  in geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle. the crusts of earth, the 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. ==earth's crust and mantle== thumb|350px|the internal structure of earth 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 mohoroviÄiÄ discontinuity, a boundary defined by a contrast in seismic velocity. the crust occupies less than 1% of earth's volume. the oceanic crust of the sheet is different from its continental crust. the oceanic crust is  to  thickstructure of the earth. the encyclopedia of earth. march 3, 2010 and is composed primarily of basalt, diabase, and gabbro. the continental crust is typically from  to  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. both the continental and oceanic crust "float" on the mantle. because the continental crust is thicker, it extends both above and below the oceanic crust. the slightly lighter density of felsic continental rock compared to basaltic ocean rock contributes to the higher relative elevation of the top of the continental crust. because the top of the continental crust is above that of the oceanic, water runs off the continents and collects above the oceanic crust. the continental crust and the oceanic crust are sometimes called sial and sima respectively. because of the change in velocity of seismic waves it is believed that on continents at a certain depth sial becomes close in its physical properties to sima, and the dividing line is called the conrad discontinuity. the temperature of the crust increases 
Plate_tectonics