Plutonic rock composed chiefly of dark-coloured minerals such as olivine and pyroxene that contain abundant magnesium and iron. It is derived from magnesium and ferric (iron). The term mafic also applies to dark-coloured minerals rich in iron and magnesium as a group. Mafic rocks usually refers to dark-coloured igneous rocks such as basalt, but can also refer to their metamorphic counterparts.