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Not Continental Drift but Plate Tectonics
(Released May 2010)

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  by Adam T. Mansur  

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A Revolution Deferred

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Despite Wegener's confidence in his theory, he failed to convince many of his peers. Philip Lake wrote that Wegener's theory was "vulnerable in every statement" and that his commitment to it blinded him "to every fact and argument that tells against it."23 The British seismologist Sir Harold Jeffreys argued that continental drift was mechanically impossible; Chester Longwell accused Wegener of iconoclasm.24 These reactions and others like them were both typical and durable: Fifty years would pass between the initial publication of The Origins of the Oceans and Continents and widespread acceptance of mobile continents. Though the flaws of the current geological theories were well-known, many geologists preferred to tinker with familiar frameworks rather than commit to Wegener's strange new theory. Rejection was quick and final in the United States; elsewhere, the theory was entertained if not immediately adopted.25

old observation station
Station NIU, Oahu Hawaii, during reobservation of World Longitude stations. Looking for evidence of continental drift, but instruments were too crude to measure small earth movements. Photo: 1934.

All aspects of Wegener's theory and many qualities of Wegener himself were subject to attack. Because Wegener was trained as a meteorologist, critics questioned the relevance of his experience. Because he relied on published studies instead of conducting his own fieldwork, he was criticized for co-opting, misinterpreting, or cherry-picking other scientists' work. Even the jigsaw fit of the continents that initially inspired continental drift was denied, with Jeffreys, Rollin Chamberlain, and others arguing that the margins of the continents matched only if they were dramatically deformed.26 (This argument is telling of the lack of care of some critics of Wegener, who failed to notice that Wegener's fit was based on the continental shelves, not the outlines of the continents as they appeared above sea level.27)

Above all geologists rejected the notion of the horizontal displacement of the continents. For many critics, the argument against continental drift boiled down to a single point: Continents could not move. Jeffreys, a pioneering seismologist, was particularly adamant that the Earth's rigidity precluded displacement.28 Bowie made a similar argument based on isostasy.29 Most geologists were not so final in their rejection, but nevertheless considered the mechanisms suggested by Wegener and his allies to be inadequate to displace a continent. Nor did they concede that displacement was necessary to explain mountain building or homologies. Other reasonable explanations existed. When attempts to measure continental displacement using astronomical observation and telegraph signals provided ambiguous results, most geologists were content to dismiss continental drift.30

The alternative theories developed during and after the debate on drift theory still struggled with the conflicting requirements of isostasy and the fossil and structural homologies. Many theories simply ignored one or the other of these lines of evidence. For example, the fissiparturition hypothesis argued that the present distribution of the continents occurred when the Moon was ripped from the Earth, splitting the then-continuous granitic crust and filling the cracks with basalt. But this theory offered no explanation for fossil homologies.31 Others tried to reconcile the problematic observations by appealing to processes operating in the deep crust. Joseph Barrell argued that loading the lower crust of land bridges with intrusions of basalt would make them dense enough to sink, a model rejected on isostatic grounds.32 Perhaps the most successful theory in this period was a different reimagining of the land bridge hypothesis, this one proposed by Charles Schuchert and Bailey Willis. They argued that land bridges were not granitic but rather thick accumulations of basalt, similar to modern ocean islands that rise above the surface of the ocean but eventually subside.33 Though this model had its flaws—chief among them that Schuchert believed that the trans-Atlantic land bridge must have been large and granitic, thus burdening it with the same problems that doomed earlier land bridge theories—it was widely regarded as a good solution to the problem of fossil homologies.

Not all geologists were content with this direction. Outside of the United States, many scientists supported continental drift. The South African field geologist Alexander du Toit, a passionate (and often frustrated) defender of drift, made detailed studies of fossil assemblages and geological structures in rocks from South Africa and South America that were adjacent in Pangaea, addressing concerns about the quality of the homologies cited by Wegener.34 But perhaps the most important adherent to the theory of continental drift was the British geologist Arthur Holmes, whose research provided the mechanism for driving plate motion lacking from Wegener's theory. Holmes argued that convection of the mantle was necessary to transfer excess radiogenic heat from the base of the mantle to the surface. Convection cells would transfer hot material from near the core to just below the crust, then travel laterally along the base of the crust until the material had cooled sufficiently to sink back into the mantle. The lateral, near-surface currents would drag the overlying continents along as they moved.35 As the historian Naomi Oreskes notes, Holmes's theory is similar and in key aspects superior to the theory of mantle convection by Harry H. Hess that is credited with sparking the plate tectonic revolution of the 1960s.36 In America, where continental drift was a theory non grata, a small number of geologists contributed work that supported the theory and foreshadowed key aspects of modern plate tectonic theory. Prominent among these were Reginald Daly and G. A. F. Molengraaf.37 Despite their efforts, however, and those of international colleagues like du Toit and Holmes, they failed to convince other American geologists of the merits of drift theory.

Wegener died during an expedition to Greenland in 1930. By the mid-1930s, the debate about mobile continents in America had petered out, and adherents would remain in the minority until the 1960s. The revival of the idea came only in response to new data from studies of the Earth's magnetic field and seafloor conducted in the 1950s and 1960s. Some forty odd years after Wegener's initial proposal, mapping of the seafloor revealed faulting and high heat flow along mid-ocean ridges that traced the midline of all the ocean basins, leading Hess to propose that the seafloor was spreading apart at these ridges as a result of deep currents in the mantle.38

Evidence consistent with mantle convection came from the new field of paleomagnetism, which looked at changes in the Earth's magnetic field over time as recorded in rocks. Certain iron-rich minerals align with the orientation of the magnetic field operating at the time of their crystallization. Sensitive magnetometers allowed geologists to measure this remanent magnetism in both continental and seafloor samples. Studies of paleomagnetism revealed three additional pieces of evidence key to the formulation of plate tectonic theory:

  1. Magnetic reversals: The polarity of the Earth's magnetic field was found to have reversed at irregular intervals throughout Earth history. Remanent magnetism in rocks could be either normal (aligned with the current magnetic field) or reversed (aligned opposite to the current field). An absolute paleomagnetic timescale was calibrated based on the history of magnetic reversals in the rock record.39
  2. Apparent polar wander: Paleomagnetic data from rocks of the same age from different continents indicated different locations for the poles, whereas rocks of different age from the same locations showed an apparent change in the poles' location over time. Both observations could be reconciled if the continents had moved relative to one another.40
  3. Seafloor striping: Basalts near the mid-ocean ridges showed a distinctive pattern of alternating bands of normal and reversed remanent magnetism. These bands were parallel to and symmetric across the ridge axis. This seafloor striping was interpreted to reflect continuous extrusion of basalts at the mid-ocean ridge in alternating periods of normal and reversed polarity of the geomagnetic field. By measuring the age and thickness of each pair of bands, geologists were able to calculate a rate for seafloor spreading of a few centimeters per year.41

ship at sea
Coast and Geodetic Survey Ship PIONEER, in service 1946 - 1966, discovered magnetic striping, the key to plate tectonics. 1952?

Unlike Wegener's theory, which failed to gain the support of the geological community, these interpretations of the new geophysical evidence were quickly assimilated into the scientific canon. The result of the 1960s research was a new theory of continental displacement, plate tectonics, which posits that the Earth's surface is divided into mobile plates driven along by deep mantle currents. Plate tectonics differs in key respects from Wegener's continental drift but shares the core, long-controversial premise of mobile continents. Today, plate tectonics enjoys nearly universally acceptance, and Wegener's continental drift is recognized as a perceptive but fatally flawed forerunner of the modern theory.

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