A dark, fine-grained igneous rock formed by cooling of iron- and magnesium-rich lava above the surface of the earth. Basalt is the main component of the oceanic crust of the earth.
GabbroA dark-colored intrusive igneous rock that consists mostly of the minerals plagioclase, pyroxene, and olivine. Gabbro has a similar composition to basalt, which is extrusive. Both gabbro and basalt are mafic rocks, composed of minerals high in iron and magnesium.