Rocks can form in many different ways and from many different materials. Rocks provide a historical record of geologic events, and they give insight into interactions among components of the Earth System.
A rock is a coherent, naturally occurring solid that consists of an aggregate of minerals or, less commonly, a body of glass. Grains rock stick together to form a coherent mass either because they are bonded by natural cement, mineral material that precipitates from water and fills the space between the grains, or because they interlock with one another. Rocks whose grains are stuck together by cement are called clastic rocks, whereas rocks whose crystals interlock with one another are called crystalline rocks. Bedrock is rock that remains attached to Earth’s crust. An exposure of bedrock is an outcrop.
Geologists recognize three basic groups of rocks: igneous rocks, which form by the freezing (solidification) of molten rock; sedimentary rocks, which form either by the cementing together of fragments (grains) that had broken off pre-existing rocks, or by the precipitation of mineral crystals out of water solutions at or near the Earth’s surface; and metamorphic rocks, which form when pre-existing rocks change character in response to a change in temperature, pressure, or chemical environment. Metamorphic change occurs in the solid state, which means that it does not require melting. Different rock types form in different geologic settings. Each of the three groups contains many individual rock types, distinguished from one another by physical characteristics such as grain size and shape, composition, texture, and layering. Some rock grains are equant (meaning they have the same dimensions in all directions), where in others the grains are inequant (meaning the dimensions are not the same in all directions). The layering in sedimentary rocks is called bedding, whereas the layering in metamorphic rocks is called metamorphic foliation.
—April 2023
Lance Jones 