Komatiite is named for a locality on the Komati River of South Africa. It consists largely of olivine, making it the same composition as peridotite, but unlike the deep-seated, coarse-grained peridotite it shows clear signs of having been erupted. Only extremely high temperatures can melt rock of that composition, and most komatiite is of Archean age, reflecting the fact that Earth's mantle was much hotter 3 billion years ago than today.
Komatiite is extremely rich in magnesium and low in silica. Nearly all examples known are metamorphosed, and we must infer its original composition by careful petrological study. One distinctive feature of some komatiites is spinifex texture, in which the rock is crisscrossed with long, thin olivine crystals. Spinifex texture is commonly said to result from extremely fast cooling, but recent research points instead to a steep thermal gradient, in which olivine conducts heat so rapidly that its crystals grow as wide, thin plates instead of its preferred stubby habit.