Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological 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 aluminium silicates (Al-Si) and has a lower density compared to the oceanic crust, called sima which is richer in magnesium silicate (Mg-Si) minerals. 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.The continental crust consists of various layers, with a bulk composition that is intermediate (SiO2 wt% = 60.6). The average density of the continental crust is about, 2.83 g/cm3 (0.102 lb/cu in), less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm3 (0.12 lb/cu in). Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm3 (0.10...