Basalt may be a dark-coloured, fine-grained, rock composed mainly of plagioclase and pyroxene minerals. It most ordinarily forms as an extrusive rock, like a lava flow, but also can form in small intrusive bodies, like an igneous dike or a skinny sill. It has a composition similar to gabbro. The difference between basalt and gabbro is that basalt may be a fine-grained rock while gabbro may be a coarse-grained rock. 

 Black Basalt : basalt is an aphanitic (fine-grained) rock with generally 45-55% silica (SiO2) and fewer than 10% feldspathoid by volume, and where a minimum of 65% of the rock is feldspar in the form of plagioclase. It is the foremost common igneous rock type on Earth, being a key component of oceanic crust also because of the principal igneous rock in many mid-oceanic islands, including Iceland, the Faroe Islands, Réunion and the islands of Hawaii. Basalt commonly features a really fine-grained or glassy matrix interspersed with visible mineral grains. The average density is 3.0 gm/cm3. 

 Basalt underlies more of surface than the other rock type. Most areas within Earth’s ocean basins are underlain by basalt.