Basalt

Basalt, the most common variety of volcanic rock, composed almost entirely of dark, fine-grained silicate minerals, chiefly plagioclase feldspar and pyroxene, and magnetite. The extrusive equivalent of gabbro, it forms by the outpouring of lava all along the world's mid-ocean ridges (see Oceanic Ridge), where seafloor spreading continually adds new crust to counterbalance that lost by subduction. Usually dark grey in colour, basalt often has a vesicular texture, preserving vestiges of bubbles produced by expanding steam as lava cools and solidifies. Also characteristic are pillow-shaped masses caused by rapid cooling of lava erupted on the sea floor. In addition to lava flows, basalt is also found in the form of dykes and sills. Columnar jointing, as exhibited by the Giant's Causeway in Ireland, is a common feature of these shallow intrusive bodies. Skye and some other Hebridean islands in Scotland are composed of plateau basalts.