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Physical
Rock Cycle
The rock cycle describes how rocks change over geologic time among the three main types: igneous, sedimentary and metamorphic.
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The idea
The rock cycle is a basic idea in geology. It describes how rocks change, over geologic time, among the three main rock types: igneous, sedimentary and metamorphic. A rock stays the same only while its surroundings stay steady. When it is forced out of that balance, for example by heat, pressure or exposure to air and water, it begins to change.
Igneous rocks form when magma or lava cools and hardens. Sedimentary rocks form when sediments, such as bits of worn-down rock or the shells of dead sea creatures, settle in layers and become cemented together. Metamorphic rocks form when existing rock is heated to more than 150 to 200 degrees Celsius, often under high pressure as well. The rock then recrystallizes into a new texture or set of minerals while staying mostly solid.
Any rock type can become any other. An igneous rock such as basalt may break down when exposed to the atmosphere, or melt as it is carried down beneath a continent. Two driving forces keep the rock cycle moving: plate tectonics and the water cycle. On planets with life, the rock cycle also counts as a biogeochemical cycle, since living things, such as shell-building sea creatures, help form some rocks.
Check yourself
Question 1 of 2
What happens to existing rock as it becomes metamorphic rock?
Composition
what it's made of
- Igneous rock — Rock formed when magma or lava cools and solidifies.
- Sedimentary rock — Rock formed when sediments settle in layers and are cemented together.
- Metamorphic rock — Existing rock changed by heat and often pressure while staying mostly solid.
- Weathering and erosion — The wearing down of existing rock into particles that become sediment.
- Plate tectonics — Movement of Earth's plates that buries, melts and lifts rock; one of the cycle's driving forces.
- Moving water — Moving water that weathers rock and carries sediment; the cycle's other driving force.
Connection
how it relates
Under Earth Systems
Change
how and why it evolves
- Past
- In 1967, J. Tuzo Wilson published an article describing the repeated opening and closing of ocean basins, a pattern that became known as the Wilson cycle.
- Now
- Today light continental crust is not usually pulled back into the mantle, so over time the continents slowly grow larger.
- Next
- Most changes in the rock cycle will likely stay far too slow for anyone to notice in a lifetime, though eruptions and landslides can reshape rock quickly.