basalt

basalt ‘Basalt’ is a term widely used and abused. It has been employed both as a specific rock name, designating a narrow range of compositions (46–50 per cent silicon dioxide, (SiO₂)), and as a general term for almost any dark, fine-grained igneous rock, more particularly for those with the minerals plagioclase feldspar and pyroxene. Not all rocks of basaltic composition are, however, dark and fine-grained, nor are all dark, fine-grained, igneous rocks of basaltic composition. While the term ‘basalt’ remains a useful field name, it should be borne in mind that whole books have been written on the finer details of the sub-classification of rocks of basaltic composition. In practice many basalts also contain easily visible crystals (phenocrysts) of feldspar, pyroxene, olivine, or amphibole.

Basalts are the most common type of rock on Earth; they are also abundant on the moon and on other planets in the solar system. Almost the entire ocean floor is made of basalt, which was, and still is, erupted at mid-ocean ridges. These mid-ocean ridge basalts (MORB for short), in common with some other types of basalt, are erupted as a result of the divergence of tectonic plates. As two plates pull apart, the Earth's mantle rises to fill the gap, is decompressed, and melts to form magma. As the melt reaches the surface of the Earth it cools to form basalt.

Most MORBs are of what is known as tholeiitic composition; i.e. they have free silica and contain calcium-poor pyroxene. The basalts of oceanic islands (OIBs) are also tholeiitic, but vary widely in composition. Of special interest are the komatiites, which are all more than 2.5 billion years old. They are rich in magnesium and contain large olivine crystals. Komatiites were evidently erupted at higher temperatures than other volcanic rocks, and it is likely that they were generated by plumes in the mantle.

Extensive flows of basalt are present on the Moon, where they form the dark areas known as the maria. These lunar basalts are in general similar to those found on Earth, although they are much older (all the available samples have of ages of more than 3000 million years), but their chemical composition is more variable and they contain some minerals not known on Earth. A notable feature of the lunar basalts is the complete absence of water.

Judith M. Bunbury