Rocktype

Rocktype is a geological term used to classify rocks into broad categories based on origin and composition. In geology, three principal rocktypes are recognized: igneous, sedimentary, and metamorphic.

Igneous rocks form when molten rock cools and solidifies. Extrusive rocks crystallize at or near the Earth’s surface (for example basalt and rhyolite), while intrusive rocks crystallize below the surface (such as granite and diorite). Their texture ranges from glassy to coarse-grained, and their mineral composition ranges from felsic to mafic (and intermediate), influencing color and hardness.

Sedimentary rocks arise from the accumulation and lithification of sediment, or from chemical and biochemical precipitation. Clastic sediments are cemented grains (as in sandstone and shale); chemical rocks form from precipitation (such as limestone and rock salt); organic rocks form from accumulated plant or animal debris (such as coal and certain limestones). They often preserve fossils and show layered structures.

Metamorphic rocks originate from existing rocks altered by heat, pressure, and chemically active fluids, without melting. Common metamorphic types include foliated rocks such as slate, phyllite, schist, and gneiss, which show planar mineral alignment due to directed pressure, and non-foliated rocks such as marble and quartzite, which lack such alignment.

Rocktype terminology is used in field descriptions, resource exploration, and construction to indicate likely mineral content, strength, and durability. Boundaries among rocktypes are not always sharp, and rocks can exhibit characteristics of more than one type in complex geological histories.