Continental drift is the movement of Earth's continents relative to each other over geologic time. When initially proposed as a scientific theory in the early 20th century, the idea that continents could move was widely rejected. However, continental drift has subsequently been extensively validated and is incorporated into the science of plate tectonics, which studies the movement of the continents as they ride on plates of the Earth's lithosphere.
The speculation that continents might have "drifted" was first put forward by Abraham Ortelius in 1596. A pioneer of the modern view of mobilism was Austrian geologist Otto Ampferer.Kalliope Verbund: Ampferer, Otto (1875–1947) Helmut W. Flügel: Die virtuelle Welt des Otto Ampferer und die Realität seiner Zeit. In: Geo. Alp., Vol. 1, 2004. The concept was independently and more fully developed by Alfred Wegener in 1912, and expanded into book form with his 1915 publication, Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans). However, at that time his hypothesis was rejected by many, largely because there was no known geological mechanism which could propel such massive movements. In 1931, English geologist Arthur Holmes proposed mantle convection for that mechanism, which is now known to be powered by radioactive decay and primordial heat and a much smaller amount of heat from tidal heating.
History
Early history
Abraham Ortelius , Theodor Christoph Lilienthal (1756), Alexander von Humboldt (1801 and 1845), Antonio Snider-Pellegrini , and others had noted earlier that the shapes of continents on opposite sides of the Atlantic Ocean (most notably, Africa and South America) seem to fit together. W. J. Kious described Ortelius's thoughts in this way:
In 1889, Alfred Russel Wallace remarked that "It was formerly a very general belief, even amongst geologists, that the great features of the earth's surface, no less than the smaller ones, were subject to continual mutations, and that during the course of known geological time the continents and great oceans had, again and again, changed places with each other." He quotes Charles Lyell as saying, "Continents, therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages." and claims that the first to throw doubt on this was James Dwight Dana in 1849.
In his Manual of Geology (1863), Dana wrote, "The continents and oceans had their general outline or form defined in earliest time. This has been proved with regard to North America from the position and distribution of the first beds of the Lower Silurian, – those of the Potsdam epoch. The facts indicate that the continent of North America had its surface near tide-level, part above and part below it (p.196); and this will probably be proved to be the condition in Primordial time of the other continents also. And, if the outlines of the continents were marked out, it follows that the outlines of the oceans were no less so". Dana was enormously influential in America—his Manual of Mineralogy is still in print in revised form—and the theory became known as the Permanence theory.
This appeared to be confirmed by the exploration of the deep sea beds conducted by the Challenger expedition, 1872–1876, which showed that contrary to expectation, land debris brought down by rivers to the ocean is deposited comparatively close to the shore on what is now known as the continental shelf. This suggested that the oceans were a permanent feature of the Earth's surface, rather than having "changed places" with the continents.
Eduard Suess had proposed a supercontinent Gondwana in 1885, and the Tethys Ocean in 1893, assuming a land-bridge between the present continents submerged in the form of a geosyncline, and John Perry had written an 1895 paper proposing that the Earth's interior was fluid, and disagreeing with Lord Kelvin on the age of the Earth.
French geographer Elisée Reclus (1830 – 1905) presented evidence for a supercontinent and theorized continental drift fifty years prior to Wegener in his 1872 work The Earth.Reclus, E., 1872, The Earth (American edition translated by B. Woodward; H. Woodward, ed.): N.Y., Harper and Brothers,Berkland, J. O. (1979). Elisée Reclus—Neglected geologic pioneer and first (?) continental drift advocate. Geology, 7(4), 189-192. James O. Berkland argued that Reclus should be recognized "in the history of the development of plate tectonic theory as one of its foremost pioneers and, perhaps, as its founder".Berkland, J. O. (1979). Elisée Reclus—Neglected geologic pioneer and first (?) continental drift advocate. Geology, 7(4), 189-192. In 2021, Reclus, a tectonics database, was named for Elisée Reclus.Markwick, P. J., Paton, D. A., & Mortimer, E. J. (2021). Reclus, a new database for investigating the tectonics of the Earth: An example from the East African margin and hinterland. Geochemistry, Geophysics, Geosystems, 22, e2021GC009897. https://doi.org/10.1029/2021GC009897
Wegener and his predecessors
Apart from the earlier speculations mentioned above, the idea that the American continents had once formed a single landmass with Eurasia and Africa was postulated by several scientists before Alfred Wegener's 1912 paper. Although Wegener's theory was formed independently and was more complete than those of his predecessors, Wegener later credited a number of past authors with similar ideas: Franklin Coxworthy (between 1848 and 1890), Roberto Mantovani (between 1889 and 1909), William Henry Pickering (1907), and Frank Bursley Taylor (1908).
