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Continental Drift

Continental Drift

If you have ever looked at a map of the Atlantic Ocean, you have probably noticed that the coastlines of Africa and South America seem to fit together like pieces of a jigsaw puzzle. The fit between the two coastlines is even better when the edges of the continental shelf are compared. For many years, scientists thought this was just a coincidence, because no one could think of a way that the continents could slide around.

Evidence for Continental Drift

Evidence that South America and Africa might once have been joined to each other came from the research of the German geographer, Alexander von Humboldt. Von Humboldt traveled throughout South America, Africa, and other parts of the world, collecting plant and animal specimens and studying geography and geology. He observed many similarities between South America and Africa in addition to the apparent fit of continental coastlines. For example, von Humboldt noticed that the mountain ranges near Buenos Aires, Argentina, match mountain ranges in South Africa.

Other mountain ranges in Brazil extend to near the seashore and stop. Similar mountain ranges begin at the corresponding seashore in Ghana in Africa. All of these mountain ranges appear to have the same age and to be formed of the same kinds of rock. The rock strata in these and other mountain ranges would match perfectly if the coastlines of the two continents were lined up. Von Humboldt also observed similar patterns among mountain ranges in Europe and North America.

Von Humboldt and other naturalists also noticed many similarities among fossils of plants and animals on either side of the Atlantic. Although fossil species in eastern South America are somewhat different from fossil species in western Africa, their similarities are often striking. Before long, similarities across other oceanic gaps were observed. Plant and animal fossils found in India, for example, are often remarkably similar to those found in Australia.

Another important piece of evidence was discovered in the early twentieth century. When molten lava freezes, it preserves traces of Earth's magnetic field. Basalt, which freezes deep underground, also records Earth's magnetic field at the time the basalt cooled. Measurements of the direction of Earth's magnetic field from many different rocks of different ages on different continents indicate either that Earth's magnetic poles have moved all over the planet or that the continents themselves have moved.

Continental drift was first proposed in 1908 by American geologist Frank B. Taylor. However, Taylor's paper was mostly ignored and soon forgotten. Then a German meteorologist, Alfred Wegener, began working on a theory of continental drift. By 1912 Wegener had developed a theory suggesting that continental rocks were stronger and lighter than seafloor rocks. He also suggested that the seafloor rocks were like very thick tar. He concluded that the stronger continents were able to drift around on the weaker seafloor rocks.

Furthermore, Wegener thought the continents had once been part of a single large land mass, which he called Pangaea. Initially, he asserted, the original land mass had broken into two parts, two supercontinents, which he called Gondwanaland and Laurasia. Over millions of years, he suggested, Gondwanaland had broken apart into South America, Africa, India, Australia, and Antarctica, while Laurasia separated into North America and Eurasia.

Unfortunately, Wegener could not suggest any mechanism that would have caused the continents to break apart and move around in this way. In contrast to Taylor's experience, Wegener's theory was met with rejection and open hostility by other scientists, probably because Wegener was a meteorologist, not a geologist.

In the mid 1930s, however, Wegener's ideas were resurrected and rehabilitated. Scientists had discovered a ridge down the middle of the Atlantic seafloor through which hot lava was flowing upward and spreading outward. Stripes of lava on either side of this ridge were progressively older the farther away they were from the ridge. This pattern of stripes of lava strongly suggested to the scientists that the floor of the Atlantic Ocean was getting steadily wider. The discovery of this spreading of the seafloor, along with other discoveries, eventually led to the modern theory of plate tec-tonics .

