The concept of the rock cycle allows us to relate together the three major categories of rocks. These are the igneous rocks produced as magma cools, the sedimentary rocks produced under various conditions as a result of denudation, weathering and deposition processes, and the metamorphic rocks that are produced when igneous and sedimentary rocks are subjected to great pressure and heat, and in some instances to chemical change.
The rock cycle visualises a sequence in which magma produces igneous material which (perhaps along with earlier-formed rock types) provides a basis for sedimentation, with rocks over time being further changed to metamorphic rocks. Each of these types may be returned to the interior of the Earth as a result of subduction at some plate margins. There is thus, over several hundred million years, a ‘rock cycle’ in operation.