Continental drift
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Continental drift is a highly supported scientific theory, originating in the early 20th century, that Earth's continents move or drift relative to each other over geologic time.<ref name="pubs.usgs.gov" /> The theory of continental drift has since been validated and incorporated into the science of plate tectonics, which studies the movement of the continents as they ride on plates of the Earth's lithosphere.<ref name="Oreskes-2002" />
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 the Austrian geologist Otto Ampferer.<ref>Kalliope Verbund: Ampferer, Otto (1875–1947) </ref><ref>Helmut W. Flügel: Die virtuelle Welt des Otto Ampferer und die Realität seiner Zeit. In: Geo. Alp., Vol. 1, 2004.</ref> The concept was independently and more fully developed by Alfred Wegener in his 1915 publication, "The Origin of Continents and Oceans".<ref name="Wegener-1912" /> 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, the English geologist Arthur Holmes proposed mantle convection for that mechanism, which is now known to be powered by radioactive decay and primordial heat Template:Citation span.
History
Early history
Abraham Ortelius Template:Harv,<ref name="Romm-1994" /> Theodor Christoph Lilienthal (1756),<ref name="Schmeling-2004" /> Alexander von Humboldt (1801 and 1845),<ref name="Schmeling-2004" /> Antonio Snider-Pellegrini Template:Harv, 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.<ref name="Brusatte-2016" /> W. J. Kious described Ortelius's thoughts in this way:<ref name="Kious-2001" /> Template:Blockquote
In 1889, Alfred Russel Wallace remarked, "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."<ref name="Wallace-1889" /> He quotes Charles Lyell as saying, "Continents, therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages."<ref name="Lyell-1872" /> 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".<ref name="Dana-1863" /> 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.<ref name="Oreskes-2002-2" />
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 them having "changed places" with the continents.<ref name="Wallace-1889" />
Eduard Suess had proposed a supercontinent Gondwana in 1885<ref name="Suess-1885" /> and the Tethys Ocean in 1893,<ref name="Suess-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.<ref name="Perry-1895" />
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.<ref name="Wegener-1912" /> 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:<ref name="Wegener-1966" /><ref name="Wegener-1929" /> Franklin Coxworthy (between 1848 and 1890),<ref name="Coxworthy-1924" /> Roberto Mantovani (between 1889 and 1909), William Henry Pickering (1907)<ref name="Pickering-1907" /> and Frank Bursley Taylor (1908).<ref name="Taylor-1910" />
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.<ref name="Mantovani-1889" /><ref name="Mantovani-1909" /><ref name="Scalera-2003" />
Continental drift without expansion was proposed by Frank Bursley Taylor,<ref name="Lane-1944" /> who suggested in 1908 (published in 1910) that the continents were moved into their present positions by a process of "continental creep",<ref name="Taylor-1910a" /><ref name="Frankel-2012" /> 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.<ref name="Powell-2015" /> 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".<ref name="Lane-1944" /><ref name="Powell-2015" /><ref name="Hansen" /><ref name="Wood-2016" />
Alfred Wegener first presented his hypothesis to the German Geological Society on 6 January 1912.<ref name="Wegener-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.<ref name="Wegenerproofs">Template:Cite web</ref>
Wegener was the first to use the phrase "continental drift" (1912, 1915)<ref name="Wegener-1912" /><ref name="Wegener-1966" /> (Template:Langx) 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 (Template:Lang) of the Earth's rotation or by a small component of astronomical precession, but calculations showed that the force was not sufficient.<ref name="PlateTectonics-2011" /> The Template:Lang 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.<ref name="pubs.usgs.gov" /> A second was that Wegener's estimate of the speed of continental motion, Template:Cvt, was implausibly high.<ref name="UniCalifMusPaleontology" /> (The currently accepted rate for the separation of the Americas from Europe and Africa is about Template:Cvt.)<ref name="Unavco-2015" /> 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.<ref name="pubs.usgs.gov" />
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.<ref name="Holmes-1931" /> His Principles of Physical Geology, ending with a chapter on continental drift, was published in 1944.<ref name="Holmes-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.<ref name="Wells-1931" />
The fixists
Hans Stille and Leopold Kober opposed the idea of continental drift and worked on a "fixist"<ref name="Sen30" /> geosyncline model with Earth contraction playing a key role in the formation of orogens.<ref name="Sen28" /><ref name="Sen29" /> Other geologists who opposed continental drift were Bailey Willis, Charles Schuchert, Rollin Chamberlin, Walther Bucher and Walther Penck.<ref name="Sen31" /><ref name="Bremer-1983">Template:Cite journal</ref> 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,<ref name="Frankel403" /> which expounded abundant criticism of continental drift and mobilism from the perspectives of tectonics, sedimentology (Nölke), paleontology (Nölke), mechanics (Lehmann) and oceanography (Troll, Wüst).<ref name="Frankel403" /><ref name="Frankel405" /> Cloos and Troll maintained that excepting the Pacific Ocean, continents were not radically different from oceans in their behaviour.<ref name="Frankel405" /> The mobilist theory of Émile Argand for the Alpine orogeny was criticized by Kurt Leuchs.<ref name="Frankel403" /> 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).<ref name="Frankel407" /> 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 Template:Cvt apart, the approximate width of the volcanic zone in Iceland.<ref name="Frankel409" />
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."<ref name="McKie-2012" />
As late as 1953—just five years before Carey<ref name="Carey-1958" /> introduced the theory of plate tectonics—the theory of continental drift was rejected by the physicist Scheidegger on the following grounds.<ref name="Scheidegger-1953" />
- 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.
