Geologists classify the rocks of earth's crust in one of three categories — igneous, metamorphic, or sedimentary — based on how the rock was created. Each type of rock has its own unique characteristics:
Igneous: Igneous rocks form from the cooling of melted rock (either lava or magma) into solid form. If the cooling occurs underground, the rock is an intrusive, or plutonic, igneous rock. If the cooling occurs on the earth's surface, the rock is an extrusive or volcanic rock.
Geologists describe different igneous rocks according to their texture and composition.
Metamorphic: Metamorphic rocks form when existing rocks are subjected to intense heat and pressure, usually deep below the earth's surface. These conditions change the original minerals of the rock into new minerals.
Geologists classify metamorphic rocks according to how much they have been changed from the original, or parent, rock. Low-grade metamorphic rocks still appear very similar to the parent rock, while high-grade metamorphic rocks have been changed so much that they look very different from the parent rock.
Sedimentary: Sedimentary rocks are either detrital or chemical. Detrital rocks are formed by the compaction of separate particles, or sediments, into a rock. The particles are pieces of a different, pre-existing rock that have been weathered and transported by wind, water, ice, or gravity. Chemical sedimentary rocks form from minerals that have been dissolved in water and precipitate out, forming a solid rock.
Geologists describe sedimentary rocks according to the size and shape of the particles in them or their mineral composition (in the case of chemical sedimentary rocks).
The rocks of earth's crust are constantly being recycled and changed into new forms through geologic processes. This continual transformation of rocks from one type to another is called the rock cycle. Through processes such as weathering, heating, melting, cooling, and compaction, any one rock type can be changed into a different rock type as its chemical composition and physical characteristics are transformed.