DRIFT encyclopedia topics

drift, deposit of mixed clay, gravel, sand, and boulders transported and laid down by glaciers. Stratified, or glaciofluvial, drift is carried by waters flowing from the melting ice of a glacier. The flowing water sorts the particles, generally depositing layers of coarser particles nearer the point of origin. Till, or boulder clay, which makes up the greater part of the drift, is unstratified, consisting of disorganized heaps of rocks that range widely in size. Till is deposited directly by the glacier itself without water transport. The drift may take the form of a drumlin, a kame, an esker, a moraine, or an outwash plain; its thickness varies noticeably from place to place and is not dependent upon topographical factors. Presence of drift proved useful in establishing the existence of time periods when large parts of the surface of continents were covered with glaciers (see glacial periods). Large sections of continental Europe and North America are covered by drift.

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continental drift

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 Wegener 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 Paleozoic and early Mesozoic 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 continent). 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 tectonics; seafloor spreading.

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).

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Licensed from Columbia University Press

North Atlantic Drift

North Atlantic Drift, warm ocean current in the northern part of the Atlantic Ocean. It is a continuation of the Gulf Stream, the merging point being at lat. 40°N and long. 60°W. Off the British Isles it splits into two branches, one going south (the Canary Current) and the other going north along the coast of W and N Europe, where it exerts considerable influence upon the climate as far as northwestern Europe.

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Licensed from Columbia University Press

genetic drift

Change in the pool of genes of a small population that takes place strictly by chance. Genetic drift can result in genetic traits being lost from a population or becoming widespread in a population without respect to the survival or reproductive value of the gene pairs (alleles) involved. A random statistical effect, genetic drift can occur only in small, isolated populations in which the gene pool is small enough that chance events can change its makeup substantially. In larger populations, any specific allele is carried by so many individuals that it is almost certain to be transmitted by some of them unless it is biologically unfavourable.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

continental drift

The theory of continental drift is based on the concept that the continental and oceanic crusts are elipsis

Large-scale movements of continents over the course of geologic time. The first complete theory of continental drift was proposed in 1912 by Alfred Wegener, who postulated that a single supercontinent, which he called Pangea, fragmented late in the Triassic Period (approximately 250–200 million years ago) and that the parts began to move away from one another. He pointed to the similarity of rock strata in the Americas and Africa as evidence to support his hypothesis. Wegener's ideas received support from the concepts of seafloor spreading and plate tectonics beginning in the 1960s. The modern theory states that the Americas were joined with Europe and Africa until circa 190 million years ago, when they split apart along what is now the Mid-Atlantic Ridge. Subsequent tectonic plate movements took the continents to their present positions.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.