The rock cycle describes the transition of rock materials through several stages over periods of geologic time (Lutgens, Tarbuck, and Tasa, 2010). This progression is part of the system of Earth. Defined as a single unit, study of the rock cycle helps to understand the many interactions and influences throughout the various pieces of Earth’s system. As an isolated series of events, the sequence helps to describe the relationships between the three main categories of rock (igneous, sedimentary, and metamorphic), as well as the forces that result in changes between these forms.
The basic rock cycle begins at the magma stage. At this point, the molten material cools and crystallizes to form igneous rock. Crystallization can occur at the surface following a volcanic eruption, but it is most commonly found beneath the ground. Igneous rock then undergoes the weathering process (atmospheric forces), which breaks down the material into sediment. Due to erosion, sediment commonly falls to the ocean floors, but can end up in other areas of deposit as well. The settled material is compacted under pressure (or cemented) in a process known as lithification, which describes the formation of sedimentary rocks. Finally, if the newly formed sedimentary rock is exposed to the pressure and temperatures found deep in the crust of the Earth, it will be converted to another type; metamorphic rock. If this material experiences even higher temperatures, it will be melted into magma to begin the cycle anew.
While the process described above is the basic rock cycle, there are alternative paths that exist. For example, igneous rock can be converted directly to metamorphic rock if it is exposed to enough heat and pressure. Similarly, exposed metamorphic and/or sedimentary rock can be reduced to sediment due to atmospheric and aquatic influences. These processes clearly demonstrate that rock is far from a stationary and static material.
Lutgens, F. K., Tarbuck, E. J., and Tasa, D. (2010). Foundations of earth science (6th ed.). Boston, MA: Prentice Hall.