Ultramafic

Ultramafic is a term used in geology to describe igneous rocks that have a very low silica content and are rich in magnesium and iron. The mineral assemblage is dominated by mafic minerals, especially olivine and pyroxene, with feldspar and quartz typically absent or present only in trace amounts. Ultramafic rocks are among the densest ordinary crustal rocks, often exceeding 3.2 g/cm3 in density.

These rocks originate from the Earth’s mantle and are commonly exposed in ophiolites, mantle xenoliths, and ultramafic intrusions. They form through processes such as partial melting of mantle material, fractional crystallization of ultramafic magmas, or hydration and alteration in subduction zones. When exposed to surface conditions, ultramafic rocks can alter to serpentine minerals in a process called serpentinization, producing greenish, slippery rocks rich in Mg and water.

Common ultramafic rock types include peridotite, dunite, harzburgite, and lherzolite, which are characterized by varying proportions of olivine and pyroxenes. Pyroxenite is another ultramafic rock where pyroxene is dominant. Ultramafic rocks also occur as volcanic equivalents, such as komatiites, which are highly magnesian and ultramafic in composition but erupted at high temperatures in Earth's early history. Ultramafic rocks are important in studies of mantle composition, tectonics, and the evolution of magmatic systems.