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the Mid-Atlantic Ridge (Original)

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Carolyn Silver
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« Reply #90 on: July 30, 2008, 10:40:45 pm »

The driving force of plate movements was initially claimed to be mantle-deep convection currents welling up beneath midocean ridges, with downwelling occurring beneath ocean trenches. Plate tectonicists expected seismotomography to provide clear evidence of a well-organized convection-cell pattern, but it has actually provided strong evidence against the existence of large, plate-propelling convection cells in the mantle. The favored plate-driving mechanisms at present are 'ridge-push' and 'slab-pull', but their adequacy is very much in doubt.
Thirteen major plates are currently recognized, ranging in size from about 400 by 2500 km to 10,000 by 10,000 km, together with a proliferating number of microplates (over 100 so far). Plate boundaries are identified and defined mainly on the basis of earthquake and volcanic activity. The close correspondence between plate edges and belts of earthquakes and volcanoes is therefore to be expected and can hardly be regarded as one of the 'successes' of plate tectonics! A major problem is that several 'plate boundaries' are purely theoretical and appear to be nonexistent, including the northwest Pacific boundary of the Pacific, North American, and Eurasian plates, the southern boundary of the Philippine plate, part of the southern boundary of the Pacific plate, and most of the northern and southern boundaries of the South American plate.

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