The rock cycle is a general model that describes how various geological processes create, modify, and influence rocks (Figure 10a-1). This model suggests that the origin of all rocks can be ultimately traced back to the solidification of molten magma. Magma consists of a partially melted mixture of elements and compounds commonly found in rocks. Magma exists just beneath the solid crust of the Earth in an interior zone known as the mantle.
Igneous rocks form from the cooling and crystallization of magma as it migrates closer to the Earth’s surface. If the crystallization process occurs at the Earth’s surface, the rocks created are called extrusive igneous rocks. Intrusive igneous rocks are rocks that form within the Earth’s solid lithosphere. Intrusive igneous rocks can be brought to the surface of the Earth by denudation and by a variety of tectonic processes.
All rock types can be physically and chemically decomposed by a variety of surface processes collectively known as weathering. The debris that is created by weathering is often transported through the landscape by erosional processes via streams, glaciers, wind, and gravity. When this debris is deposited as a permanent sediment, the processes of burial, compression, and chemical alteration can modify these materials over long periods of time to produce sedimentary rocks.
A number of geologic processes, like tectonic folding and faulting, can exert heat and pressure on both igneous and sedimentary rocks causing them to be altered physically or chemically. Rocks modified in this way are termed metamorphic rocks.
All of the rock types described above can be returned to the Earth’s interior by tectonic forces at areas known as subduction zones. Once in the Earth’s interior, extreme pressures and temperatures melt the rock back into magma to begin the rock cycle again.