’s Note: Serious students of contemporary problems in mantle convection should be frequent visitors to Don Anderson’s instructive web site, found at: http://www.mantleplumes.org/Convection.html.
Slab-Pull Theory proposes that gravity and the plates themselves are responsible for tectonic plate movement through subduction. Although geoscientists first described slab-pull and slab-suction in the 1970s it was generally believed that the details of plate movement would never be fully understood since those forces are buried so deeply that no driving mechanism could be tested directly and proven beyond reasonable scientific doubt. But by 1994 Seiya Uyeda, a world-renowned Japanese plate tectonics expert, was able to conclude that “subduction . . . plays a more fundamental role than sea-floor spreading in shaping the Earth’s surface features” and in “running the plate tectonic machinery.” A key observation made in the mid-1970s by geoscientists studying plate movement was that oceanic plates move toward subduction zones roughly between three and four times faster than continental plates. Although the reasons for that difference remain deep under the surface, so to speak, research published in Science in the early 2000s by Clinton Conrad and Carolina Lithgow-Bertelloni, geophysicists at the University of Michigan, suggests that the interaction of slab-pull and slab-suction may be responsible for the observed plate movements. As background, readers should remember that subduction zones are found at the outer edges of oceanic plates where the rock mass is cool and dense (as rock ages it cools and with that cooling becomes more dense).
Since oceanic plates are thinner and denser than continental plates, a collision between the two typically results in the ocean plate being pulled down into the mantle below it by gravitational forces, with its leading edge forming the downward moving slab. As the subducting portion of the plate (the slab) is pulled down into the mantle it drags the rest of the (mechanically) attached plate with it, causing tectonic plate movement. The density of the slab will affect the velocity of its subduction and thus the force it applies on the plate. Consequently, a very dense slab will sink faster than a less dense slab because of gravitational pull and will exert a greater force on the plate to which it is attached.
This theory explains mantle convection as a product, rather than a cause, of plate movement. The outward movement of the plate allows hot magma to bubble up from the Earth’s mantle at the center ridges of the plate, forming new crust where the older crust used to be. The computer model of viscous flow in the mantle created by the Michigan geophysicists integrated upper mantle slab-pull and lower mantle slab-suction into a system capable of making specific predictions. As a result of the application of that model, researchers determined that when either the pull force or the suction force acted alone, their model was unable to predict the observed difference in the rates at which oceanic and continental plates head toward subduction zones. Only when the two forces were combined together were the predictions able to account for between 60 percent to 40 percent of the observed reality. For more information and better than decent illustrations, see: http://www.soest.hawaii.edu/GG/FACULTY/conrad/resproj.html
Additional Author’s Note: Please note that research into this fascinating topic is ongoing and likely will not reach conclusion for a decade or more. In addition, future research may result in the combination of both the major theories on plate tectonic drivers into a single cohesive unit rather than supporting one while rejecting the other. All I can advise is for interested readers to keep focused on the key professional journals for updates as they are published. For more directly related information see my post of 7-16-12.
