Basalt is a mafic extrusive rock, is the most widespread of all igneous rocks, and comprises more than 90% of all volcanic rocks. Because of its relatively low silica content, basalt lava has a comparatively low viscosity, and forms thin flows that can travel long distances. It is also found as intrusive dikes and sills. Many moon rocks brought back by Apollo astronauts are of basaltic composition. Basalt is the volcanic equivalent of gabbro.


Other specimens - Click the thumbnails to enlarge

Group - volcanic.
Colour - dark grey to black.
Texture - aphanitic (can be porphyritic).
Mineral content - groundmass generally of pyroxene ( augite), plagioclase and olivine, possibly with minor glass; if porphyritic the phenocrysts will be any of olivine, pyroxene or plagioclase.
Silica (SiO 2) content - 45%-52%.
Uses - aggregate, fill etc. in the construction and roading industries (best if olivine content is low); armour rock for seawalls; dimension stone, e.g. stone walls, curb stones, paving stones etc.
New Zealand occurrences - Northland, Auckland, Banks Peninsula, Timaru, Dunedin, Chatham Islands, Campbell Island, Auckland Island.