The crust is a thin layer of rock that floats on the mantle. It is made mainly of silicate minerals (minerals made of silicon and oxygen) such as quartz.
There are two kinds of crust: oceanic and continental.
Oceanic crust is the crust beneath the oceans. It is much thinner – just 7 km thick on average. It is also young, with none being over 200 million years old.
Continental crust is the crust beneath the continents. It is up to 80 km thick and mostly old.
Continental crust is mostly crystalline ‘basement’ rock up to 3800 million years old. Some geologists think at least half of this rock is over 2500 million years old.
It is estimated that approximately one cubic kilometer of new continental crust is probably being created each year.
The ‘basement’ rock has two main layers: an upper half of silica-rich rocks such as granite, schist and gneiss, and a lower half of volcanic rocks such as basalt which have less silica. Ocean crust is mostly basalt.
Continental crust is created in the volcanic arcs above subduction zones (see converging plates). Molten rock from the subducted plate oozes to the surface over a period of a few hundred thousand years.
The boundary between the crust and the mantle beneath it is called the Mohorovicic discontinuity.