An igneous rock consisting dominantly of mafic mineral, containing tiron and magnesium and with much less silicon and aluminum than most crustal rocks. Most come from the Earth's mantle. Includes dunite, peridotite, amphibolite, and pyroxenite.
Typically, dark coloured rocks that have been intruded into the Earth or extruded underwater in a marine environment. May have been altered by heat and pressure producing foliation in the rock. They can be low strength, sheared and altered and a potential source of challenging ground conditions.
Ultramafic (or ultrabasic) rocks are igneous and meta-igneous rocks with very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). The Earth's mantle is considered to be composed of ultramafic rocks.