Wegener Proposes the Theory of Continental Drift

Date January, 1912

Alfred Wegener proposed that all landmasses on the earth were once part of the supercontinent of Pangaea, which then fragmented, with the pieces drifting apart to form the present-day continents.

Locale Frankfurt, Germany

Key Figures

• Alfred Wegener (1880-1930), German meteorologist and earth scientist
• Frank Bursley Taylor (1860-1938), American student of geology and astronomy
• Alexander Logie Du Toit (1878-1948), South African geologist
• Arthur Holmes (1890-1965), British geologist
• Harry Hammond Hess (1906-1969), American geologist

Summary of Event

The concept of continental drift was developed, at least in part, to explain the striking parallelism between the Atlantic coasts, which seem as though they could fit together like pieces of a giant jigsaw puzzle. In particular, the fit between the eastern coast of South America and the western coast of Africa is very striking. The idea that the continents were once joined together as part of a single landmass predates Wegener’s treatise. As early as 1620, Sir Francis Bacon had discussed the possibility that the Western Hemisphere had once been joined to Africa and Europe. In 1668, P. Placet expressed similar ideas. Antonio Snider-Pellegrini, in his book La Création et ses mystères dévoilés (1859; creation and its mysteries revealed), recognized the similarities between American and European fossil plants of the Carboniferous period (about 300 million years ago) and proposed that all continents were once part of a single landmass. By the end of the nineteenth century, Austrian geologist Eduard Suess had noticed the close correspondence between geological formations in the lands of the Southern Hemisphere and had fitted them together into a single landmass he termed “Gondwanaland.” In 1908, Frank Bursley Taylor of the United States, and in 1910, Alfred Wegener of Germany independently suggested mechanisms that could account for large, lateral displacements of the earth’s crust and therefore explained how continents could be driven apart. Wegener’s work became the center of the debate that has lasted until the present.

The concept of continental drift was best expressed by Wegener in his book Die Entstehung der Kontinente und Ozeane (1912; The Origin of Continents and Oceans, 1924). He based the theory not only on the shape of the continents but also on geological evidence found around the world. Wegener specifically cited similarities in fossil fauna and flora (extinct animals and plants) found in Brazil and Africa. He developed a series of maps to show three stages in the drift process and named the original supercontinent Pangaea (a word meaning “all lands”). Wegener believed that the continents, composed of light-density granitic rocks, were independently propelled and plowed through the denser basalts of the ocean floor driven by forces related to the rotation of the earth. He provided evidence based on detailed correlations of geological features and fossils indicating a common historical record on both sides of the Atlantic. He also proposed that the supercontinent of Pangaea existed before the beginning of the Mesozoic era (about 200 million years ago).

Wegener visualized the split of Pangaea as beginning during the Jurassic period (about 190 million years ago) with the southern continents moving westward and toward the equator. South America and Africa began to drift apart during the Cretaceous period (70 million years ago). The opening of the northern Atlantic was accomplished during the Pleistocene epoch (approximately 2.5 million years ago). Greenland and Norway started to separate as recently as 1.5 million years ago. The Indian peninsula drifted northward, colliding with the Asian continent and giving rise to the folded mountains of the Himalayas. Similarly, the European Alps and the Atlas Mountains of North Africa were explained as a westward extension of the Himalayan Chain. Wegener also suggested that as the drifting continents met the resistance of the ocean floor, their leading edges were compressed and folded into mountains. In this way, he also explained the Western Cordillera of the Americas and the mountains of New Zealand and New Guinea. He visualized the tapering ends of Greenland and South America and the island arcs of the Antilles and East Asia as stragglers trailing behind the moving continents. Periods of glaciation found in the southern part of South America, Africa, Australia, peninsular India, and Madagascar provided further evidence of drift.

Detailed studies by the South African geologist Alexander Logie Du Toit provided strong support for Wegener’s concepts. Du Toit postulated two continental masses rather than the single entity of Pangaea: the northern supercontinent of Laurasia and its southern counterpart Gondwanaland, separated by a seaway called Tethys. Du Toit was also the first to propose that the continental masses of the Southern Hemisphere had moved relative to the position of the South Pole. His ideas were published in Our Wandering Continents (1937), a book he dedicated to Wegener. Both Wegener and Du Toit utilized four main lines of evidence to support continental drift: the geometric shape of the continents, matching rock types and geological structures, fossil evidence, and paleoclimatological evidence (evidence based on ancient patterns of climate). It was the last of these that led to the early conclusion that the geographic poles had shifted relative to the position of the continents through time. This “polar wandering” could best be explained by continental masses drifting over the fixed poles.

Continental drift was debated among earth scientists of the Southern Hemisphere and the leaders of geophysical thought of the Northern and Western Hemispheres. The eminent geophysicist Sir Harold Jeffreys of the University of Cambridge voiced opposition to the drift concept based on the concept that the earth’s crust and underlying mantle were too rigid to permit such large motions. Although Wegener and Du Toit had provided compelling evidence in favor of the drift theory, one monumental problem remained: What forces could be strong enough to rupture, fragment, and cause the continents to drift? It was precisely this question that resulted in the decrease in popularity of the continental drift theory until its rebirth in the 1960’s as the new seafloor spreading and plate tectonics. Wegener had visualized the continents as being raised above the seafloor on a spinning, spheroidal earth and argued that the continents had been propelled toward the equator. Drift was explained as the result of gravitational attraction between the continents and the equatorial bulge, resulting in the movement away from the poles toward the equator. A westward drift was explained as resulting from the differential attraction of the Moon and the Sun on the continents, causing them to lag behind the rotation of the earth.

