Building Mountain Ranges: A Plate Tectonics Model

Abstract

The processes that raise mountains crumple the earth's surface have traditionally posed some of the more baffling problems in the earth sciences. One of the better-developed explanations has been the principle of isostasy -that areas of the crust being loaded with sediments tend to sink that areas b2ing relieved of some of their burden by erosion tend to rise. Another proposal is that thermal convection in the mantle beneath the continents produces rising subsurface movements that push the mountains upward. But most major mountain belts seem to have been formed primarily by horizontal compressions. These lateral movements rumple sediments laid down over millions of years, wrinkling them like a pile of blankets pushed from one side. Many early geologists believed that the wrinkling was a result of a slow, steady contraction of the earth as it cooled. The evidence against a shrinking earth, however, is formidable. The distribution of natural radioactivity in the interior, for instance, indicates that the earth is probably not cooling at all. Then the opposite hypothesis arose, that mountain building is a result of a gradual heating up of the earth's interior, causing the rocks to decrease in volume as they pass from a solid to a molten state, like water in a melting ice cube. Even an expanding-earth hypothesis was proposed, but met little favor. In the last decade earth scientists interested in mountain building began taking an increased interest in the mounting evidence for continental drift. The original theory became greatly modified by the hypothesis of sea-floor spreading then, in 1968, was generalized by the sweeping new theory of plate tectonics (SN: 11/8/68, p. 430). In this view, the earth's lithosphere -the crust uppermost mantleis segmented into a number of rigid plates bordered by the world's major seismic zones. Plates grow at the ocean ridges by accretion of magma, are consumed in the ocean trenches, move horizontally at an average rate of several centimeters per year. Most of the evidence has come from seismic, magnetic heat-flow studies of the ocean floor. was then that geologists began to realize what they had been given by their geophysicist colleagues: the first unifying worldwide explanation for continental tectonic processes such as the creation of mountain belts. In early 1969, Dr. John F. Dewey of Cambridge University in England Dr. John M. Bird of the State University of New York at Albany, both geologists, were struck by the revolutionary implications of plate theory for continental geology. They quickly began applying the new ideas to the geology of mountain belts; their colaborative studies have helped lead to a new understanding of mountain building. It is tremendously exciting, says Dr. Bird. We can now legitimately say that we know how the world's mountain belts are formed. They are al due to plate motions. He Dr. Dewey put it more formally in the May 10 JOURNAL OF GEOPHYSICAL RESEARCH, one of the several papers they have published on the subject in the last few months. Analysis of the sedimentary, volcanic, structural metamorphic chronology in mountain belts, they write, and consideration of the implications of the new global tectonics (plate tectonics) strongly indicate that mountain belts are a consequence of plate evolution. Dr. Bird finds it ironic, instructive, that the fundamental answers about the formation of mountain belts on the continents were discovered primarily by studies of the ocean floor. To geologists, who historically have been bound to the continents to tradition more than other scientists, the lesson is one they probably will not forget. It was ridiculous for us to