Basalt, buh SAWLT, is a hard, dark volcanic rock. It is formed by lava that flows from volcanoes or from cracks in the earth, and hardens. There are many types of basalts. They all have similar chemical compositions, but they vary in crystallinity. They range from fine-grained rocks with uniform crystals to crystal-poor glasses. Basalt consists chiefly of two minerals, pyroxene and plagioclase feldspar.