batholith, enormous mass of intrusive igneous rock, that is, rock made of once-molten material that has solidified below the earth's surface (see rock). Batholiths usually are granitic (see granite) in composition, have steeply inclined walls, have no visible floors, and commonly extend over areas of thousands of square miles. Batholiths are formed either as one large mass or many smaller masses at great depths in the earth's crust and are exposed at the surface only after considerable erosion of the overlying mountain mass. They are commonly associated with lithospheric plate boundaries, where the interactions between plates can produce sufficient heat to melt crustal rocks on a large scale and form batholiths (see plate tectonics). One of the largest single batholiths in North America is the Coast Range batholith of W Canada and Alaska, encompassing an area of about 73,000 sq mi (182,500 sq km). Important batholiths in the United States include the Idaho batholith, 18,000 sq mi (45,000 sq km), and the Sierra Nevada batholith, 16,000 sq mi (40,000 sq km).