Continental drift

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File:Plates tect2 en.svg
Plates in the crust of the earth, according to the plate tectonics theory

Continental drift refers to the movement of the Earth's continents relative to each other.

Frank Bursley Taylor had proposed the concept in a Geological Society of America meeting in 1908 and published his work in the GSA Bulletin in June 1910.<ref>Frank Bursley Taylor, Bearing of the Tertiary Mountain Belt on the Origin of the Earth's Plan, GSA Bulletin, June 1910; Taylor FB (2005) WHEN THE CONTINENTS CREPT AWAY. GSA Today: Vol. 15, No. 7 pp. 29 http://www.gsajournals.org/perlserv/?request=get-document&doi=10.1130%2F1052-5173(2005)015%5B29b%3AWTCCA%5D2.0.CO%3B2 </ref> Abraham Ortelius, Francis Bacon, Antonio Snider-Pellegrini, Benjamin Franklin, and others had noted earlier that the shapes of continents on either side of the Atlantic Ocean (most notably, Africa and South America) seem to fit together. The similarity of southern continent fossil faunae and some geological formations had led a small number of Southern hemisphere geologists to conjecture as early as 1900{{fix-{{#switch:{{{style}}} |box|page=box |line|section=line |inline|#default=inline}} |{{#if:|image=}} |{{#if:|size=}} |{{#if:WikiPilipinas:Citing sources|link=WikiPilipinas:Citing sources}} |{{#if:noprint Template-Fact|class=noprint Template-Fact}} |{{#if:This claim needs references to reliable sources|title=This claim needs references to reliable sources}} |{{#if:|pre-text=}} |{{#if:citation needed|text=citation needed}} |{{#if:|post-text=}} |{{#if:|special=}} |{{#if:February 2007|date=February 2007}} |cat= |{{#if:|cat-date=}}}} that all the continents had once been joined into a supercontinent known as Pangaea. Frank Bursley Taylor suggested that the continents were dragged towards the equator by increased lunar gravity during the Cretaceous, thus forming the Himalaya and Alps on the southern faces.

Alfred Wegener was the first to formally publish the hypothesis that the continents had somehow "drifted" apart. However, he was unable to provide a convincing explanation for the physical processes which might have caused this drift. His suggestion that the continents had been pulled apart by the centrifugal pseudoforce of the Earth's rotation was considered unrealistic by the scientific community.<ref>Plate Tectonics: The Rocky History of an Idea</ref>

The hypothesis received support through the controversial years from South African geologist Alexander Du Toit as well as from Arthur Holmes. The idea of continental drift did not become widely accepted even as theory until the late 1950s. By the 1960s, geological research conducted by Robert S. Dietz, Bruce Heezen, and Harry Hess, along with a rekindling of the theory including a mechanism by J. Tuzo Wilson led to widespread acceptance of the theory among geologists.

The hypothesis of continental drift became part of the larger theory of plate tectonics. This article deals mainly with the historical development of the continental drift hypothesis before 1950. See: plate tectonics for information on current ideas underlying concepts of continental drift.

Contents

Various data

South America and Africa are moving apart at an average of 5.7 cm per year because the seafloor is spreading along the Mid-Atlantic Ridge. This is comparable to the growth speed of a fingernail.

The fastest recorded seafloor spreading takes place along the East Pacific Rise at 17.2 cm per year.

Evidence

Template:Seesubarticle Note: This section contains evidence available to Wegener's contemporaries and predecessors

Fossil patterns across continents.
Pangaea separation animation

The notion that continents have not always been at their present positions was suggested as early as 1596 by the Dutch map maker Abraham Ortelius in the third edition of his work Thesaurus Geographicus. Ortelius suggested that the Americas, Eurasia and Africa were once joined and have since drifted apart "by earthquakes and floods", creating the modern Atlantic Ocean. For evidence, he wrote: "The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three continents." Francis Bacon commented on Ortelius' idea in 1620, as did Benjamin Franklin and Alexander von Humboldt in later centuries.

Evidence for continental drift is now extensive, in the form of plant and animal fossils of the same age found around different continent shores, suggesting that these shores were once joined: the fossils of the freshwater crocodile, found in Brazil and South Africa, are one example. Another is the discovery of fossils of the aquatic reptile Lystrosaurus from rocks of the same age from locations in South America, Africa, and Antarctica. There is also living evidence - the same animals being found on two continents. An example of this is a particular earthworm found in South America and South Africa.

