2.3: The Rock Cycle

What Are Rocks, Exactly?

There three main types of rocks: igneous, sedimentary, and metamorphic. These different types form in different ways and they have specific properties that help us to distinguish them. Through geological processes the rocks on Earth can be transformed from one type to another. These relationships and processes are illustrated using the rock cycle (Figure \(\PageIndex{1}\)). The three types of rock and some of the intermediate forms of rocky materials (magma, outcrop, and loose sediments) are depicted in the rock cycle, along with the processes that are important in their transformations (indicated as arrows). Those processes include:

• Uplift (such as mountain formation) that results in rock that was present at depth in the crust being brought to surface and exposed to weathering
• Erosion and transportation of weathered products (e.g., sand, clay, and ions in solution) and then deposition as sediments (e.g., within rivers or the ocean)
• Burial of those sediments beneath other sediments, followed by compaction (squeezing) and cementation to make sedimentary rock
• Further burial which results in heating and more squeezing and then mineral transformations to form metamorphic rock
• Further heating or other changes in conditions that result in melting to form magma
• Movement of magma towards Earth’s surface where cooling forms igneous rock. It can cool slowly to make intrusive igneous (plutonic) rock, or it can cool quickly to make extrusive igneous (volcanic) rock.

The rock cycle can be disrupted at any point by a change in conditions brought about by uplift, or burial, or in rare cases by the nearby presence of magma. Table \(\PageIndex{1}\) summarizes some key processes that are included in the rock cycle.

Figure \(\PageIndex{1}\): A schematic view of the rock cycle. The rock cycle takes place both above and below the Earth’s surface. The rock deepest beneath the earth’s surface, and under extreme heat and pressure, is metamorphic rock. This metamorphic rock can melt and become magma. When magma cools below the earth’s surface, it becomes “intrusive igneous rock.” If magma cools above the earth’s surface, it is “extrusive igneous rock” and becomes part of the outcrop. The outcrop is subject to weathering and erosion, and can be moved and redeposited around the earth by forces such as water and wind. As the outcrop is eroded, it becomes sediment which can be buried, compacted, and cemented beneath the Earth’s surface to become sedimentary rock. As sedimentary rock gets buried deeper and comes under increased heat and pressure, it returns to its original state as metamorphic rock. Rocks in the rock cycle do not always make a complete loop. It is possible for sedimentary rock to be uplifted back above the Earth’s surface and for intrusive and extrusive igneous rock to be reburied and become metamorphic rock. This work by Karla Panchuk is licensed under CC BY-SA 4.0.

Table \(\PageIndex{1}\): Common geologic processes that take place on Earth.

Process	Description
Burial	The process by which more sediments are deposited onto existing sediments.
Compaction	The process whereby sediment is converted to consolidated rock.
Cementation	The process by which clastic sediments become lithified or consolidated into hard, compact rocks, usually through deposition or precipitation of minerals in the spaces between the individual grains of sediment; may occur simultaneously with sedimentation or at a later time.
Metamorphism	The mineralogical, chemical, and structural changes that transform solid rock into other types of solid rock, generally imposed at depth below the surface zones of weathering and cementation.
Melting	The change of a material from solid to liquid. In the case of rocks, the change from solid rock to liquid magma.
Cooling/Crystallizing	The process of liquid rock freezing into solid rock.
Uplift	An increase in the vertical elevation of Earth's surface in response to natural causes.
Weathering	The chemical breakdown or physical breakup of rock.
Erosion	The general process or group of processes that loosen, dissolve, wear away, and simultaneously move from one place to another, the materials of Earth’s crust; includes weathering, solution, abrasive actions, and transportation, but usually excludes slope movements.
Transportation	The movement of material across Earth's surface by water, wind, ice, or gravity.
Deposition	The process by which sediments, soil, and rocks are added to a landform or landmass. Or, what happens when transportation stops.

Query \(\PageIndex{1}\)

References

1. Bates, R. L., and J. A. Jackson, editors. 1984. Dictionary of geological terms. Third edition. American Geological Institute, Alexandria, Virginia.
2. Earle, S. (2019). Physical Geology – 2nd Edition. Victoria, B.C.: BCcampus. Retrieved from https://opentextbc.ca/ March 2024
3. Neuendorf, K. K. E., J. P. Mehl Jr., and J. A. Jackson. 2005. Glossary of geology. Fifth edition. American Geological Institute, Alexandria, Virginia. https://www.nps.gov/subjects/geology/gri-glossary-of-geologic-terms.htm