crust

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crust In geology, the thin, outermost solid layer of the Earth. The crust represents less than 1% of the Earth's volume and varies in thickness from c.5km (3mi) beneath the oceans to c.70km (45mi) beneath mountain chains. Oceanic crust is generally thinner averaging 7km (4.5mi) thick and is basaltic in composition, whereas continental crust is mainly between 30–40km (20–25mi) thick and of granitic composition. The crust is defined by its seismicity. The lower boundary of the crust is defined by a marked increase in seismic velocity, known as the Mohorovičič discontinuity. See also Moho

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