The complementary arrangement of the facing sides of South America and Africa is obvious, but is a temporary coincidence. In millions of years, seafloor spreading, continental drift, and other forces of tectonophysics will further separate and rotate those two continents. It was this temporary feature which inspired Alfred Wegener to study what he defined as continental drift. He never lived to see his hypothesis be proved true.

Permo-Carboniferous

Permo-Carboniferous was a period of great glaciation that occurred about 250 million years ago. It is one of the many ice ages that has occurred on Earth. This is also an era that has been used to submit proof that the continents were once a large land mass called Pangaea. Permo-Carboniferous rocks are widely distributed in Pangaea. 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. Glacial activity spanned virtually the whole of Carboniferous and Early Permian time (A.G. Smith 1997). Toward the end of the Carboniferous, and around 290 million years ago, Gondwanda hovered over the south polar regions, where glacial centres expanded across the continents, as evidenced by glacial deposits of tillites along with striations in ancient rocks. Those heavily grooved by the advancing glaciers showed lines of ice flow away from the equator and toward the poles, which is the opposite direction if the continents were situated where they are today. Overall, the southern continents drifted together over the South Pole, and massive ice sheets radiating outward from a central point crossed the present continental boundaries. The Permo-Carboniferous ice sheet is so extensive that it can fit within a latitude circle of 50 degrees (A.G.Smith 1997)

Debate

Before geophysical evidence started accumulating after World War II, the idea of continental drift caused sharp disagreement among geologists. Wegener had introduced his theory in 1912 at a meeting of the German Geological Association. His paper was published that year and expanded into a book in 1915. In 1921 the Berlin Geological Society held a symposium on the theory. In 1922 Wegener's book was translated into English and then it received a wider audience. In 1923 the theory was discussed at conferences by Geological Society of France, the Geological Section of the British Association for the Advancement of Science, and the Royal Geological Society. The theory was carefully but critically reviewed in the journal Nature by Philip Lake. On November 15, 1926, the American Association of Petroleum Geologists (AAPG) held a symposium at which the continental drift hypothesis was vigorously debated. The resulting papers were published in 1928 under the title Theory of continental drift. Wegener himself contributed a paper to this volume (Friedlander 1995:21-27).

One of the main problems with Wegener's theory was that he believed that the continents "plowed" through the rocks of the ocean basins. Most geologists did not believe that this could be possible. In addition, Wegener did not have an acceptable theory of the forces that caused the continents to drift. He also ignored counter-arguments and evidence contrary to his theory and seemed too willing to interpret ambiguous evidence as being favorable to his theory (Williams 2000:59). For their part, the geologists ignored Wegener's copious body of evidence, allowing their adherence to a theory to override the actual data, when the scientific method would seem to demand the reverse approach - a common obstacle to the advancement of knowledge (see paradigm shift and confirmation bias).

Plate tectonics, a modern update of the old ideas of Wegener about "plowing" continents, accommodates continental motion through the mechanism of seafloor spreading. New rock is created by volcanism at mid-ocean ridges and returned to the Earth's mantle at ocean trenches. Remarkably, in the 1928 AAPG volume, G. A. F. Molengraaf of the Delft Institute (now University) of Technology proposed a recognizable form of seafloor spreading in order to account for the opening of the Atlantic Ocean as well as the East Africa Rift. Arthur Holmes (an early supporter of Wegener) suggested that the movement of continents was the result of convection currents driven by the heat of the interior of the Earth, rather than the continents floating on the mantle. In the words of Carl Sagan (1995:302-03), it is more like the continents are being carried on a conveyor belt than floating or drifting. The ideas of Molengraaf and of Holmes led to the theory of plate tectonics, which replaced the theory of continental drift, and became the accepted theory in the 1960s (based on data that started to accumulate in the late 1950s).

However, acceptance was gradual. Nowadays it is universally supported; but even in 1977 a textbook could write the relatively weak: "a poll of geologists now would probably show a substantial majority who favor the idea of drift" and devote a section to a serious consideration of the objections to the theory (Davis, 1977).

References

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External links

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