Although appealing, Wegener’s theory of continental drift remained controversial and was not widely accepted until Harry Hammond Hess and Robert Sinclair Dietz introduced the theory of seafloor spreading in the early 1960’s. In fact, although the forces proposed by Wegener (and also by Taylor) are known to exist, they are minuscule and no one has considered them seriously as the mechanism capable of rupturing and moving continents. It was Du Toit who first proposed the idea that continents could slide over the mantle under the action of gravity (an idea that came to be used by some scientists in defense of plate tectonics), but Arthur Holmes of the University of Edinburgh was the originator of the now-popular concept of thermal convection in the earth’s mantle as the main cause of drift. Holmes’s model, published in 1931, is very similar to models that came to be used in the widely accepted theory of plate tectonics. Holmes was also the first to introduce the idea that the continents themselves do not play an active role in the drift, but act as passive members being carried along by a moving mantle in sort of a conveyor-belt motion.

Significance

Wegener’s theory of continental drift, first introduced during the second decade of the twentieth century, remains one of the most fascinating, inspiring, and controversial topics in the field of earth science. The drift theory polarized the scientific community and became the center of a heated debate until its rebirth during the second half of the century, when the concepts of seafloor spreading and plate tectonics were developed. Most earth scientists in the United States and around the globe subscribe to the theory of plate tectonics, a modern version of Wegener’s continental drift.

Although the theory of continental drift was supported by a variety of geological evidence, a poor understanding of the nature of the oceanic crust prevented the development of a complete theory of earth dynamics until the 1960’s. It was then that the topography of the ocean floor was mapped and magnetic and seismic characteristics established. In plate tectonics, the crust and uppermost upper mantle constitute what is now known as the lithosphere. The lithosphere lies above a soft, plastic (or ductile) zone in the mantle known as the asthenosphere. Thermal convection within the asthenosphere provides the mechanism for splitting the rigid lithosphere into distinct plates and propelling them into motion. The lithospheric plates are bounded by ocean ridges, ocean trenches, and major faults. At the ocean ridges, plates move apart where the convecting mantle rises and spreads laterally. It is at these divergent boundaries that a new ocean floor is generated. The moving plates converge at the site of oceanic trenches, where the basaltic (oceanic) lithosphere descends into the mantle under the lighter-density, granitic (continental) lithosphere. In those regions where two lithospheric plates move past each other, large transform fault boundaries are generated.

Plate tectonics provides an appealing explanation for zones of earthquakes, active volcanism, and mountain building. These events take place along plate boundaries, where they either diverge, collide, or move relative to each other. The theory has gained wide acceptance among earth scientists worldwide. Its concepts are deeply rooted on Wegener’s original ideas of continental drift. The new theory provides answers to many of the critical questions of global tectonics, but, as commonly occurs in science, new questions and problems continue to arise. Just as in Wegener’s time, notable and respected workers voiced opposition to the new theory. V. V. Belousov of the Soviet Union and Arthur Augustus Meyerhoff of the United States both provided well-founded and thought-provoking arguments against the new theory, but they failed to provide a theory of global tectonics as well integrated as plate tectonics. Partly as a result of this, continental drift and its modification into the concepts of seafloor spreading and plate tectonics remained one of the most significant theories in earth science. Although it is not without its problems, the theory is one of the most complete explanations of global tectonics in existence.

Bibliography

Du Toit, Alexander Logie. Our Wandering Continents. New York: Hafner, 1937. Du Toit provides strong support to Wegener’s ideas on continental drift and advances important concepts that have gained acceptance in the theory of plate tectonics.

Stein, Seth, and Michael Wysession. An Introduction to Seismology, Earthquakes, and Earth Structure. New York: Blackwell Science, 2002. Introductory textbook intended for advanced undergraduate and first-year graduate courses in seismology. Heavily illustrated. Includes suggestions for further reading and index.

Tarbuck, Edward J., and Frederick K. Lutgens. The Earth: An Introduction to Physical Geology. 8th ed. Upper Saddle River, N.J.: Prentice Hall, 2004. Introductory-level text offers a very good overview of the earth’s interior. Well written and illustrated with color graphics.

Taylor, Frank Bursley. “Bearing of the Tertiary Mountain Belt on the Origin of the Earth’s Plan.” Geological Society of America 21 (1910): 179-226. Classic reference outlines the concept of continental drift developed by Taylor independently but concurrent with the timing of Wegener’s treatise.

Wegener, Alfred Lothar. The Origin of Continents and Oceans. Translated by J. G. A. Skerl. New York: E. P. Dutton, 1924. This is the original treatise on continental drift, a monumental and revolutionary contribution to the field of earth science, and a key reference that enhances the understanding of global tectonics.

Wilson, J. Tuzo. “A Revolution in Earth Science.” Geotimes 13 (December, 1968): 10-16. In this excellent article, Wilson summarizes the importance of continental drift, seafloor spreading, and plate tectonics. The theory’s impact on Charles Darwin’s contribution to biology and major and revolutionary advances in other sciences are compared also.

Wyllie, Peter J. The Dynamic Earth: Textbook in Geosciences. New York: John Wiley & Sons, 1971. This outstanding reference contains separate chapters on continental drift, seafloor spreading, and plate tectonics. It also provides a comprehensive discussion of the evolution of geologic thought and an understanding of the earth’s tectonic framework.