rocks

Rocks are naturally occurring solid aggregates of minerals or mineraloids. They form the Earth’s crust and interior and vary in composition, texture, and structure. Geologists classify rocks by origin into three main groups: igneous, sedimentary, and metamorphic.

Igneous rocks form when molten rock cools and solidifies. Intrusive (plutonic) rocks crystallize below the surface and tend to be coarse-grained (for example, granite); extrusive (volcanic) rocks erupt or crystallize at the surface and are fine-grained or glassy (for example, basalt, rhyolite, obsidian). The mineral composition of igneous rocks is often described as felsic, intermediate, mafic, or ultramafic, reflecting silica-rich light-colored minerals versus dark-colored minerals.

Sedimentary rocks form from the accumulation and consolidation of sediments or from the precipitation of minerals from water. Clastic types (sandstone, shale) are formed from fragments of other rocks; chemical rocks (limestone, rock salt) form by mineral precipitation; organic rocks (coal, some limestones) form from accumulated plant or animal matter. Features such as bedding and fossils aid interpretation.

Metamorphic rocks arise from existing rocks transformed by heat, pressure, and chemically active fluids without melting. They are broadly foliated (slate, schist, gneiss) or non-foliated (quartzite, marble).

The rock cycle describes how rocks may change from one type to another over time through weathering, erosion, burial, metamorphism, melting, and solidification. Rocks are used for building materials, raw minerals, and studies of Earth’s history.