In geology, a crust is the outer layer of a planet, part of its lithosphere. Planetary crusts are generally composed of a less dense material than that of its deeper layers.
On partially-molten planets, such as Earth, the lithosphere is floating on fluid interior layers. Because of the partially-fluid upper mantle, or asthenosphere, underneath, floating lithospheres are often broken into tectonic plates that move. On Earth there are two types of crust: continental crust (sial) and the more dense oceanic crust (sima).
