Basalt
Volcano Learning zone > Volcano Glossary
Basalt lava is responsible for the dramatic rivers of lava we see on the TV. Basalt lava is a black mafic extrusive rock derived from partial melting of the mantle. It is commonly found as the uppermost layer of oceanic crust. It largley consists of fine grained crystals of olivine,feldspar and pyroxene with other accessory minerals. It may contain gas bubbles filled with other minerals or porphry minerlas which are larger crystals formed in the magma. It is usually black with small gas bubbles.
Under water basalt forms pillow lavas when it pushes out elongate pillow shapes of lava around which a chilled skin forms on contact with cold sea water. As more lava arrives then the pillow bursts and the new outer skin is cooled.
On land basalt forms long fluid lava flows or high fountains of glowing lava. Columnar basalt can form if the lava flow is thick enough. Polyganol columns of lava which look man made almost.
Found in most volcanic areas but mainly in areas where a constructive ridge breaks the surface, rifting is occuring or hot spot activity. Can be found in Destructive margin setting.
