Credit: U.S. Geological Survey.
A rock is a naturally occurring mixture of minerals, mineral matter, or organic materials. Three main types occur: sedimentary rocks, formed by weathering and mechanical sorting on the Earth's surface; metamorphic rocks, which are rocks that have been transformed by the effects of high temperature and pressure; and igneous rocks, derived from magma (for example, volcanic rocks ).
The primary constituents of rocks are oxygen and silicon, combined with each other to form silica (see quartz) and with each other and further elements (e.g.,aluminum, iron, calcium, potassium, sodium, and magnesium) to form silicates. Together, silica and silicates make up about 95% of the Earth's rocks.
Igneous rocks form from magma, a molten, subsurface complex of silicates. They are the primary source of all the Earth's rocks.
Sedimentary rocks are consolidated accumulations of fragmented inorganic and organic material. They are of three types: classic, formed of weathered (see erosion) particles of other rocks (e.g., sandstone); organic deposits (e.g., coal, some limestones); and chemical precipitates (e.g., evaporites). (See also fossil; stratigraphy.)
Metamorphic rocks have undergone change within the Earth under heat, pressure, or chemical action. Sedimentary, igneous, and even previously metamorphosed rocks may change structure or composition in this way.