<P> By definition, basalt is an aphanitic (fine - grained) igneous rock with generally 45 - 55% silica (SiO) and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase . 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 Hawaii . 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> <P> Basalt with a vesicular texture is called vesicular basalt, when the bulk of the rock is mostly solid; when the vesicles are over half the volume of a specimen, it is called scoria . This texture forms when dissolved gases come out of solution and form bubbles as the magma decompresses as it reaches the surface, yet are trapped as the erupted lava hardens before the gases can escape . </P> <P> The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are generally referred to as diabase (also called dolerite) or, when more coarse - grained (crystals over 2 mm across), as gabbro . Gabbro is often marketed commercially as "black granite ." </P>

What is the origin of vesicles in a basalt