<Tr> <Td_colspan="2"> Mafic: amphibole and pyroxene, sometimes plagioclase, feldspathoids, and / or olivine . </Td> </Tr> <P> Basalt (/ bəˈsɔːlt, ˈbæsɒlt, - sɔːlt /) is a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface of a planet or moon . Flood basalt describes the formation in a series of lava basalt flows . </P> <P> By definition, basalt is an aphanitic (fine - grained) igneous rock with generally 45 - 53% silica (SiO) and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase . This is as per definition of the International Union of Geological Sciences (IUGS) classification scheme . It is the most common volcanic rock type on Earth, being a key component of oceanic crust as well as the principal volcanic rock in many mid-oceanic islands, including Iceland, the Faroe Islands, Réunion and the islands of Hawaiʻi . Basalt commonly features a very fine - grained or glassy matrix interspersed with visible mineral grains . The average density is 3.0 g / cm . </P> <P> Basalt is defined by its mineral content and texture, and physical descriptions without mineralogical context may be unreliable in some circumstances . Basalt is usually grey to black in colour, but rapidly weathers to brown or rust - red due to oxidation of its mafic (iron - rich) minerals into hematite and other iron oxides and hydroxides . Although usually characterized as "dark", basaltic rocks exhibit a wide range of shading due to regional geochemical processes . Due to weathering or high concentrations of plagioclase, some basalts can be quite light - coloured, superficially resembling andesite to untrained eyes . Basalt has a fine - grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow; it is often porphyritic, containing larger crystals (phenocrysts) formed prior to the extrusion that brought the magma to the surface, embedded in a finer - grained matrix . These phenocrysts usually are of olivine or a calcium - rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt . </P>

Where do most rocks have a basaltic composition