tholeiite,fine-grained extrusive igneous rock, a basalt that contains plagioclase feldspar (labradorite), clinopyroxene (augite with pigeonite or hypersthene), and iron ore (magnetite and ilmenite). Tholeiitic lavas often contain glass, but little or no olivine. Tholeiite occurs as extensive plateaus (volumes on the order of 200,000 to 800,000 cubic km [50,000 to 200,000 cubic miles]) in the Pacific Northwest and the Deccan, western India, as well as in the lavas of the Lake Superior region, South Africa, Western Australia, and South America.