- Third, there was the problem of why some parts of the Earth's surface (crust) should have solidified while other parts were still fluid. Various attempts to explain this foundered on other difficulties.
Road to acceptance
From the 1930s to the late 1950s, works by Vening-Meinesz, Holmes, Umbgrove, and numerous others outlined concepts that were close or nearly identical to modern plate tectonics theory, which has encompassed and superseded continental drift. The English geologist Arthur Holmes proposed in 1920 that plate junctions might lie beneath the sea, and in 1928 that convection currents within the mantle might be the driving force.<ref name="Holmes-1928" /> Holmes's views were particularly influential: in his bestselling textbook, Principles of Physical Geology, he included a chapter on continental drift, proposing that Earth's mantle contained convection cells which dissipated radioactive heat and moved the crust at the surface.<ref name="Wessel-2007" /><ref name="Vine-1966" /> Holmes's proposal resolved the phase disequilibrium objection (the underlying fluid was kept from solidifying by radioactive heating from the core). However, scientific communication in the 1930s and 1940s was inhibited by World War II, and the theory still required work to overcome the orogeny and isostasy objections. Worse, the most viable forms of the theory predicted the existence of convection cell boundaries reaching deep into the Earth, which had not been observed.Template:Citation needed
In 1947, a team of scientists led by Maurice Ewing confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the sediments was chemically and physically different from continental crust.<ref name="Lippsett-2001" /><ref name="Lippsett-2006" /> As oceanographers continued to bathymeter the ocean basins, a system of mid-oceanic ridges was detected. An important conclusion was that along this system, new ocean floor was being created, which led to the concept of the "Great Global Rift".<ref name="Heezen-1960" />
Meanwhile, scientists began recognizing odd magnetic variations across the ocean floor using devices developed during World War II to detect submarines.<ref name="LATimes-2009" /> Over the next decade, it became increasingly clear that the magnetization patterns were not anomalies, as had been originally supposed. In a series of papers published between 1959 and 1963, Heezen, Dietz, Hess, Mason, Vine, Matthews, and Morley collectively realized that the magnetization of the ocean floor formed extensive, zebra-like patterns: one stripe would exhibit normal polarity and the adjoining stripes reversed polarity.<ref name="Mason-1961" /><ref name="Korgen-1995" /><ref name="Spiess-2003" /> The best explanation was the "conveyor belt" or Vine–Matthews–Morley hypothesis. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. The new crust is magnetized by the Earth's magnetic field, which undergoes occasional reversals. Formation of new crust then displaces the magnetized crust away from the rift, akin to a conveyor belt.<ref name="Heirtzler-1966" />
Without workable alternatives to explain the stripes, geophysicists were forced to conclude that Holmes had been right: ocean rifts were sites of perpetual orogeny at the boundaries of convection cells.<ref name="LePichon-1968" /><ref name="McKenzie-1967" /> By 1967, barely two decades after discovery of the mid-oceanic rifts, and a decade after discovery of the striping, plate tectonics had become axiomatic to modern geophysics.
In addition, Marie Tharp provided essential corroboration using her skills in cartography and seismographic data. She collaborated with Bruce Heezen, who was initially sceptical of Tharp's assertions that her maps confirmed continental drift.<ref name="Barton-2002" /><ref name="Blakemore-2016" /><ref name="Evans-2002" /><ref name="Doel-2006" /><ref name="Wills-2016" />
Modern evidence
Geophysicist Jack Oliver provided seismologic evidence for plate tectonics with the 1968 article "Seismology and the New Global Tectonics", using data from seismologic stations including those he set up in the South Pacific.<ref name="NYTimes-2011" /><ref name="Isacks-1968" /> The modern theory of plate tectonics, refining Wegener, explains that there are two kinds of crust of different composition, continental and oceanic, both floating on a much deeper "plastic" mantle. Continental crust is inherently lighter. Oceanic crust is created at spreading centers and descends back into the mantle at subduction zones, driving the system of plates chaotically, with continuous orogeny and areas of isostatic imbalance.
