Columnar Basalt. Image Source:

Basalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition.

Basalt is a mafic, extrusive and fine grained igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a differentiated parent body such as large asteroids and moons, planetesimals, and planets. Basalts can also form in small intrusive bodies, such as an igneous dike or a thin sill. It is composed of approximately equal amounts of plagioclase feldspar and clinopyroxene, with smaller amounts of other minerals usually including olivine and magnetite. Basalt magma contains less than ~52 wt. % SiO2 (silica).

Due to the rapid cooling of the extruded magma exposed at the surface, large mineral crystals are unable to form in basalts leading to the fine-grained mineral texture. However, basalts can display a porphyritic texture containing larger crystals (phenocrysts) that formed prior to the final extrusion and are embedded in the fine-grained matrix. Other textural varieties include  glassy, massive, vesicular and scoriaceous.