The Rock Cycle
The rock cycle is a process in which rocks are continuously transformed between the three rock types igneous, sedimentary and metamorphic.
Rocks of any type can be converted into any other type, or into another rock of the same type, as this diagram illustrates:
Conversion to metamorphic rocks requires conditions of increased temperature and/or increased pressure, conversion to sedimentary rocks occurs via the intermediate stage of sediments, and conversion to igneous rocks occurs via the intermediate stage of magma:
Increased temperature and pressure occurs in subduction zones and in areas where two plates of continental lithosphere collide to produce a mountain range, while increased pressure without increased temperature is produced when sedimentary rocks are deeply buried under more sediments. Sediments are produced when rocks are uplifted, weathered and eroded, and the resulting detrital material deposited in marine or terrestrial basins. If the sediments are buried under further layers of sediment, they can become lithified to produce a sedimentary rock. Magma is produced when rocks are melted. This melting can occur when a lithospheric plate descends into the Earth's crust at a subduction zone, or when a mid-ocean ridge opens up and produces decompression melting in the athenosphere under the ridge. When the magma solidifies, it becomes an igneous rock.
The rock cycle has many alternative pathways. The following diagram illustrates one of these and gives an indication of the plate tectonic setting where it occurs: