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 and has a lower density compared to the oceanic crust, called sima which is richer in magnesium silicate minerals. Changes in seismic wave velocities have shown that at a certain depth , there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character.