The Crust

The Earth's crust is a thin layer of rock that floats on the mantle. It is made mainly of silicate minerals such as quartz. There are two kinds of crust:

• oceanic
• 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 aproximately one cubic kilometre 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 geneiss
• and a lower half of volcanic rock as basalt which have less silica(e.g. 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 hundreed thousand years.