Ultrabasic rocks are a large group of igneous rocks that are usually formed by the intrusion of mantle magma into the Earth’s crust or by the eruption of magma onto the ground surface. Ultrabasic rocks contain less than 45% SiO2 and higher contents of MgO and FeO but no quartz. The main minerals are olivine, pyroxene, and mafic plagioclase. Ultrabasic rocks have dark colours and high densities. Common types are peridotite and pyroxenite.