The rock cycle shows how the three rock types relate to each other. 
The four processes: weathering, crystallisation, lithification and
metamorphism are the links between the rock types, sediment and magma
(molten rock). Weathering is the gradual wearing down of all rock types
once they are exposed at the surface. Crystallisation is the cooling of
magma to form an igneous rock. Lithification is the transformation of
loose sediment into a rock by a slight heating and/or cementation.
Metamorphism occurs when a rock is subjected to heat and pressure,
transforming a rock. This can occur on any rock, even a metamorphic rock.
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