The rock cycle is an interconnected chain of events that keeps rock constantly on the move around the Earth. Like other cycles in nature, such as the water cycle and the carbon cycle, it ensures a steady recycling of geological materials, and it explains the origins of various rock types. This geologic cycle also interplays with other cycles, illustrating the interdependent nature of the various systems on Earth.
James Hutton, a geologist who worked during the 1700s, is generally credited with developing the first version of the rock cycle. Over time, geologists have refined the cycle to address new information, and some versions of it get extremely minute and very complex.
There are three main types of rock: igneous, sedimentary, and metamorphic. Igneous rock is formed when magma solidifies and cools, forming rocks like obsidian and basalt. Sedimentary rock, like sandstone, forms from an aggregation of sediments, while metamorphic rock is created when rock undergoes very high pressure inside the Earth's crust. These rock types are all interrelated.
Rock starts out in a molten form, as magma under the Earth's crust. When that magma is extruded in a lava flow, volcanic eruption, or seafloor seep, it solidifies and cools, turning into igneous rock. Over time, igneous rock erodes into sediments with the assistance of wind and water, and it is transported to new locations, where it becomes compacted and turns into sedimentary rock.
When sedimentary rock is transported into the Earth's crust, it undergoes compression, becoming metamorphic rock. Eventually, the metamorphic rock will travel so far into the Earth's crust that it will come into contact with magma and extreme heat, becoming molten again and restarting the rock cycle.
This cycle takes millions of years to make a complete loop, and a number of factors play into it, including the movement of tectonic plates and interaction with water. Tectonic plates can collide, pull apart, or subduct, with one plate sliding under another, changing the face of the Earth along with the composition of the Earth's rocks. Water plays a major role by actively eroding rock and transporting sediments to new locations. The rock cycle can also be affected by things like the movement of glaciers and earthquakes.
Geologists use their knowledge of this cycle to gather information about the age of the Earth, and to research specific rock formations. For example, the age of fossil deposits can sometimes be estimated by looking at the rock the fossils are embedded in, along with the surrounding material.