Plate Tectonics

Wegener's idea of continental drift had the continents floating around on semisolid oceanic rock. In contrast, plate tectonics suggests that Earth's entire crust is composed of a number of large plates that are in constant motion relative to each other. Some plates are sliding under other plates, some are sliding past each other, others are pulling apart, and still others are colliding. Each of these types of interactions produces unique geological consequences. The Himalayas are formed as two continental plates collide. Along the northwest coast of North America, an oceanic plate is sliding under the North American plate. The resulting geological characteristic is a chain of volcanoes. As one plate is forced under the other, friction causes enormous amounts of heat that builds up until a volcano forms and erupts. Earthquakes are often the result of sudden movement of two adjacent plates. The plates "lock up" until enough force is generated to break them apart, causing the quake. One of the world's most famous earthquake zones, the San Andreas Fault, lies at the boundary of the Pacific and the North American plates.

After being initially rejected and ridiculed, the concept of continental drift (and plate tectonics) is now widely accepted as one of the fundamental unifying ideas of geology. This shift in thinking among geologists depended not only on the discovery of an adequate explanation for continental movement (seafloor spreading, rifts, and trenches) but also on the discovery of more and more similarities between continents.

Evolution and Biological Diversity

Early explorers, mapmakers, and traders were often accompanied on their travels by naturalists (people who studied all the natural sciences). These naturalists made two striking observations. They found that fossils of exactly the same plants and animals were located on continents that are separated by thousands of miles of oceans. For example, the tropical fossil fern, Glossopteris, was found in South America, Africa, India, and Australia. Similarly, fossils of the land vertebrate, Kannemeyrid, were found in Africa, North and South America, and Asia.

While the ancient fossils on different continents were often similar or identical, the exploring naturalists were finding out that living plants and animals on the different continents were often very different. The naturalists were discovering whole new groups of animals and plants on nearly every island and continent they visited. Most biological species seemed to be unique to the region or continent in which they were found. How could these seemingly contradictory observations be reconciled? Plate tectonics provided the answer. When the different land masses were connected, the same or closely related plants and animals inhabited each. After the land masses were separated, the different populations were geographically isolated from each other by great distances of ocean. Life on the different continents had apparently evolved into different species, because the populations were isolated from each other by such great distances.

It is possible to correlate, or link, the breakup of the continents with the types of animals found on each. The longer the period of separation, the more differences between species were found. For example, all of the indigenous (native) mammals found in Australia are marsupials. There are no naturally occurring placental mammals. This suggests that Australia broke away before placental mammals had evolved. In geographic isolation from the rest of the world, Australia's mammals were able to evolve into many highly sophisticated forms found nowhere else.

Has the diversity of life on Earth increased as a result of the breakup of the supercontinents? This idea was first proposed in 1970 by the American geologists James W. Valentine and Eldridge M. Moores. They suggested that the diversity of life increased as continents broke up and moved apart and decreased as land masses moved together and joined.

Since 1970 the study of plate activity as a force in the evolution of life has substantially added to our understanding of evolution. For example, during the Permian period (around 286 million years ago), there was a decrease in the variety of species of animals living in the shallow seas around Pangaea. In contrast, when the Atlantic Ocean began to open during the middle Mesozoic era (144 million years ago), the differences between the species living on opposite shores gradually increased. The greater the distance, the smaller the number of families in common. Differences accumulated more rapidly in the South Atlantic than in the North Atlantic, because a land connection between Europe and North America remained until the Cenozoic era (66 million years ago). The opposite happened when North and South America became connected at the Isthmus of Panama. In South America, there were many different marsupials and few large predators. After the isthmus emerged, many large herbivores migrated south. They adapted well to the new environment and were more successful than the local fauna in competing for food. Large predators also moved south and contributed to the extinction of at least four orders of South American land mammals. Only a few species, such as the armadillo and the opossum, migrated in the opposite direction. Many of the invading northerners, such as the llama and tapir, died out in North America and are now found only in the south.

see also Biogeography.

Elliot Richmond

Bibliography

Cox, Allan, ed. Plate Tectonics and Geomagnetic Reversals. San Francisco: W. H. Freeman, 1973.

Dott, Robert H., Jr., and Roger L. Batten. Evolution of the Earth, 3rd ed. New York: McGraw-Hill, 1994.

