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Oceanic crust is the part of Earth's lithosphere which surfaces in the ocean basins. Oceanic crust is composed of mafic basaltic rocks, or sima. It is thinner, generally less than 10 kilometres thick, but more dense than the continental crust, or sial, having a mean density of about 3.3 grams per cubic centimeter.
Most of the present day oceanic crust is less than 200 million years old because it is continuously being created at oceanic ridges and destroyed by being pulled back under the continental crust in subduction zones by the convection currents in the lower mantle. Plate tectonics is the study of these processes.
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