Alfred Wegener Introduces the Concept of Continental Drift

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Alfred Wegener Introduces the Concept of Continental Drift

Overview

The theory of continental drift holds that the great landmasses are slowly moving, and have done so continually over the long span of geologic time. According to the theory, a single landmass called Pangaea split up about 200 million years ago, and the resulting continents eventually drifted to their present locations. The German meteorologist Alfred Wegener (1880-1930) introduced the theory of continental drift in 1912.

Background

Looking at a world map, it is easy to see that the edges of Europe and North America, Africa and South America, would fit together nicely, almost like pieces in a jigsaw puzzle. This was noticed almost as soon as Europeans began traveling to the Americas; Francis Bacon wrote of it in 1620. But the early mapmakers had no real understanding of how the Earth formed and changed over time, and made no attempt to explain the shapes of the continents.

Nineteenth-century naturalists began to find other intriguing evidence linking opposite sides of the Atlantic Ocean. Certain rock formations stopped at the western coast of Africa and picked up again on the eastern coast of South America. In 1858, Antonio Snider, an American living in Paris, cited similarities between fossil plants and proposed that Europe and North America had once been part of the same landmass. He suggested that the continents had been separated by the biblical flood.

On December 29, 1908, the American geologist Frank B. Taylor first suggested the slow drifting of continents in a presentation to the Geological Society of America. He described a scenario in which two large sheets of the Earth's crust, originally centered at the North and South poles respectively, crept inexorably forward like the glaciers of the Ice Age, leaving behind great tears that became the South Atlantic and Indian Oceans and the Arctic Basin. His ideas went largely unnoticed.

The man who is remembered as the "father of continental drift" is the German professor Alfred Wegener. Wegener was an adventurer who enjoyed exploring and going aloft in balloons. His field of study was meteorology, but it is for his contributions to geology that he is best remembered. He put forth a theory of moving continents "not for the first time," commented a Mr. F. Debenham, attending a 1923 presentation by Wegener at the Royal Geographical Society in London, "but for the first time boldly." He also marshaled a great deal of evidence in its defense.

Impact

Wegener was intrigued by the fit of the Atlantic coastlines, and had read about the similarities in the American and European fossils. In 1912, he proposed the theory that the continents had drifted into place, and elaborated upon it in his book The Origins of Continents and Oceans (1915).

The theory of continental drift contradicted the way most geologists of the time believed the crust of the Earth had taken shape. The standard view was that the Earth had formed in a molten state and, as it cooled, it contracted. This contraction was thought to have resulted in the sinking of the ocean basins and the crumbled crust of mountain ranges, just as the skin of an apple becomes wrinkled and pitted as the fruit dries up and shrinks.

Some scientists were willing to listen to Wegener, however, in part because the contraction theory was itself crumbling. They now knew that recently discovered radioactive elements in the Earth's crust, such as radium, would decay and produce heat that would build up below the surface. This continual source of heat meant that cooling, and therefore contraction, was unlikely.

Wegener began accumulating evidence to support his theory. First, there were the fossils. For example, skeletons of the small reptile Mesosaurus, about 270 million years old, had been found in only two places, Brazil and South Africa. It strained credulity to think that this reptile could swim the Atlantic, but if its range was indeed this large, it should have been found in many places.

Living animals were also distributed in ways that were suggestive. Lemurs, for instance, the most primitive primates, are found only in southeast Africa, the nearby island of Madagascar, and across the Indian Ocean in south Asia. The usual explanation for these oddities in animal distribution were ancient land bridges that had since sunk beneath the ocean. However no traces of such bridges were found, nor was a convincing reason advanced for them to sink if their material had been lighter than the crust of the ocean floor to begin with. Very old rock formations, too, matched up on both sides of the ocean, and they certainly did not wander across any land bridge.

Finally, Wegener was able to employ his own discipline, meteorology, by seeking evidence of climatic conditions millions of years ago. The remains of tropical plants found in coal samples from Alaska provided evidence of a warm climate in the past. Large boulders deposited by glaciers were evidence of a polar climate in what is now Africa. In 1915, Wegener proposed that the continents had split off from a single landmass he called Pangaea, or "All Land." Surrounding Pangaea was a great ocean, which would later become the Pacific.

Wegener believed that Pangaea began breaking up about 40 million years ago because of forces created by slight irregularities in the Earth's rotation around its axis. Many scientists rejected this theory because his explanation could not begin to account for the movement of continents—the forces were far too weak. A mistaken claim that Greenland had moved 120 feet (36 m) in a single year, based on erroneous data, led to widespread ridicule, and the fact that Wegener was not a geologist did not help his cause. When he died in 1930 on a dogsled expedition to Greenland, the idea of continental drift all but died with him. It was revived after World War II, when new scientific techniques provided not only new data but, finally, a plausible explanation.

In the 1950s, paleomagnetic evidence supporting continental drift became available. When rock is in a molten state, the magnetic particles it contains are free to move around, and they orient themselves toward the Earth's magnetic poles. Once the rock cools and hardens, the orientation of the individual magnetic particles becomes fixed. When the position of the rock changes, the magnetic polarity changes with it. By studying the magnetic properties of ancient rocks, the British geophysicist S.K. Runcorn was able to determine that Europe and North America were at one time connected.

At about the same time, oceanographers had mapped features such as deep trenches, arcs of islands, and ridges or mountain ranges on the ocean floor. Seismologists noted that volcanoes tended to occur along the ocean ridges, and that earthquakes were common beneath the ocean trenches.

In 1961, the American scientists H.H. Hess and Robert Dietz explained continental drift in terms of sea-floor spreading. Their hypothesis was confirmed in 1963 by the British geophysicists F.J. Vine and D.H. Mathews using paleomagnetic measurements. In sea-floor spreading, molten rock from a layer of the Earth's interior called the mantle rises up through convection currents and breaks through the crust at the ocean ridges. It pushes out from the ridges as it hardens, spreading the ocean floor and forcing the continents farther apart.

While the rocks on the Earth's surface are billions of years old, the rocks on the ocean floor have existed for no more than 200 million years. This figure has thus replaced Wegener's 40 million year estimate for the date of Pangaea's breakup. The first split created a northern continent called Laurasia, and the southern landmass scientists named Gondwanaland. Further breaks resulted in the familiar continents of today.

The upper surface of the Earth's crust, or lithosphere, moves like a number of rigid plates floating on a soft zone of the mantle called the asthenosphere. As the plates collide, their edges crumble, pushing up to form mountain ranges or down into the mantle to create ocean trenches, island arcs—and earthquakes. The study of these changes in the Earth's surface is called plate tectonics.

Plate tectonics allows scientists to predict what the continents might look like millions of years in the future. The California coast may separate from the mainland and head toward Alaska. Australia may also move north until it runs into Asia. The size of the Atlantic Ocean might increase, with Africa and South America moving farther apart. We shouldn't expect to see any drastic changes in our lifetimes, however, as the continents move at a rate of only a few centimeters per year.

SHERRI CHASIN CALVO