Ultramafic rocks are igneous rocks with very low silica content and high levels of magnesium and iron. These rocks, which include peridotite and dunite, are significant in petrology for understanding the composition of the Earth’s mantle and the processes of mantle melting and magma formation.
Reference: Le Maitre, R. W. (2002). “Igneous Rocks: A Classification and Glossary of Terms.” Cambridge University Press.