Komatiites are very rare igneous rocks. They are one variety of extrusive ultramafic igneous rock (although a komatiite in Canada has been interpreted to be an intrusive sill). They are named after the Komati River 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), when Earth's heat flux was much higher.
Based on experiments, it's been determined that komatiite lava, when originally erupted, was considerably hotter than any modern lava type on Earth. Komatiite lava also had a very low viscosity, flowed like an ultradense gas, and could solidify into individual lava flows only 1 cm thick.
The classic texture of komatiites is spinifex texture, named after clumps of long, spiky (& painful!) grasses common in Australia. Komatiites with spinifex texture are characterized by having long blades of olivine mixed with smaller-scale blades of pyroxene.
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.
Komatiite (6.3 cm across) showing spinifex texture - long blades of olivine mixed with pyroxene. This sample is from northeastern South Africa's Barberton Greenstone Belt. It comes from the Komati Formation (Onverwacht Volcanics Group). Published dates on this unit range from 3.482 to 3.657 billion years (Paleoarchean). Research on the komatiites from here indicates that these ultramafic lavas erupted on an ancient oceanic plateau (a submarine lava plain eruption).
Komatiite (same sample as above; field of view: ~1.8 cm across) showing spinifex texture (long blades of olivine mixed with pyroxene).
Komatiite (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.
Komatiite (same sample as above; field of view: 3.0 cm across)