basalt

basalt A dark‐coloured, very fine‐grained igneous rock derived from volcanic upwellings, and mainly composed of plagioclase feldspar, pyroxene, and magnetite. Molten basalt spreads very rapidly to form lava sheets often hundreds of kilometres across, such as the Deccan Plateau of India, and basalt flows cover about 70% of the earth's surface. Basalt may crack into hexagonal columns, as at the Giant's Causeway, Co. Antrim.

Basalt is the principal rock of the sea bed and is associated with oceanic plate margins. It accounts for 90% of all volcanic extrusions.