A rock which originated as molten magma from beneath the earth's surface and subsequently came to the surface as an extrusion, or remained below ground as an intrusion. The nature of the rock depends in part on the rate at which it cooled; as intrusions of magma slowly solidify, enough time elapses for large crystals to form whereas extrusions cool quickly, leaving little time for crystal growth. Thus, a coarse-grained, intrusive igneous rock has a fine-grained, extrusive counterpart; granite is coarse rhyolite and gabbro is coarse basalt. Igneous rocks are also classified as acid or basic, according to whether their silica content is high (e.g. granite), or low (e.g. basalt).