Formation of rocks

In geology, terrestrial rocks are formed by three main mechanisms:

• Lithification: the gradual accumulation and compaction of sediments to form sedimentary rock.
• Fractional crystallisation: the solidification of melt or magma to form igneous rock.
• Metamorphism: the subjection of rock to varying degrees of pressure and heat within the Earth's crust, changing the composition and fabric to form metamorphic rock.

Rock can also form in the absence of a substantial pressure gradient as material that condensed from a protoplanetary disk, without ever undergoing any transformations in the interior of a large object such as a planet or moon. Astrophysicists classify this as a fourth type of rock: primitive rock. This type is common in asteroids and meteorites.