Rocks explained 2: Basalt

Basalt is a fairly familiar word, although few laymen could define it. In fact, basalts are the commonest rocks of the Solar System. On Earth, they are found in all tectonic settings and of all ages throughout geological time. Basalts are volcanic rocks, rich in magnesium and poor in silica, consisting of plagioclase (with a composition in the labradorite range) and the mafic minerals: olivine, pyroxene, iron oxides and sometimes hornblende. They are dark-coloured, fine-grained (although larger crystals, known as ‘phenocrysts’ or ‘megacrysts’ may occur) and are typically found as lava flows. Basalt makes up enormous accumulations, often over 1 million square kilometres in size, known as ‘large igneous provinces’ or LIPs, which may be associated with mass extinctions. Other, more siliceous, rocks are hypothesized to be derived from basaltic magmas by the process of igneous differentiation.