![]() ![]() ![]() ![]() Only a small number of major lithospheric plates exist, which grind and scrape against each other as they move independently like rafts of ice on water. The brittle lithosphere is broken into a mosaic of internally rigid plates that move horizontally across the Earth's surface relative to one another. The asthenosphere, or low-velocity zone, extends from the base of the lithosphere to a depth of about 400 mi (700 km). The theory is based on a simple model of the Earth in which a rigid outer shell 30–90 mi (50–150 km) thick, the lithosphere, consisting of both oceanic and continental crust as well as the upper mantle, is considered to lie above a hotter, weaker semiplastic asthenosphere. Numerous other geological phenomena such as lateral variations in surface heat flow, the physiography and geology of ocean basins, and various associations of igneous, metamorphic, and sedimentary rocks can also be logically related by plate tectonics theory ( Fig. The theory explaining the movement across the Earth's surface of the rigid plates that make up the Earth's lithosphere. Meanwhile, an additional sloping accretionary wedge forms from scraped crustal material buildup. Plate tectonics have led to continental rearrangements throughout geological history, resulting in the past existence of massive continents, sea-level changes, and resultant evolutionary development of organisms.Īlong convergent zones, the subducted plate submerges beneath the upper plate, creating a sloped erosion-prone region with an accompanying trench. Transform plate boundaries allow for lateral ground movements that do not produce volcanic activity. Internal thermal circulations drive plate movements, with plate recycling occurring along subduction and convergent zones at plate boundaries, which are volcanically and seismically active.Įarly evidence supporting plate tectonic theory came from the discovery of datable magnetic sea-floor banding, which was indicative of sea-floor spreading, and magnetic-pole switching events throughout time. Earth's lithosphere is divided into seven major plates, and several minor ones, which continuously ride on a semiplastic asthenosphere. ![]()
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