In geology, a crust is the outermost layer of a planet.
The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks.
The crust is underlain by the mantle.
The upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust.
The boundary between the crust and mantle is conventionally placed at the Mohorovicic discontinuity, a boundary defined by a contrast in seismic velocity.
Earth's crust occupies less than 1% of Earth's volume.
The oceanic crust of the Earth is different from its continental crust.
The oceanic crust is 5 km (3 mi) to 10 km (6 mi) thick and is composed primarily of basalt, diabase, and gabbro.
The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick, and it is mostly composed of less dense rocks than is the oceanic crust.
Some of these less dense rocks, such as granite, are common in the continental crust but rare to absent in the oceanic crust.
The temperature of the crust increases with depth, reaching values typically in the range from about 500 °C (900 °F) to 1,000 °C (1,800 °F) at the boundary with the underlying mantle.
The crust and underlying relatively rigid mantle make up the lithosphere.
Because of convection in the underlying plastic, although non-molten, upper mantle and asthenosphere, the lithosphere is broken into tectonic plates that move.
The common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%.
Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen.
It occurs principally in combination as oxides, of which the chief are silicon, aluminum, iron, calcium, magnesium, potassium and sodium oxides.
Silica is a major constituent of the crust occurring as the silicate minerals, which are the most common minerals of igneous and metamorphic rocks.
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