Large masses cool slowly to form a polygonal joint pattern.

Basalt is a common grey to black rock. It is pushed out (extruded) by volcanos. The lava is rich in magnesium and iron. It has olivine and pyroxene minerals which make it dark in colour. Basalt is usually fine-grained because the lava cools rapidly on the Earth's surface. Flood basalts are commonplace, formed by lava pouring out and solidifying.

On Earth, most basalt magmas have formed by the hot molten mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta.

The crustal portions of oceanic tectonic plates are made mostly of basalt, produced from upwelling mantle below mid-ocean ridges.

Unweathered basalt is black or grey. Basalt blocks are often used for roads and pavements (sidewalks), and for houses and other buildings. It is hard-wearing and heat resistant.

Basalt can be made into fibre. Basalt fibre is often used as a safe replacement for asbestos. Intense heat and pressure can change basalt into its metamorphic rock equivalent. The rocks known as greenstone are basalt which has been metamorphised.