External layer of the earth.

The oceanic crust measures about five to six kilometres and consists of material rich in iron and magnesia. This part of the crust is continously rebuilt in a process called continental drift. It is formed at the oceanic ridges and torn back into the internal of the earth at the deep-sea trenches. The continental crust, in contrast, is considerably thicker (35-40 kilometres, in the case of mountains up to 70 kilometres) consisting mainly of light elements like aluminium, potassium, sodium, calcium and silicon.