Continental crust

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because there is more felsic, or granitic, bulk composition, which lies in contrast to the oceanic crust, called sima because of the mafic or basaltic rock. (Based on the change in velocity of seismic waves, it is believed that at a certain depth sial becomes close in its physical properties to sima. This line is called the Conrad discontinuity.) Consisting mostly of granitic rock, continental crust has a density of about 2.7 g/cm3 and is less dense than the material of the Earth’s mantle (density of about 3.3 g/cm3), which consists of ultramafic rock. Continental crust is also less dense than oceanic crust (density of about 2.9 g/cm3), though it is considerably thicker; mostly 25 to 70 km versus the average oceanic thickness of around 7–10 km. About 40% of the Earth’s surface is now overlaid by continental crust. Continental crust makes up about 70% of the volume of Earth’s crust.