<P> The 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 more felsic compared to the oceanic crust, called sima which has a more mafic bulk composition . 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> <P> Because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life . Its existence also provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time, in what is now called the Cambrian explosion . </P>

Which parts of the crust are thicker oceanic or continental