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Plate Techonics
Ever since the first world maps became reasonably accurate; men have noticed and wondered about the shape of the continents and the corresponding pattern of the coastlines.
Geologists were well aware of the vertical movements of portions of the earths crust, but horizontal forces were unknown. Geologists recognized isostatic adjustments, in which all elements were in gravitational equilibrium. The continents are composed of lighter rock (2.7g./cm3) than the ocean basins (3.5g./cm3). Consequently, the continents were regarded as being much thicker in order to equal the weight of the denser ocean floor.
Alfred L. Wegener, a German professor of meteorology at Graz, Austria and an important explorer of Greenland was the greatest proponent of continental drift. He showed that the theory of isostasy alone was inadequate to explain many of the geological structures of the earth, Wegener was influenced by the shape of South America and Africa. He mapped the types of rocks of the earth and from these, determined earlier climates. The tillites of South America and Australia showed earlier glacial movement; the coal-bearing beds of the United States and China showed these had been formed under tropical conditions; sand and salt deposits of desert regions suggested an earlier trade wind location. These various rocks could not have been formed in their present latitudes. Wegener also noted that ancient beds of rock in Africa and South America would line up precisely if the two continents could be placed together.
Wegener studied fossil evidence. The distribution of the Glossopteris flora from New Zealand and Australia to India, Madagascar, Southern Africa and South America and the distribution of the small fresh water reptile Mesosaurus, known only from South Africa and Southern Brazil, lends further support to the concept of one landmass in the southern hemisphere during ancient Pennsylvanian and Permian times. From his broadly based studies, Wegener postulated that at an earlier time all the continents formed one large land mass wich he called Pangaea. This land mass separated into two-the equivalent of the northern hemisphere, which he called Laurasia and the equivalent of the southern hemisphere, which he called Gondwanaland. Wegener worked with the edge of the continental slope which is a much more logical margin than the edge of the continent. (Today some investigators use the mid-point of the continental slope.) Wegener also anticipated the shifting of the position of the poles which has been confirmed.
Continental drift has had a solid foundation for the past four decades since the concept of sea-floor spreading was advanced by Harry H. Hess of Princeton University. Sea floor spreading considers the sea floor as being continuously pulled apart along a narrow ridge. As the sides are pulled apart the crack is filled with molten lava from the asthenosphere. The oceanic ridge can now be traced through all of the ocean basins.
Magnetometer surveys showed that the rocks of the ocean floor differed in their magnetism. F. J. Vine and D. H. Matthews of the University of Cambridge believed that as new rocks are formed along the ridge they would assume a direction of magnetism in accordance with the then present position of the poles. They believed that there would be parallel strips of newer and still newer rocks along either side of the ridge in a symmetrical pattern. This has been confirmed and further, it has been shown that the age of the ocean basins increases with the distance from the ridge. Nearly all of the present ocean basins have been formed within the past 200 million years.
Continental drift and sea floor spreading are now combined in the newer concept of plate tectonics. A plate, such as North America or Africa consists of the continent plus a portion of the adjacent ocean basins. The plates are in motion and they may either diverge, converge or slide by one another. In divergence, land can be split by a ridge as in present day Iceland or as in the Great Rift Valley of Africa. In the case of two continents as in North America and Eurasia with a mid-oceanic ridge, they will be rafted apart.
In convergence as two plates move together the lithosphere of one plate is pulled down into the asthenosphere (subduction) where it is consumed. When this occurs with a section of ocean basin, a trench may be formed. Where continental blocks meet, the lighter weight of the materials prevents them from sinking into the asthenosphere. Instead there is a gradual overriding with accompanying mountain building as in the Alps or as in the meeting of India with Eurasia and the forming of the Himalayas. Two plates can slide by one another as at the San Andreas Fault in California.