The term sima designates the lowest layer of the Earth's crust and the ocean floors (basal crust, basal layer, basalt layer). Sima comprises magnesium rich silicon minerals and when it rises to the surface as lava sima forms mafic rocks (usually basalts) or rocks with mafic minerals. The densest sima forms ultramafic rocks.
Constituent elements include Mg, Fe, Ca, and Na. Mafic minerals are usually dark in color, with a specific gravity greater than 3 (2800 to 3300 kg/m^3). Common rock-forming mafic minerals include amphibolte, biotite, olivine, pyroxene, and other micas, augite and the calcium-rich plagioclase feldspars. Common mafic rocks include basalt and gabbro.
Because of its lower silica content, molten mafic lava has a lower viscosity than felsic lava. Mafic volcanoes are less explosive than felsic lava eruptions because water and other volatiles more easily and gradually escape from mafic lava. Most mafic lava volcanoes are oceanic volcanoes, like Hawaii.