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/cm
3 and is less
dense than the material of the Earth's
mantle (density of about 3.3 g/cm
3), which consists of mafic rock. Continental crust is also less dense than oceanic crust (density of about 2.9 g/cm
3), 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.