Continental drift

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Image:Pangea animation 03.gif The concept of continental drift was first proposed by Alfred Wegener. In 1912 he noticed that the shapes of continents on either side of the Atlantic Ocean seem to fit together (for example, Africa and South America). Francis Bacon, Antonio Snider-Pellegrini, Benjamin Franklin, and others had noted much the same thing earlier. The similarity of southern continent fossil faunae and some geological formations had led a relatively small number of Southern hemisphere geologists to conjecture as early as 1900 that all the continents had once been joined into a supercontinent known as Pangaea. The concept was initially ridiculed by most geologists, who felt that an explanation of how a continent drifted was a prerequisite and that the lack of one made the idea of drifting continents wholly unreasonable. The theory 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 as theory until the 1950s in Europe. 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 acceptance among North American 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.

1 Various data
2 Evidence for continental drift
3 Permo-Carboniferous
4 The debate over continental drift
5 Further reading
6 References
7 External links

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Various data

South America and Africa are moving apart at an average of 5.7 cm per year, due to the seafloor 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.

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Evidence for continental drift

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Sir Francis Bacon first noticed this peculiarity in the 17th century.

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

Image:Snider-Pellegrini Wegener fossil map.gif 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. For example the fossils of the freshwater crocodile found in Brazil and South Africa. Another illustrative example 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 shapes 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.

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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 this 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) (Rahul Megharaj 1985).

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The debate over continental drift

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 counterarguments and evidence contrary to his theory and seemed too willing to interpret ambiguous evidence as being favorable to his theory (Williams 2000:59).

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).

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References

Davis, Richard A. (1977). Principles of Oceanography, 2nd edition, Addison-Wesley. ISBN 0-201-01464-5
Friedlander, Michael W. (1995). At the Fringes of Science, pages 21-27, Westview, ISBN 0-8133-2200-6. 1998 edition with new epilog: ISBN 0-8133-9060-5.
Sagan, Carl. (1997). The Demon-Haunted World, Science As a Candle in the Dark. Ballantine Books, ISBN 0345409469. 1996 hardback edition: Random House, ISBN 0-394-53512-X. 302-03.
William F. Williams, editor (2000). Encyclopedia of Pseudoscience: From Alien Abductions to Zone Therapy. Facts on File. ISBN 0-8160-3351-X.