<P> Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves . This layer is sometimes called sial because its bulk composition is richer in silicates and aluminium minerals and has a lower density compared to the oceanic crust, called sima which is richer in magnesium silicate minerals and is denser . Changes in seismic wave velocities have shown that at a certain depth (the Conrad discontinuity), there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character . </P> <P> The continental crust consists of various layers, with a bulk composition that is intermediate to felsic . The average density of continental crust is about 2.7 g / cm, less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g / cm . Continental crust is also less dense than oceanic crust, whose density is about 2.9 g / cm . At 25 to 70 km, continental crust is considerably thicker than oceanic crust, which has an average thickness of around 7--10 km . About 40% of Earth's surface is currently occupied by continental crust . It makes up about 70% of the volume of Earth's crust . </P>

What is formed on top of continental crust