Evidence for Plate Tectonics.

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Evidence for Plate Tectonics

Into the 1900's, many scientists believed that as the earth cooled after the Big Bang, the planet's surface contracted and wrinkled like the skin of a raisin. The 'raisin' theory implied that mountain ranges like the Himalayas were forced up by the wrinkling process. This theory assumed that all of the features on earth had formed during one cooling event and that the planet was relatively static, changing little as the cooling (and wrinkling) slowed to a halt over billions of years.

Alfred Wegener, a German geophysicist and meteorologist, was not satisfied by this explanation. His ideas drew on the widely recognized fact that Africa and South America appeared to fit together like jigsaw puzzle pieces. He collected data from the continents on both sides of the Atlantic, finding that fossils and rock types along the eastern coast of South America matched those on the western coast of Africa. When he added the northern continents to the puzzle, Wegener realized that the chain of Appalachian Mountains in North America continued as the Caledonian Mountains in northern Europe.  

To explain these data, Wegener proposed the theory of continental drift    in his book The Origins of the Continents and the Oceans, published in German in 1915. His theory stated that all of the continents had originally been joined together in a super continent called Pangaea. About 200 million years ago, the theory continued, Pangaea broke apart and the continents slowly drifted to their present positions.

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One of the main problems with his theory was that he did not propose a driving mechanism for the motion of the continents. After Wegener published his theory, major technical and scientific developments allowed scientists to map the ocean floor and to detect paleomagnetic reversals in the rocks on the ocean floor. These two sets of data provided geologists with additional evidence for the process of continental drift.

In a 1962 paper entitled "History of Ocean Basins," Harry Hess, a geologist at Princeton University, proposed that the mid-ocean ridges marked regions where hot magma rose close to the ...

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