Faul, Henry, and Carol Faul. It Began with a Stone: A History of Geology from the Stone Age to the Age of Plate Tectonics. New York: Wiley, 1983.

Foster, Robert. Geology, 3rd ed. Columbus, OH: Charles E. Merrill, 1976.

Hallam, Anthony. A Revolution in the Earth Sciences: From Continental Drift to Plate Tectonics. Oxford, U.K.: Clarendon Press, 1973.

Matthews, William H., et al. Investigating the Earth, 3rd ed. Boston: Houghton Mifflin, 1981.

Stanley, Stephen. Earth and Life through Time. New York: W. H. Freeman, 1989.

Toulmin, Stephen, and June Goodfield. The Discovery of Time. New York: Harper and Row, 1965.

Uyeda, Seiya. The New View of the Earth: Moving Continents and Moving Oceans. San Francisco: W. H. Freeman, 1978.

Wilson, J. Tuzo, ed. Continents Adrift and Continents Aground. San Francisco: W. H. Freeman, 1976.

Wood, Robert Muir. The Dark Side of the Earth: The Battle for the Earth Sciences, 1800-1980. London: Allen and Unwin, 1985.

Wyllie, Peter J. The Way the Earth Works: An Introduction to the New Global Geology and Its Revolutionary Development. New York: Wiley, 1976.

A marsupial is a member of the mammalian subclass Metatheria, which includes a wide variety of mammals that give birth to undeveloped young. The young complete their development outside the mother's body, attached to a nipple. Most marsupials have a pouch that covers the nipples and protects the young while they are developing.

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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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continental drift

continental drift The theory that the earth's continents once formed a single mass and have since moved relative to each other. It was first postulated by A. Snider in 1858 and greatly developed by Alfred Wegener (1880–1930) in 1912. He used evidence, such as the fit of South America into Africa and the distribution of rock types, flora, fauna, and geological structures, to suggest that the present distribution of the continents results from the breaking up of one or two greater land masses. The original land mass was named Pangaea and it was suggested that this broke up into the northerly Laurasia and the southerly Gondwanaland (see illustration). The theory was not accepted for about 50 years by the majority of geologists but during the early 1960s, the seafloor-spreading hypothesis of Harry Hess (1906–69) and the subsequent development of plate tectonics produced a mechanism to explain the drift of the continents.

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continental drift

continental drift Theory that the continents change position very slowly, moving over the Earth's surface at a rate of a only a few centimetres per year, adding up to thousands of kilometres over time. Early supporters of continental drift claimed that the jigsaw shapes of the present day continents could be pieced together to form an ancient land mass which, at sometime in the past, split and drifted apart. Evidence for this theory included matching the outlines of continents, rock types, geological structures and fossils. Continental drift became accepted with the development of plate tectonics in the 1960s. In recent years, continental movement has been measured by global positioning satellites. See also Gondwanaland; Pangaea

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continental drift

continental drift Hypothesis proposed around 1910 to describe the relative movements of continental masses over the surface of the Earth. A major theorist of continental drift, and certainly the one who gave the hypothesis scientific plausibility, was Alfred Wegener (1880–1930). His work was based on qualitative data, but has been vindicated in recent years by the development of the plate tectonics theory, which has provided geologists with a viable mechanism to account for continental movements. See also polar wander.

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continental drift

continental drift The hypothesis proposed around 1910 to describe the relative movements of continental masses over the surface of the Earth. A major theorist of continental drift, and certainly the one who gave the hypothesis scientific plausibility, was Alfred Wegener (1880–1930). His work was based on qualitative data, but has been vindicated in recent years by the development of the plate tectonics theory, which has provided geologists with a viable mechanism to account for continental movements.

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continental drift

con·ti·nen·tal drift • n. the gradual movement of the continents across the earth's surface through geological time.See plate tectonics.

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