Komatiite

An ultramafic volcanic rock derived from the Earth's mantle, crystallized from lava that contained at least 18 percent magnesium oxide and that erupted at extremely high temperatures, potentially exceeding 1600 degrees Celsius, well above typical basaltic lava. Its composition is notably low in silicon, potassium and aluminum and high, sometimes extremely high, in magnesium, a result of a high degree of partial melting, often over 50 percent, of the mantle that produced magma fundamentally different from ordinary basalt. Komatiites are exceptionally rare today because almost all of them formed during the Archean Eon, between about 4.03 and 2.5 billion years ago, when the mantle is estimated to have been 100 to 250 degrees Celsius hotter than today due to residual heat from the planet's accretion and a greater abundance of radioactive isotopes; only a few younger examples are known. The rock is largely restricted to Archean shield areas and occurs within greenstone belts, with notable localities including the Komati Valley in South Africa, its type locality, as well as sites in Western Australia, Canada and Colombia's Gorgona Island, home to the youngest known komatiites.