Continental drift

Continental drift—the horizontal displacement or rotation of continents relative to one another—is the modern paradigm that describes and accounts for the distribution of present-day continents and associated geological formations and phenomena, including mountain ranges, mineral deposits, volcanoes, and earthquakes.

Early Continental Drift Theories

Continental drift is the guiding model for the mechanisms driving the geologic forces near the surface of the earth. This theory is the simplest explanation for the behavior of the earth's crust and the distribution of continents and their associated topographic features. The theory is useful not only in decoding the history of the earth but also in predicting future geologic occurrences.

The idea that continents may have occupied different locations in the past was first fully developed by German geophysicist Alfred L. Wegener as early as 1910. Wegener observed that the coastlines of the Americas corresponded with those of Europe and Africa in a jigsaw-puzzle fashion, building off of similar observations beginning with those of cartographer Abraham Ortelius in the late sixteenth century. He learned that similar fossils had been discovered on both sides of the Atlantic Ocean, and he proposed the idea of continental drift, in which the splitting up of a supercontinent and the drift of its pieces could explain these data. Wegener continued to refine his ideas and published a book, Die Entstehung der Kontinente und Ozeane (1915; The Origin of Continents and Oceans, 1924). By his account, about 200 million years ago, at the end of the Permian period, there existed a single supercontinent that Wegener called Pangaea. This supercontinent, he theorized, broke apart, and the various pieces drifted; for example, North America and South America moved westward from Europe and Africa, creating the Atlantic Ocean.

Wegener had an American rival, Frank B. Taylor, who in 1910 published his own theory of mobile continents. Interestingly, Taylor's starting point was not the physical similarity of the Atlantic coastlines but the pattern of mountain belts in Eurasia and Europe. Yet Taylor's hypothesis, like Wegener's, soon faded from scientific memory.

There was, however, much physical evidence to suggest that continental drift was a feasible theory. First, the physical fit of the coastlines of the modern-day continents was a good one. In addition, the discovery of similar fossils, mineral deposits, glacier deposits, and mountain ranges seems to show a correlation across the oceans. Wegener described these correlations: “It is just as if we were to refit the torn pieces of a newspaper by matching their edges and then check[ing] whether the lines of print run smoothly across.” Efforts to confirm the hypothesis were interrupted by World War I and, soon thereafter, the Depression and World War II. The theory of continental drift thus retained its marginal status until the 1950s.

The major stumbling block to the theory's acceptance was the need to describe a plausible mechanism for driving the motion of the continents. Pushing continents around requires a tremendous amount of energy, and no model proposed was acceptable to the geophysics community. The only plausible suggestion came from British geologist Arthur Holmes, who tentatively proposed that thermal convection within the mantle could split the continents and drive them across the surface. Holmes was one of the most respected geologists of his time, and the scientific community paid attention to his idea. Yet he could offer no evidence to support it, and continental drift thus remained merely an interesting possibility in the eyes of most scientists.

Wegener had a more difficult time gathering an audience. To most of the geologic community, he seemed to be an outsider attempting to restructure the science. For example, in the publication of the 1928 American Association of Petroleum Geologists symposium, R. T. Chamberlain quotes a remark made by a colleague:

“If we are to believe Wegener's hypothesis we must forget everything which has been learned in the last 70 years and start all over again.”

That, however, is exactly what happened in the 1950s, as the strength of the hypothesis eventually became evident. The hypothesis thus lived on the fringes of the scientific community and was supported by a minority of geologists, most of whom worked in the Southern Hemisphere.

Worldwide interest in the origins and evolution of the planet's features culminated in the observation of the International Geophysical Year from July 1957 to December 1958. The result of this effort was that in almost every area of research, and especially in geology, scientists found the earth and particularly its oceans to be very different from what they had imagined. One of the most interesting features studied was the Mid-Atlantic Ridge, an investigation that would lead to the understanding of plate tectonics.

The existence of a submarine ridge in the Atlantic had been recognized in the 1850s by Matthew Maury, director of the US Navy's Department of Charts and Instruments. The British expedition aboard the HMS Challenger (1872–1876) also recorded a submarine mountain. The next advance came in the 1920s with a German expedition led by Nobel laureate Fritz Haber. The expedition utilized an echo sounder to map the ocean floor. In 1933, German oceanographers Theodor Stocks and Georg Wust produced a detailed map of the ridge, and they noted a valley that seemed to be bisecting it. In 1935, geophysicist Nicholas H. Heck found a strong correlation between earthquakes and the Mid-Atlantic Ridge.

