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A tectonic plate is a piece of the Earth's Crust and uppermost mantle (together referred to as the Lithosphere ). The surface of the Earth consists of ten major tectonic plates and many more minor ones. The plates are around 100 km (60 miles) thick and consist of two principal types of material: oceanic crust (also called ''sima'' from Silicon and Magnesium ) and continental crust (''sial'' from silicon and Aluminium ). Under both lies a relatively plastic, although non-molten, layer of the Earth's Mantle called the Asthenosphere . The composition of the two types of crust differs markedly. Oceanic Crust consists largely of Basaltic rocks (" Mafic "), while the Continental Crust consists principally of lower Density Granitic rocks rich in aluminium and silica (" Felsic "). The two types of crust also differ in thickness, with continental crust considerably thicker than oceanic. The churning of the asthenosphere carries the plates along in a process known as Continental Drift , which is explained by the theory of Plate Tectonics . Interaction between the plates creates Mountain s and Volcano es, as well as giving rise to Earthquake s and other geological phenomena. The boundaries of the plates do not coincide with those of the continents. For instance, the North American Plate covers not only North America , but also far eastern Siberia and northern Japan . As far as is known, the Earth is the only planet in the Solar System to possess active plate tectonics, although there are suggestions that different styles of plate tectonics were in operation on Mars , Venus , and some of the Galilean Satellites in the past. See also External links
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