ultramafic rock

Which Rock Is Low In Silica? Mafic rocks have a relatively low silica content and are composed mostly of pyroxenes, olivines and calcic plagioclase. These rocks (basalt, gabbro) are usually dark coloured, and have a higher density than felsic rocks. Ultramafic rock is very low in silica, with more than 90% of mafic minerals (komatiite,