<Tr> <Td> Shield Platform Orogen Basin Large igneous province Extended crust </Td> <Td> Oceanic crust: 0--20 Ma 20--65 Ma> 65 Ma </Td> </Tr> <P> In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet or natural satellite, which is chemically distinct from the underlying mantle . The crusts of Earth, Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in incompatible elements than their respective mantles . The Earth's crust is composed of distinctly different continental crust and oceanic crust, which have different chemical compositions and physical properties, and which were formed by different geological processes . </P> <P> 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 Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity . The crust occupies less than 1% of Earth's volume . </P> <P> The crust of the Earth is of two distinctive types: oceanic and continental . 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 is mostly composed of slightly less dense rocks than those of 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 . </P>

What percentage of the earth is the crust