Basic Rocks

igneous rocks that are comparatively low in silica (45–55 percent) and rich in magnesium and calcium. Basic plagioclases (labradorite, bytownite) are characteristic of the mineral composition of basic rocks; olivine and other minerals that are undersaturated with silica are also present. Basic rocks may be either intrusive (gabbro, norites, anorthosites) or extrusive (basalts). Most of the present-day volcanoes erupt basic lavas, and basic lavas are also characteristic of all fissure eruptions.