Oceanic exploration resumed after World War II as a predominantly American venture. The data collected pointed to an array of seemingly unrelated phenomena. In 1950, Maurice Ewing of the Lamont-Geological Observatory discovered that no continental crust existed beneath the ocean basins. In 1952, Roger Revelle, the director of the Scripps Institute of Oceanography, and his student A. E. Maxwell measured the heat flow from the earth's interior and discovered that it was hotter over the oceanic ridges. Additional data from Jean P. Rothe, director of the International Bureau of Seismology, revealed a continuous belt of earthquake centers associated with this submarine mountain range, which extends from Iceland through the mid-Atlantic, around South Africa, and into the Indian Ocean to the Red Sea. In 1956, Maurice Ewing and Bruce C. Heezen mapped a large area of this submarine mountain range and confirmed the existence of a rift valley bisecting the mountain crest. A peculiar faulting style was discovered in association with the range in 1959 by Victor Vacquier. The mountain range was offset by a large transverse fault that ran for hundreds of miles but did not extend into the continents. In 1961, Ewing and Mark Landisman discovered that this ridge system extended throughout the world's oceans, was seismically and volcanically active, and was mostly devoid of sediment cover.

The paleomagnetic research of University of Manchester scientist Patrick M. S. Blackett and his student Keith Runcom proved central to understanding the relationships among these phenomena. Their studies of fossil magnetism suggested that the position and polarity of the earth's magnetic field had once been very different from its present orientation. These data could only make sense if one assumed that the continents had shifted relative to the poles and to one another.

By the end of the 1950s, it was clear that then-current geologic theories had failed to predict or explain these seemingly unrelated phenomena, and the new data required a new theory. In 1960, Harry Hammond Hess proposed a simple model to explain the data. He suggested that seafloor spreading powered by convection currents within the mantle might be the cause of the motion of the continents. Hess's theory, though simple, was radical; it bore out Chamberlain's earlier insight that previous geologic models would have to be discarded and that the geologic community would have to reinterpret and test all of its data in the light of the new model. This did not come easily to the science community. However, the theory of plate tectonics, incorporating continental drift, eventually emerged as the dominant paradigm for the earth sciences.

History of Drift Theories

The lengthy process leading to the acceptance of the theory of continental drift is not unusual in the history of science. Often, a hypothesis has to wait upon the development of technology or upon accidents of timing for its observations to be tested.

The observation that there was a relationship between the coastlines of the Americas and those of Europe and Africa can be traced to Ortelius as well as English philosopher and scientist Francis Bacon, who in 1620 noted the similarities of shape. The idea was further enhanced by a French monk, François Placet, who in 1666 suggested that the earth's landmasses had split as a result of the biblical flood, thus separating Europe and Africa from the Americas. The idea was repeated by a German theologian, Theodor Lilenthal, during the eighteenth century. In 1800, Alexander von Humboldt suggested that the oceans had eroded the land to further divide the continents. The drift theory was then supported by Evan Hopkins, who in 1844 proposed the existence of a “magnetic fluid” that circulated to drive the continents. In 1857, American geologist Richard Owen published Key to the Geology of the Globe, in which he proposed that the earth was originally a tetrahedron that had expanded in a great cataclysm, breaking the crust and expelling the Moon from the Mediterranean.

These ideas and hypotheses were interesting but largely unsupported by physical evidence. Wegener's and Taylor's hypotheses, proposed in the early twentieth century, were thus fundamentally different from those of the past. Still, it was the development of new technological tools that illuminated the phenomena and eventually supported the drifting continent theory. Much of this new technology was developed as a result of World Wars I and II and the technological race of the Cold War.

Wegener collected data from paleontology to show a correlation between continents and to illustrate that the continents had been in different latitudes in the geologic past. Further, he suggested an experiment to confirm the theory; the experiment failed not because the idea was inappropriate but because the experimental error resulting from his crude equipment was greater than the phenomena he was trying to measure. His experiment, conducted in 1922 and again in 1927 and 1936, involved the measurement of the time it took radio signals to travel across the Atlantic. The measurements failed to reveal a widening of the Atlantic through progressively longer travel times. Upon the advent of satellite and laser technology, however, widening was detected.

