Lexikon

see Earth crust

the outermost layer of the Earth is the crust. The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. The crust comprises the continents and ocean basins. It has a variable thickness, being 35–70 km thick in the continents and 5-10 km thick in the ocean basins. The crust is composed mainly of alumino-silicates. The oceanic crust of the Earth is different from its continental crust. The oceanic crust is composed primarily of basalt, diabase, and gabbro. The continental crust 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. The crust is underlain by the Mantle. The boundary between the crust and mantle is conventionally placed at the Mohorovièiæ discontinuity, a boundary defined by a contrast in seismic velocity. The crust and the underlying relatively rigid mantle make up the lithosphere.