komatiite An extrusive igneous rock of peridotite composition, dominated by essential magnesium olivine accompanied by lesser amounts of aluminous clinopyroxene and chromite, and found as lava flows and shallow sills within Archaean and Proterozoic rock successions. Many komatiite flows were erupted above their liquidus temperature and may have had temperatures in excess of 1600°C. Because their melting temperature is much higher than that of basalts, sediments, etc., they were quite capable of melting the rocks over which they flowed. Flows commonly show an upper zone containing magnesium olivine or aluminous clinopyroxene crystals with spinifex texture, indicating extreme undercooling of the upper part of the flow by contact with the atmosphere on extrusion. Komatiites represent the only known examples of peridotitic magma.

spinifex texture An array of crisscrossing sheafs of subparallel, blade- or plate-like, skeletal, magnesium-rich olivine or aluminous pyroxene, between which is found a finer-grained aggregate of devitrified glass, skeletal pyroxene, and skeletal chromite. The texture is usually found as the product of extreme undercooling of magnesium-rich komatiite lava.