continental drift


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continental drift,

geological theory that the relative positions of the continents on the earth's surface have changed considerably through geologic time. Though first proposed by American geologist Frank Bursley Taylor in a lecture in 1908, the first detailed theory of continental drift was put forth by German meteorologist and geophysicist Alfred WegenerWegener, Alfred Lothar
, 1880–1930, German geologist, meteorologist, and Arctic explorer. Early in his life, he was on the staff of the aeronautical observatory at Lindenberg; was a professor of geophysics and meteorology at Hamburg from 1919 to 1924; was professor of
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 in 1912. On the basis of geology, biology, climatology, and the alignment of the continental shelf rather than the coastline, he believed that during the late PaleozoicPaleozoic era
, a major division (era) of geologic time (see Geologic Timescale, table) occurring between 570 to 240 million years ago. It is subdivided into six periods, the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian (see each listed individually).
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 and early MesozoicMesozoic era
[Gr.,=middle life], major division of geologic time (see Geologic Timescale, table) from 65 to 225 million years ago. Great crustal disturbances that marked the close of the Paleozoic and the beginning of the Mesozoic eras brought about drastic changes in the
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 eras, about 275 to 175 million years ago, all the continents were united into a vast supercontinent, which he called Pangaea. Later, Pangaea broke into two supercontinental masses—Laurasia to the north, and Gondwanaland to the south. The present continents began to split apart in the latter Mesozoic era about 100 million years ago, drifting to their present positions.

As additional evidence Wegener cited the unusual presence of coal deposits in the South Polar regions, glacial features in present-day equatorial regions, and the jigsaw fit of the opposing Atlantic continental shelves. He also pointed out that a plastic layer in the earth's interior must exist to accommodate vertical adjustments caused by the creation of new mountains and by the wearing down of old mountains by erosion (see continentcontinent,
largest unit of landmasses on the earth. The continents include Eurasia (conventionally regarded as two continents, Europe and Asia), Africa, North America, South America, Australia, and Antarctica.
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). He postulated that the earth's rotation caused horizontal adjustment of rock in this plastic layer, which caused the continents to drift. The frictional drag along the leading edges of the drifting continents results in mountain building.

Wegener's theory stirred considerable controversy during the 1920s. South African geologist A. L. Dutoit, in 1921, strengthened the argument by adding more exacting details that correlated geological and paleontological similarities on both sides of the Atlantic. In 1928, Scottish geologist Arthur Holmes suggested that thermal convection in the mantle was the mechanism that drove the continental movements. American geologist David Griggs performed scale model experiments to show the mantle movements.

The theory of continental drift was not generally accepted, particularly by American geologists, until the 1950s and 60s, when a group of British geophysicists reported on magnetic studies of rocks from many places and from each major division of geologic time. They found that for each continent, the magnetic pole had apparently changed position through geologic time, forming a smooth curve, or pole path, particular to that continent. The pole paths for Europe and North America could be made to coincide by bringing the continents together.

See plate tectonicsplate tectonics,
theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.
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; seafloor spreadingseafloor spreading,
theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges. First proposed in the early 1960s by the American geologist Harry H.
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.

Bibliography

See E. H. Colbert, Wandering Lands and Animals: The Story of Continental Drift and Animal Populations (1985); T. H. Van Andel, New Views on an Old Planet: A History of Global Change (2d ed. 1994); W. Sullivan, Continents in Motion: The New Earth Debate (1995); N. Oreskes, The Rejection of Continental Drift: Theory and Method in American Earth Science (1999).

continental drift

[¦känt·ən¦ent·əl ′drift]
(geology)
The concept of continent formation by the fragmentation and movement of land masses on the surface of the earth. Also known as continental displacement.

continental drift

Geology the theory that the earth's continents move gradually over the surface of the planet on a substratum of magma. The present-day configuration of the continents is thought to be the result of the fragmentation of a single landmass, Pangaea, that existed 200 million years ago

continental drift

In 1980 David Turner remarked that KRC ran "at the speed of the continental drift".
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