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The crust, the uppermost layer of the solid Earth, is a region of interaction between surface pro­cesses brought about by the heat of radioactive reactions deep in the Earth. It is the most complex layer of the lithosphere in its physical and chemical nature.

The Earth's crust contains a wide var­iety of rock types, ranging from sedimentary rocks dominated by single minerals, such as sandstone (which is mainly silica) and limestone (which is mainly calcite), to the mineral-chemical mixtures of igneous rocks such as basalt lavas and granite intrusions.

The crust is divided into ocean crust and con­tinental crust. The average height of the two dif­fers by about 4.5km and the difference in their average total thickness is more exaggerated (con­tinental crust is about 40km thick, and oceanic crust about 7km).

The boundary between the crusts and the mantle is almost everywhere defi­ned sharply by the Mohorovicic seismic discon­tinuity. But there the similarity between oceanic and continental crust ends: they contrast strongly in structure, composition, average age, origin and evolution.

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