The similarity of southern continent geological formations had led Roberto Mantovani to conjecture in 1889 and 1909 that all the continents had once been joined into a supercontinent; Wegener noted the similarity of Mantovani's and his own maps of the former positions of the southern continents. In Mantovani's conjecture, this continent broke due to volcanic activity caused by thermal expansion, and the new continents drifted away from each other because of further expansion of the rip-zones, where the oceans now lie. This led Mantovani to propose a now-discredited Expanding Earth theory.
Continental drift without expansion was proposed by Frank Bursley Taylor, who suggested in 1908 (published in 1910) that the continents were moved into their present positions by a process of "continental creep", later proposing a mechanism of increased tidal forces during the Cretaceous dragging the crust towards the equator. He was the first to realize that one of the effects of continental motion would be the formation of mountains, attributing the formation of the Himalayas to the collision between the Indian subcontinent with Asia. Wegener said that of all those theories, Taylor's had the most similarities to his own. For a time in the mid-20th century, the theory of continental drift was referred to as the "Taylor-Wegener hypothesis".
Alfred Wegener first presented his hypothesis to the German Geological Society on 6 January 1912. He proposed that the continents had once formed a single landmass, which he called Pangaea, before breaking apart and drifting to their present locations.
Wegener was the first to use the phrase "continental drift" (1912, 1915) () and to publish the hypothesis that the continents had somehow "drifted" apart. Although he presented much evidence for continental drift, he was unable to provide a convincing explanation for the physical processes which might have caused this drift. He suggested that the continents had been pulled apart by the centrifugal pseudoforce () of the Earth's rotation or by a small component of astronomical precession, but calculations showed that the force was not sufficient. The hypothesis was also studied by Paul Sophus Epstein in 1920 and found to be implausible.
Rejection of Wegener's theory, 1910s–1950s Continental drift is now generally accepted, and it always had a minority of scientific proponents since Wegener. However, the theory was largely rejected for many years, with evidence in its favor considered insufficient, and Wegener did not live to see his hypothesis triumph. One problem was that a plausible driving force was missing. A second was that Wegener's estimate of the speed of continental motion, , was implausibly high. (The currently accepted rate for the separation of the Americas from Europe and Africa is about .) Furthermore, Wegener was treated less seriously because he was not a geologist. Even today, the details of the forces propelling the plates are poorly understood.
The English geologist Arthur Holmes championed the theory of continental drift at a time when it was deeply unfashionable. He proposed in 1931 that the Earth's mantle contained convection cells which dissipated heat produced by radioactive decay and moved the crust at the surface. His Principles of Physical Geology, ending with a chapter on continental drift, was published in 1944.
Geological maps of the time showed huge land bridges spanning the Atlantic and Indian oceans to account for the similarities of fauna and flora and the divisions of the Asian continent in the Permian period, but failing to account for glaciation in India, Australia and South Africa.
The fixists Hans Stille and Leopold Kober opposed the idea of continental drift and worked on a "fixist" geosyncline model with Earth contraction playing a key role in the formation of orogens. Other geologists who opposed continental drift were Bailey Willis, Charles Schuchert, Rollin Chamberlin, Walther Bucher and Walther Penck. Willem van der Gracht was virtually the only tectonicist who supported mobilism. In 1939 an international geological conference was held in Frankfurt, organized by the fixist Hans Cloos, which expounded abundant criticism of continental drift and mobilism from the perspectives of tectonics, sedimentology (Nölke), paleontology (Nölke), mechanics (Lehmann) and oceanography (Troll, Wüst). Cloos and Troll maintained that excepting the Pacific Ocean, continents were not radically different from oceans in their behaviour. The mobilist theory of Émile Argand for the Alpine orogeny was criticized by Kurt Leuchs. The few drifters and mobilists at the conference appealed to biogeography (Kirsch, Wittmann), paleoclimatology (Kurt Wegener, brother of Alfred Wegener), paleontology (Gerth) and geodetic measurements (Kurt Wegener). F. Bernauer correctly equated Reykjanes in south-west Iceland with the Mid-Atlantic Ridge, arguing with this that the floor of the Atlantic Ocean was undergoing extension just like Reykjanes. Bernauer thought this extension had drifted the continents only apart, the approximate width of the volcanic zone in Iceland.
David Attenborough, who attended university in the second half of the 1940s, recounted an incident illustrating the dismissal of the theory: "I once asked one of my lecturers why he was not talking to us about continental drift and I was told, sneeringly, that if I could prove there was a force that could move continents, then he might think about it. The idea was moonshine, I was informed."
As late as 1953—just five years before Carey introduced the theory of plate tectonics—the theory of continental drift was rejected by the physicist Adrian Scheidegger on the following grounds. First, it had been shown that floating masses on a rotating geoid would collect at the equator, and stay there. This would explain one, but only one, mountain-building episode between any pair of continents; it failed to account for earlier orogenic episodes. Second, masses floating freely in a fluid substratum, like icebergs in the ocean, should be in isostatic equilibrium (in which the forces of gravity and buoyancy are in balance). But gravitational measurements showed that many areas are not in isostatic equilibrium.