Rocktypes

Rock types are the major natural categories of rocks, classified by their origin. The three broad kinds are igneous, sedimentary, and metamorphic. The classification reflects the conditions under which the material formed or transformed, from cooling magma to surface deposition and deep burial under pressure.

Igneous rocks form when molten material cools and solidifies. If cooling occurs underground, the rocks are intrusive or plutonic, with coarse crystals such as granite or diorite. If eruption produces rapid cooling at the surface, the rocks are extrusive or volcanic, often fine-grained or glassy, as in basalt, rhyolite, obsidian. Texture and mineral content separate rocks into felsic, intermediate, mafic, and ultramafic groups, reflecting feldspar and quartz content versus dark minerals.

Sedimentary rocks originate from weathered and eroded fragments or from precipitation of minerals from water. Clastic varieties form by compaction of sand, silt, and clay into sandstone, siltstone, and shale. Chemical and biochemical varieties form from mineral precipitation or biological activity, yielding rocks such as limestone, chert, rock salt, and coal. Sedimentary rocks often preserve fossils and layered structures.

Metamorphic rocks arise when existing rocks are altered by heat, pressure, or chemically active fluids without melting. Regional metamorphism produces foliated rocks such as slate, phyllite, schist, and gneiss, associated with mountain-building zones. Contact metamorphism forms non-foliated rocks like marble and quartzite adjacent to hot intrusions. Metamorphism changes mineralogy and texture, often strengthening rocks and altering permeability.

Together, rock types form a connected cycle. Studying rocks informs about Earth's history, tectonics, and resource distribution, including building materials, aggregates, and energy resources. Identification relies on mineral content, texture, layering, and relationships to surrounding rocks.