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| Plate tectonics | ||||||||||||||||||||||||
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Introduction Plate Tectonics is the Theory formulated in the 1960s to explain the phenomena of continental drift and sea floor spreading, and the formation of the major physical features of the Earth's surface. The Earth's outermost layer is regarded as a jigsaw of rigid major and minor plates up to 100 km/62 mi thick, which move relative to each other, probably under the influence of convection currents in the mantle beneath. Major land forms occur at the margins of the plates, where plates are colliding or moving apart for example, volcanoes, fold mountains, ocean trenches, and ocean ridges. The rate of drift is at most 15 cm/6 in per year. | |||||||||||||||||||||||
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Background The plate-tectonics revolution in geologic thought occurred in the 1960s and 1970s, but the roots of the theory were established by earlier observation and deduction. In one such discovery, James Hall, a New York geologist, observed that sediments accumulated in mountain belts are at least ten times thicker than those in continental interiors. This planted the seed for the later geosynclinal theory that continental crust grows by progressive additions that originate as ancient and folded geosynclines hardened and consolidated into plates. This theory was well established by the 20th century. Another 19th-century discovery was a mid-ocean ridge in the Atlantic; by the 1920s scientists had concluded that this ridge was continuous almost all the way around the world. | |||||||||||||||||||||||
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