Komatiites are very rare, magnesium-rich, extrusive, ultramafic igneous rocks. They are named after the Komati River Valley in South Africa, the type locality. Komatiite is an exceedingly rare type of lava. No volcano on Earth erupts this material today. Komatiites are essentially restricted to the Archean (4.55 to 2.5 billion years ago). Experimental evidence has shown that komatiite lavas, when originally erupted, were considerably hotter (~1600º C) than any modern lava type on Earth. This indicates that Earth’s mantle was much hotter than now. Other geologic evidence also indicates that early Earth’s heat flux was much higher than today’s.
Komatiite lava had a very low viscosity - it could flow like an ultradense gas. This property permitted the solidification of some individual lava flows that are only 1 cm thick.
The classic texture of komatiites is spinifex texture, named after clumps of long, spiky (& painful!) grasses. Komatiites with spinifex texture have short to long blades of olivine mixed with smaller-scale blades of pyroxene.
All Archean komatiites are metamorphosed - the original igneous mineralogy (olivine, pyroxene, minor chromite, etc.) is gone to mostly gone. Such rocks are best termed metakomatiites, but the prefix “meta-” is usually not specified.
Komatiites have economic significance, as many are closely associated with copper-nickel minerals (chalcopyrite & pentlandite), plus minor platinum-group elements, arsenides, bismuthides, and maybe a little gold and silver. Komatiites are a world-class source of nickel in Canada and Western Australia.
Two komatiite samples from South Africa and Ontario, both having excellent spinifex texture, are shown below.
Metakomatiite (6.3 cm across) showing spinifex texture. The original igneous rock consisted of long blades of olivine mixed with pyroxene. Greenschist-facies regional metamorphism has altered the original olivine spinifex komatiite into black serpentinite.
Stratigraphy & age: Komati Formation, Onverwacht Volcanics Group, Swaziland sequence, Barberton Greenstone Belt, 3.481-3.482 b.y.
Locality: Komati River Valley, northeastern South Africa.
Komati Formation komatiites are the oldest well-preserved examples in the world. Published research indicates that these ultramafic lavas erupted on an ancient oceanic plateau (a submarine lava plain environment).
Metakomatiite (same sample as above; field of view: ~1.8 cm across) showing spinifex texture. The rock is now a black serpentinite.
Metakomatiite (field of view: 3.0 cm across) with spinifex texture. This rock is from the Pyke Hill Komatiite in Munro Township, east of Timmins, southern Cochrane District, eastern Ontario, southeastern Canada. Pyke Hill is a world-class locality for komatiites. The rocks there are part of the Abitibi Greenstone Belt of Late Archean age. Published dates on komatiites from Pyke Hill range from 2.675 to 2.746 billion years. The best available age for this rock appears to be between 2.703 and 2.715 billion years.
Metakomatiite (same sample as above; field of view: 3.0 cm across)