Basalt is a fine-grained, dark-colored mafic volcanic rock composed of plagioclase, ortho- or clinopyroxene, and minor Fe-Ti oxides, with or without olivine. Porphyritic samples contain large crystals (phenocrysts) of olivine, pyroxene, or plagioclase dispersed in a fine-grained glassy matrix, also called groundmass. Gas-rich samples contain abundant vesicles. Basalt contains 45–52 % SiO2 and 40–90 % ferromagnesian minerals. Basalt erupts as pillow lava, thick sheet flows, or fragmental scoria. It is the most common rock of the Earth’s oceanic crust and in lunar maria (ancient flood-basalt plains corresponding to the dark surfaces of the Moon). Basalt is also present in the crust of Mars and Venus. It forms by partial melting of the mantle and erupts in diverse tectonic settings: mid-ocean ridges, oceanic islands, subduction zones, continental rifts, and volcanic plateaus.