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.

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 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.^[1] 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^{[citation needed]} that all the continents had once been joined into a supercontinent (now 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 use the phrase "continental drift" (in German "die Verschiebung der Kontinente") and 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.^[2]

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.

Evidence

For more details on this topic, see Plate tectonics.

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

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.

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 deposits called tillites, 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, in modern coordinates, and was a good indicator of the fact that the southern continents had previously been in dramatically different locations, as well as contiguous with each other.

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.^[3] 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.^[4]

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 fact, the biggest objection to Wegener was that he 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.^[5] 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.

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,^[6] 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.^[7]

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.^{[citation needed]}

The fastest recorded seafloor spreading takes place along the East Pacific Rise at 17.2 cm per year.^{[citation needed]}

Notes and references

• Le Grand, H. E. (1988). Drifting Continents and Shifting Theories. Cambridge University. ISBN 0-521-31105-5. 

External links

• A brief introduction to Plate Tectonics, based on the work of Alfred Wegener.
• Maps of continental drift, from the Precambrian to the future
• Four main evidences of the Continental Drift theory
• Wegener and his proofs

Donate to Wikimedia