Evidence for the movement of continents on tectonic plates is now extensive. Similar plant and animal fossils are found around the shores of different continents, suggesting that they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile, found both in Brazil and South Africa, are one example; another is the discovery of fossils of the land reptile Lystrosaurus in rocks of the same age at locations in Africa, India, and Antarctica.<ref name="USGS" /> There is also living evidence, with the same animals being found on two continents. Some earthworm families (such as Ocnerodrilidae, Acanthodrilidae, Octochaetidae) are found in South America and Africa.
The complementary arrangement of the facing sides of South America and Africa is an obvious and temporary coincidence. In millions of years, slab pull, ridge-push, and other forces of tectonophysics will further separate and rotate those two continents. It was that temporary feature that inspired Wegener to propose continental drift.
The widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence for the theory of continental drift. The continuity of glaciers, inferred from oriented glacial striations and tillite deposits, suggested the existence of the supercontinent of Gondwana, which became a central element of the concept of continental drift. Striations indicated glacial flow away from the equator and toward the poles, based on continents' current positions and orientations, and supported the idea that the southern continents had previously been in dramatically different locations contiguous with one another.<ref name="Wegener-1966" />
GPS evidence
Today continental drift can be directly measured with Global Positioning Satellite systems. A GPS device placed in Maui, Hawaii moved about 48 cm latitudinally and about 84 cm longitudinally over 14 years.<ref>Template:Cite web</ref>
See also
Citations
<references> <ref name="pubs.usgs.gov">Template:Cite web</ref> <ref name="Schmeling-2004">Template:Cite web</ref> <ref name="Wallace-1889">Template:Citation</ref> <ref name="Wegener-1912">Template:Citation</ref> <ref name="Wegener-1966">Template:Citation</ref> <ref name="Lane-1944">Template:Citation</ref> <ref name="Powell-2015">Template:Cite book</ref> <ref name="Sen30">Şengör (1982), p. 30</ref> <ref name="Sen28">Şengör (1982), p. 28</ref> <ref name="Sen29">Şengör (1982), p. 29</ref> <ref name="Sen31">Şengör (1982), p. 31</ref> <ref name="Frankel403">Frankel (2012), p. 403</ref> <ref name="Frankel405">Frankel (2012), p. 405</ref> <ref name="Frankel407">Frankel (2012), p. 407</ref> <ref name="Frankel409">Frankel (2012), p. 409</ref> <ref name="Snider-Pellegrini-1858">Antonio Snider-Pellegrini, La Création et ses mystères dévoilés (Creation and its mysteries revealed) (Paris, France: Frank et Dentu, 1858), plates 9 and 10 Template:Webarchive (between pages 314 and 315).</ref> <ref name="Oreskes-2002">Template:Harvnb.</ref> <ref name="Romm-1994">Template:Citation</ref> <ref name="Brusatte-2016">Template:Citation</ref> <ref name="Kious-2001">Template:Citation</ref> <ref name="Lyell-1872">Template:Citation</ref> <ref name="Dana-1863">Template:Citation</ref> <ref name="Oreskes-2002-2">Template:Harvnb</ref> <ref name="Suess-1885">Eduard Suess, Das Antlitz der Erde (The Face of the Earth), vol. 1 (Leipzig, (Germany): G. Freytag, 1885), page 768. From p. 768: "Wir nennen es Gondwána-Land, nach der gemeinsamen alten Gondwána-Flora, ... " (We name it Gondwána-Land, after the common ancient flora of Gondwána ... )</ref> <ref name="Suess-1893">Edward Suess (March 1893) "Are ocean depths permanent?" Template:Webarchive, Natural Science: A Monthly Review of Scientific Progress (London), 2 : 180- 187. From page 183: "This ocean we designate by the name "Tethys", after the sister and consort of Oceanus. The latest successor of the Tethyan Sea is the present Mediterranean."</ref> <ref name="Perry-1895">Perry, John (1895) "On the age of the earth", Nature, 51 : 224–227 Template:Webarchive, 341–342, 582–585.</ref> <ref name="Wegener-1929">Template:Citation</ref> <ref name="Coxworthy-1924">Template:Cite book</ref> <ref name="Pickering-1907">Template:Citation</ref> <ref name="Taylor-1910">Frank Bursley Taylor (3 June 1910) "Bearing of the Tertiary mountain belt on the origin of the earth's plan", Bulletin of the Geological Society of America, 21 : 179–226.