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Komatiite

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Komatiite (ko-MOTTY-ite) is a rare and ancient ultramafic lava, the extrusive version of peridotite. (more below)
Rare and ancient ultramafic lava
Photo courtesy GeoRanger via Wikimedia Commons
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. It is thought that only extremely high temperatures can melt rock of that composition, and most komatiite is of Archean age, in line with the assumption that Earth's mantle was much hotter 3 billion years ago than today. However, the youngest komatiite is from Gorgona Island, off the coast of Colombia, and dates from about 60 million years ago. There is another school that argues for the influence of water in allowing young komatiites to form at lower temperatures than usually thought. Of course, this would throw into doubt the usual argument that komatiites must be extremely hot.

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.

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