Mounting Evidence for Wegener-Hess

Eventually, wartime technology such as sonar was applied to scientific applications. Sonar is the underwater version of radar; an energy pulse is sent out, and its reflection from the sea floor is recorded. These data can be translated via computers to create either a profile or contour map. Literally thousands of soundings were made over thousands of kilometers of ocean in an effort to construct a map of the ocean floor in the greatest possible detail. These data began to produce a map that revealed rather remarkable features, including a continuous 64,000-kilometer mountain range that had a valley running along its crest. Hess realized the significance of the valley on top of the mountain range: It was a tensional, or “pull-apart,” feature. The same forces that formed the mountain chain were also pulling it apart.

Other technologies were also contributing to the investigation. After World War II, a worldwide network of seismographs was deployed, not so much for recording earthquakes as to listen for atomic explosions. These new and sensitive instruments mostly recorded earthquakes at plate boundaries and revealed their outlines. (Interestingly, no nuclear powers camouflaged their atomic blasts as earthquakes by detonating them at a plate boundary.) The earthquake pattern was a fingerprint of plate activity and evidence of a dynamic crust.

Other compelling data came from monitoring internal heat flow. The temperature of the earth increases with depth. At the core, the temperature is more than 4,000 degrees Celsius. This heat flowing from the interior can be measured; the hottest crustal areas were found to be above the junction of plates that are spreading centers.

More traditional geologic sampling of the subsurface was conducted by retrieving core samples from below the ocean depths. These physical samples were analyzed according to the type of sediment and the age of fossils present. The findings were surprising: The oceans are very young compared to the continents, and the sediments on the mid-oceanic ridges are thin to nonexistent, while the sediments next to the continents are kilometers thick. Therefore, not only are the oceans young, but they are also youngest in the middle and oldest next to the continents.

Fossil Magnetism

The straw that broke the back of opposition to the theory of continental drift came with the study of fossil magnetism. Sedimentary and igneous rocks offer a record of the orientation of the earth's magnetic field through time, as iron particles within them are incorporated into their structures as they form. The decoding of these fossil magnetic fields suggested that the magnetic poles have reversed themselves and that the continents have wandered through the latitudes. If the poles reversed and the continents wandered, then a mirror image of polar reversal correlating with submarine topography should be present on both sides of a spreading center such as the Mid-Atlantic Ridge. This was exactly what was observed. The model was no longer merely an explanation of previously observed phenomena; it had also been shown to be capable of predicting future observations. Earth scientists thus came to perceive the idea of drifting continents and plate tectonics as the unifying model of geologic phenomena.

The development of the continental drift theory is a story of how science works in a period of paradigm revolution. This period began with Alfred Wegener's work in 1912 and ended with Harry Hess's discoveries in 1960. The model Hess developed gave a new explanation of virtually all geologic phenomena at or near the surface of the earth. Within its field, the theory has had an impact comparable to that of Charles Darwin's theory of evolution in the field of biology. In essence, the theory of continental drift as an effect of plate tectonics accounts for the global distribution of the continents, the birth and death of oceans, and the distribution of earthquakes, volcanoes, and mountain ranges. It also leads explorers to mineral and fossil-fuel deposits. Although the exact nature of the forces that cause plate movement remain highly debated, the general concept has become a foundational theory in geology.

Principal Terms

continental crust: the outermost part of the lithosphere, consisting of granite and granodiorite

convection cell: a mechanism of heat transfer in a flowing material in which hot material from the bottom rises because of its lesser density, while cool surface material sinks

earthquake: the violent motion of the ground caused by the passage of a seismic wave radiating from a fault along which sudden movement has occurred

fault: a fracture in the earth's crust along which there has been relative displacement

Gondwanaland: a hypothetical supercontinent made up of approximately the present continents of the Southern Hemisphere

Laurasia: a hypothetical supercontinent made up of approximately the present continents of the Northern Hemisphere

lithosphere: the outer layer of the earth, situated above the asthenosphere and containing the crust, continents, and tectonic plates

oceanic crust: the outer part of the lithosphere, consisting mostly of basalt

paleomagnetism: the science of reconstructing the earth's former magnetic fields and the former positions of the continents from the magnetization in rocks

Pangaea: a supercontinent made up of all presently known continents, which began to break up in the Mesozoic era

plate: a large segment of the lithosphere that is internally ridged and moves independently over the interior, meeting in convergence zones and separating at divergence zones

tectonics: the study of the movements and deformation of the earth's crust on a large scale

Bibliography

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