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The Rock Cycle. Credit: USGS
The answer to ‘what is the rock cycle’ is very straight forward: it is a geological concept that describes the transitions among the three main types of rock: sedimentary, metamorphic, and igneous. Each type of rock is altered or destroyed when it is forced out of equilibrium. The water cycle and plate tectonics are the major forces behind the rock cycle.
When rocks are pushed deep under the surface of the Earth, they may melt and become magma. When magma cools it becomes a type of igneous rock. A rock that cools within the Earth is called intrusive or plutonic and will cool very slowly, producing a coarse-grain. If magma cool very rapidly on the Earth’s surface it is called extrusive or volcanic rock. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass like obsidian. Any of the three main types of rocks can melt into magma and cool into igneous rocks.
Rocks exposed to high temperatures and/or pressures can be changed physically or chemically to form a different rock. This is called metamorphic(it metamorphs into another rock). Regional metamorphism refers to the effects on large masses of rocks over a wide area, typically associated with mountain building. These rocks commonly exhibit distinct bands of mineralogy and colors(foliation). Another main type of metamorphism happens when a body of rock comes into contact with an igneous intrusion. The intrusion heats up the surrounding rock. This is called contact metamorphism and results in a rock that is altered and re-crystallized by the extreme heat of the magma and/or by the addition of chemical rich fluids from the magma(metasomatism). Any type of rock can be modified by metamorphism.
Rocks exposed to the atmosphere are unstable and subject to weathering and erosion. Weathering and erosion breaks the rock down. This fragmented material accumulates and is buried by additional material. Each grain of sand is still a member of the class of rock it was formed from, but a rock made up of such grains fused together is sedimentary. Sedimentary rocks can be formed from the lithification(process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock)of these buried smaller fragments(clastic), the accumulation and lithification of material generated by living organisms(biogenic/fossils), or lithification of chemically precipitated material from a mineral bearing solution due to evaporation(precipitate).
The rock cycle can account for every thing on our planet from sand to the Earth’s types of crust. It is greatly influenced by the other elements on our planet: water and air mainly. The most exciting thing about the rock cycle is that it exists on other planets.
We have written many articles about the Rock Cycle for Universe Today. Here’s an article about the lithosphere, and here’s an article about how rocks are formed.
If you’d like more info on the Rock Cycle, take a look at the U.S. Geological Survey Homepage, and here’s a link to an article about Soil Forming Factors.
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