Plate Tectonics: A whole new way of looking at your planet
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by J. Tuzo Wilson
Continental Drift, the theory that continents move slowly about the earth's surface, changing their positions relative to one another and to the poles of the earth. In the past the theory has been discussed but not generally accepted, most geologists believing the continents to be fixed in place and subject only to vertical movements, such as those observed during mountain uplift. In recent years, however, a sound body of evidence in support of a modified form of the drift theory has been found. Ideas are becoming precise and unified, with emphasis on a moving, evolving ocean floor. The new theory is called plate tectonics.

Early Theories Soon after the Atlantic Ocean had been mapped, about three hundred years ago, it was noticed that the opposite coasts had similar shapes, but it was not until the middle of the 19th century that accurate maps were published demonstrating that the two coasts could be fitted together quite closely. Some geologists then suggested that the fit of the coasts was not an accident--that the continents were once joined and had subsequently drifted apart. None of the suggestions were taken seriously.

In 1912, however, the German meteorologist Alfred Wegener investigated the fit of the Atlantic coasts more carefully than had his predecessors and grouped all the continents together into one great land mass, which he called Pangaea. He supposed that the mass began to break apart about 200 million years ago. He also showed that some geological features on the opposite coasts could have fitted together, and that there were many striking similarities between the fossil plants and reptiles on the opposite coasts, particularly the coasts of Africa and South America. If the continents were pushed together, the geological, fossil, and other lines of evidence would join together accurately in the way that lines of print on a torn newspaper would join when the paper was reassembled. Wegener also pointed out that ancient climatic zones seemed to have lain in different places from the present zones. He pointed out that where great ice sheets have melted in recent geological times in Scandinavia and North America, the land is rising as fast as a centimeter a year. This vertical uplift, he said, requires horizontal inflow of matter below and implies that flow and motion do take place within the earth.

Wegener's arguments led to heated controversy about continental drift in the 1920's and 1930's. Opponents regarded the idea of continents moving about through solid rock as so preposterous that they ignored all his other arguments, many of which, it is now clear, were essentially correct. Only a few geologists accepted the theory. Among the first of these was the South African geologist Alex L. Du Toit, who suggested that there had been two ancient continents: the southern continent of Gondwanaland, comprising Antarctica, Australia, the Indian peninsula, Africa, Madagascar, and South America; and the northern continent of Laurasia.

Modern Evidence Although Du Toit and the few other advocates of continental drift kept the subject alive, most geologists and geophysicists either ignored or condemned it. However, two new developments changed this situation. One was the study of paleomagnetism, or the magnetism in ancient rocks, carried out from the early 1950's. The other was the discovery, made between 1956 and 1960 by the Americans Maurice Ewing and Bruce C. Heezen, of a continuous mid-ocean ridge, a vast submarine mountain system lying along the middle of the oceans. Related to this system is a line of deep trenches, island arcs, and young mountains, where earthquake activity occurs--for example, along the boundaries of the Pacific Ocean.

Certain rocks, when they are formed, are magnetized in the direction of the earth's magnetic field. Examination of this paleomagnetism in rocks of various ages revealed the startling fact that the earth' s magnetic field has reversed its direction many times. If a core is drilled through undisturbed rock, one would find that the young rock on top is magnetized in the present normal direction, the older layer underneath is magnetized in the reverse direction, the next layer is again normal, the next layer is again reversed, and so forth. Investigations carried out all over the world show that the earth' s magnetic field has reversed direction every few hundred thousand years.

Reversals of the magnetic field have left a particularly fortuitous record on the ocean floors. The mid-ocean ridges are found throughout the oceans of the world, and there is reason to believe that they represent places where molten material from the mantle, a deep region of the earth beneath the solid crust, is rising. As it surfaces, the material heaps up to form the ridge, and it also moves out sideways away from the ridge like a pair of giant conveyor belts. At the ridge the rock being formed from mantle material is magnetized in the direction of the magnetic field, and as it moves away from the ridge it carries with it a record of the direction.

When the field reverses, the new rock being formed at the ridge is magnetized in the opposite direction, and as it moves away it too carries a record of the direction, opposite to the direction of the previous region. Therefore, one would expect to find alternating regions of normal and reversed magnetic directions symmetrically disposed on both sides of the ridge. Indeed, this pattern is found. It provides strong evidence for the main mechanism of continental drift --that is, spreading of the sea floor as new material wells up from the mantle.

