basic
basic
Said of an igneous rock having a relatively low silica content, sometimes delimited arbitrarily as 44% to 51% or 45% to 52%; e.g., gabbro, basalt. Basic rocks are relatively rich in iron, magnesium, and/or calcium, and thus include most mafic rocks as well as other rocks. Basic is one of four subdivisions of a widely used system for classifying igneous rocks based on their silica content: acidic, intermediate, basic, and ultrabasic.