</ref> <ref name="Mantovani-1889">Template:Citation</ref> <ref name="Mantovani-1909">Template:Citation</ref> <ref name="Scalera-2003">Template:Citation</ref> <ref name="Taylor-1910a">Template:Citation</ref> <ref name="Frankel-2012">Henry R. Frankel, "Wegener and Taylor develop their theories of continental drift", in The Continental Drift Controversy Volume 1: Wegener and the Early Debate, pp. 38–80, Cambridge University Press, 2012. Template:ISBN Template:Doi</ref> <ref name="Hansen">Hansen, L. T., Some considerations of, and additions to the Taylor-Wegener hypothesis of continental displacement, Los Angeles, 1946. Template:OCLC</ref> <ref name="Wood-2016">R. M. Wood, Coming Apart at the Seams Template:Webarchive, New Scientist, 24 January 1980</ref> <ref name="PlateTectonics-2011">Template:Cite web</ref> <ref name="UniCalifMusPaleontology">University of California Museum of Paleontology, Alfred Wegener (1880–1930) Template:Webarchive (accessed 30 April 2015).</ref> <ref name="Unavco-2015">Unavco Plate Motion Calculator Template:Webarchive (accessed 30 April 2015).</ref> <ref name="Holmes-1931">Template:Cite journal</ref> <ref name="Holmes-1944">Template:Cite book</ref> <ref name="Wells-1931">See map based on the work of the American paleontologist Charles Schuchert in Template:Citation</ref> <ref name="McKie-2012">Template:Cite news</ref> <ref name="Carey-1958">Template:Cite news</ref> <ref name="Scheidegger-1953">Template:Citation</ref> <ref name="Holmes-1928">Template:Cite journal; see also Template:Cite book and Template:Cite journal.</ref> <ref name="Wessel-2007">Template:Citation</ref> <ref name="Vine-1966">Template:Cite journal</ref> <ref name="Lippsett-2001">Template:Cite journal</ref> <ref name="Lippsett-2006">Template:Cite book</ref> <ref name="Heezen-1960">Template:Cite journal</ref> <ref name="LATimes-2009">Template:Citation.</ref> <ref name="Mason-1961">Template:Cite journal</ref> <ref name="Korgen-1995">Template:Cite journal</ref> <ref name="Spiess-2003">Template:Cite journal</ref> <ref name="Heirtzler-1966">See summary in Template:Cite journal</ref> <ref name="LePichon-1968">Template:Cite journal</ref> <ref name="McKenzie-1967">Template:Cite journal</ref> <ref name="Barton-2002">Template:Cite journal</ref> <ref name="Blakemore-2016">Blakemore, Erin (30 August 2016). "Seeing Is Believing: How Marie Tharp Changed Geology Forever". Smithsonian.</ref> <ref name="Evans-2002">Evans, R. (November 2002). "Plumbing Depths to Reach New Heights". Retrieved 2 June 2008.</ref> <ref name="Doel-2006">Template:Cite journal</ref> <ref name="Wills-2016">Wills, Matthew (8 October 2016). "The Mother of Ocean Floor Cartography". JSTOR. Retrieved 14 October 2016. While working with the North Atlantic data, she noted what must have been a rift between high undersea mountains. This suggested earthquake activity, which then [was] only associated with [the] fringe theory of continental drift. Heezen infamously dismissed his assistant's idea as "girl talk." But she was right, and her thinking helped to vindicate Alfred Wegener's 1912 theory of moving continents. Yet Tharp's name isn't on any of the key papers that Heezen and others published about plate tectonics between 1959 and 1963, which brought this once-controversial idea to the mainstream of earth sciences.</ref> <ref name="NYTimes-2011">Template:Cite news</ref> <ref name="Isacks-1968">Template:Cite journal</ref> <ref name="USGS">Template:Cite web</ref> </references>
General and cited sources
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- Template:Cite book (pb: Template:ISBNT)
- Template:Cite encyclopedia
- Template:Cite book (First edition published 1570, 1587 edition online)
- Template:Cite book
- Template:Cite book.
External links
Template:Library resources box Template:Wikibooks
- Benjamin Franklin (1782) and Ralph Waldo Emerson (1834) noted Continental Drift
- A brief introduction to Plate Tectonics, based on the work of Alfred Wegener
- Animation of continental drift for last 1 billion years
- Maps of continental drift, from the Precambrian to the future
- 3D visualization of what did Earth look like from 750 million years ago to present (at present location of your choice)