Scientists have also used the magnetism in ancient rocks to support in another way the idea that the continents may once have been closer together. Evidence from rocks of the same age in different continents indicates different ancient positions for the earth's magnetic poles. However, if one hypothetically pushed these continents together, the direction of magnetism in rocks of the same age points to almost identical positions for ancient poles. Thus, one may postulate that the land masses were once joined. The direction of magnetism was fixed as a permanent record in the rocks as they cooled and solidified. Then the continents drifted apart, with the result that the magnetic directions now indicate different pole positions.

Modern Theories In one model of continental drift consistent with modern evidence, a current of mantle material rises under an ancient continent, causing it to break apart. The current then spreads out horizontally and carries the pieces of the continent away from each other. Between funny pictures the separating pieces an ocean is opened up. In one instance of this, the pieces are South America and Africa; the ocean opened up between them is the Atlantic. Along the middle of the Atlantic lies the Mid-Atlantic Ridge, which represents the region where the mantle current surfaces, forming new oceanic crust and producing much volcanic activity.

The horizontal mantle current eventually meets an opposing current, and they both turn downward into the mantle again at the site of one of the trenches mentioned above. Enormous pressures are produced in this region, for the continental crust and the oceanic crust are moving toward each other. (The continental crust is being carried, so to speak, on the back of the mantle current. On the other hand, the oceanic crust is believed by some geophysicists to be identical with the mantle current--it is simply the solid and somewhat altered top layer of the mantle current.) The descending mantle current tends to drag the crust down with it, forming a deep trench or piling up young mountains. At the same time, the continental crust tends to ride over the oceanic crust, for it is the lighter of the two. The trenches may be filled as the advancing edge of the continental crust is thrust up to form mountains, and numerous earthquakes originate from the plane along which the oceanic crust is forced to slip under the continental crust.

The Mid-Atlantic Ridge is part of the worldwide system of mid-ocean ridges. It is interesting to note the striking similarity that exists between the shape of the Mid-Atlantic Ridge and the shape of the coastlines on both sides. Indeed, if one were to push the continents bordering the Atlantic together (reversing the drift that is going on at present), the continents would meet at the Mid-Atlantic Ridge and close up on the ocean that now separates them.

Such a model of continental drift is confirmed by many lines of evidence in addition to those already mentioned. If the ocean floors are spreading, they will be very young near the mid-ocean ridge and progressively older toward the coasts. Thus, although young sediments and young volcanoes can form anywhere on the ocean floors, older sediments and old volcanic islands should only be found toward the coasts. It has become clear through oceanographic research that no old islands are located near the mid-ocean ridge, but that as the coasts are approached some progressively older islands are found. It has also been shown that the maximum age of sediments decreases toward the mid-ocean ridges, and that the total thickness of sediments increases from zero over the ridges to a few miles near the continents.

Plate Tectonics The essential difference between Wegener's original theory of continental drift and the more modern ideas called plate tectonics is that Wegener believed that each continent was propelled like a ship through the solid ocean floor. Many geologists thought this impossible. The theory of plate tectonics holds that the entire crust of the earth is broken into six large pieces (plates) and many smaller ones. Any plate may consist, in part, of ocean floor and, in part, of a continent or islands. The boundaries between plates are the mid-ocean ridges, where new oceanic crust is formed and plates move apart; ocean trenches and young mountains, where plates come together and older ocean crust is overriden and returned to the interior; and large faults, such as the San Andreas in California, along which one plate moves horizontally past another. An example of a plate is the Africa plate bounded on the west, south, and east by the mid-ocean Atlantic Ridge, the southern and Indian Oceans, and the Red Sea and, on the north, by the faults and young mountains extending from the Red Sea north along the Jordan Valley to Turkey and by the young alpine mountains of the Mediterranean region extended by submarine faults from Morocco to the Azores. Thus, Africa is carried like a raft embedded in a larger plate, not propelled as an isolated continent.

Although Wegener and Du Toit proposed that the primitive continents began to break up about 200 million years ago, there is much evidence that drift began long before then, and that continental blocks have slowly been moving about the earth's surface throughout much of geological time. It seems that before the continents drifted apart and opened up the Atlantic, they had drifted together and closed up an earlier ocean. Another place where continents seem to have bumped into each other and piled up mountains between them is the Himalayas, which may have been produced when the Indian Peninsula detached itself from Gondwanaland and gradually drifted into Asia.

Vol. 7, Colliers Encyclopedia CD-ROM; Feb. 1996

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