ROCK

Rock is a naturally occurring solid aggregate of minerals or mineraloids. It forms the bulk of the Earth's crust and can be identified by its mineral composition, texture, and mode of origin. A rock differs from a mineral in that it is typically a composite material composed of two or more minerals, may include organic matter, and lacks a definite chemical formula.

Rocks are traditionally classified by origin into three main groups: igneous, sedimentary, and metamorphic. Igneous rocks form by cooling and solidification of molten material. They are divided into intrusive (plutonic) rocks, such as granite, which cool slowly below the surface to produce large crystals, and extrusive (volcanic) rocks, such as basalt or rhyolite, which erupt or extrude at the surface and cool rapidly to form fine crystals or glass. Sedimentary rocks form by deposition and lithification of sediments, or by chemical precipitation. Clastic examples include sandstone and shale; chemical/biochemical types include limestone, coal, and rock salt. Metamorphic rocks arise from the transformation of existing rocks under high heat, pressure, or chemically active fluids, producing new minerals and textures. They are commonly foliated (slate, schist, gneiss) or non-foliated (marble, quartzite).

The rock cycle describes how rocks weather, erode, redeposit, lithify, melt, and metamorphose over time. Rocks have practical uses in construction, industry, and art, and they provide information about geological history through fossils and mineral content.