The Rock Cycle

All rocks falls into one of three categories: - igneous, sedimentary or metamorphic. See the Rocks page for details.

The three rock types are transformed into one another by processes of weathering and erosion, melting and exposure to heat and pressure.


Image adapted from Strahler and Strahler (2002)

Igneous rocks are injected into the Earth’s crust as molten magma. The magma either cools to form bodies of rock (e.g. granite) within the continental crust, or it reaches the surface on the land or ocean floor where it forms volcanoes.

The erosion of rocks produces sediment. Erosion products may eventually be deposited in layers called beds, forming a stack of sediment, with the oldest sediment at the base. As these beds become buried under younger sediments they are lithified by a combination of processes such as compaction, due to the weight of overlying sediment, and cementation, arising form fluids with dissolved salts flowing through the sediment to form sedimentary rocks.

Metamorphic rocks originate from igneous or sedimentary rocks. Metamorphic rocks are also sometimes re-metamorphosed. Metamorphic rocks result from recystallisation due to the pressure and heat generated when rocks are buried deep within the crust, or are formed locally in a zone around an igneous intrusion due to the heating effect.

The formation of igneous, sedimentary and metamorphic rocks is an integral part of the plate tectonic processes pf seafloor spreading, subduction and continental collision. The heat dissipated from the Earth’s molten interior drives this plate tectonic rock cycle.

The processes of weathering and erosion follow the raising of a mountain range like the Himalayas by continental collision. These processes over time will reduce the mountains to much lower levels as the detritus is carried to the sea and deposited in sediments later to be lithified into sedimentary rocks. These sedimentary rocks in turn may be buried deep with in the crust and metamorphosed. They may be carried down a subduction zone and the metamorphosed rocks partially melted to be re-injected into the crust as igneous rocks. Thus the tectonic rock cycle goes full circle (Sleeman et al., 2004).

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