crust

The crust of the Earth is made up of plates with different kinds of margins. In mid-ocean, there are constructive plate margins, where magma wells up from the Earth's interior, forming new crust. On continent-continent margins, mountain ranges are flung up by the collision of two continents. At an ocean-continent destructive margin, ocean crust is forced under the denser continental crust, forming an area of volcanic instability.

Rocky outer layer of the Earth, consisting of two distinct parts - the oceanic crust and the continental crust. The oceanic crust is on average about 10 km/6 mi thick and consists mostly of basaltic rock overlain by muddy sediments. By contrast, the continental crust is largely of granitic composition and has a more complex structure. Because it is continually recycled back into the mantle by the process of subduction, the oceanic crust is in no place older than about 200 million years. However, parts of the continental crust are over 3.5 billion years old.

Beneath a layer of surface sediment, the oceanic crust is made up of a layer of basalt, followed by a layer of gabbro. The continental crust varies in thickness from about 40 km/25 mi to 70 km/45 mi, being deepest beneath mountain ranges, and thinnest above continental rift valleys. Whereas the oceanic crust is composed almost exclusively of basaltic igneous rocks and sediments, the continental crust is made of a wide variety of sedimentary rocks, igneous rocks, and metamorphic rocks.