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From Earthquakes to Continental Drift |
 In the early 1890s, John Milne, a geologist teaching at the Imperial College of Engineering in Tokyo, developed with colleagues the first accurate seismograph, an instrument used to record ground shaking, a frequent and sometimes devastating occurrence in Japan. A few years later a fire destroyed Milne's home, his scientific observatory, and the earthquake data he had collected over more than a decade while working in Japan. Discouraged but not defeated, Milne returned to his native Britain, where by the turn of the century he had established a bigger and bolder approach to the study of earthquakes--a network of 27 instruments throughout the British empire. By the time Milne died in 1913, 40 stations around the world were beginning to define the global pattern of earthquake location.
A seismograph records vibrations produced by sudden motion along a fault that generates several kinds of seismic, or "shaking," waves--vibrations back and forth, side to side, and up and down. The first seismologists could distinguish two kinds of seismic waves, which move at different speeds. Primary, or P waves, which alternately compress and dilate the matter in their path, arrive at the instrument first, inscribing a wavy line on a chart. Secondary, or S waves, which tend to oscillate snakelike, at right angles to their direction of motion, travel more slowly and have a more ragged seismographic signature. The interval between the arrival of P waves and S waves can be used to calculate the distance of the monitoring station from the earthquake's epicenter, the point on Earth's surface above the subterranean focus, or source of the shock. The distances from three seismographic stations can be used to triangulate the epicenter and place it precisely on a map.
Milne's network ushered in the ability to detect and locate earthquakes by remote sensing, a significant contribution to science and society in itself. But seismologists soon realized that the instruments also offered a way to probe the mysterious interior of the planet. By the start of World War I, a succession of researchers had studied the behavior of seismic waves to infer a planetary structure composed of concentric layers: an inner core (although there was disagreement as to whether it was solid or fluid) covered by an intermediate layer of dense rock, the mantle, that began about 30 miles beneath the outermost surface crust.
Against this background of knowledge, a German meteorologist caused an uproar in the world of geology with his bold theory about the nature of Earth's surface. In 1915, Alfred Wegener published The Origins of Continents and Oceans, in which he addressed the puzzling match-up between the bulge of Brazil and the dent of southwestern Africa. He argued that the two continents had once been joined and had drifted apart. As additional evidence for continental displacement, or continental "drift" as the original German word was translated, Wegener cited fossils of the Mesosaur, a 270-million-year-old reptile found only in eastern South America and western Africa. Most geologists of his generation explained these similarities by postulating a connecting land bridge that had later sunk out of sight into the  ocean. Wegener postulated, rather, that the Mesosaur bones were found in these distant places because those regions had come apart about 125 million years ago, slowly separating the groups of Mesosaur fossils. The continents of today drifted apart from a super-continent, which he called Pangaea.
The meteorologist was unsure as to how these huge slabs move around, but suggested that they were propelled through the oceanic crust by Earth's centrifugal force and the gravitational pull of the Sun and Moon. Many influential geophysicists argued convincingly that such mechanisms were insufficient to the task. However, in 1929, one supporter, Arthur Holmes of England, suggested that convective flow of heated rock in the mantle beneath the crust might provide the required driving force--that is, as rocky material deep in the mantle heats, it becomes less dense and rises toward the surface, where it cools and sinks, to be reheated and rise again. Without more evidence of such a mechanism, however, the theory of continental drift gained few adherents. |
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