Crust

The crust is the outermost solid shell of the Earth, the chemically distinct layer above the Moho that caps the lithosphere. It splits into two types: continental crust, granitic and roughly 30 to 50 km thick, and oceanic crust, basaltic and only about 7 km thick. The boundary with the mantle is the Mohorovicic discontinuity, defined by a jump in P-wave velocity from near 6.8 km/s to about 8.0 km/s. Crust holds the lowest-density rock in the solid Earth, so it floats isostatically; continents stand high and ocean basins sit low because of this density and thickness contrast.