basalt

One of the most abundant rock types underlying the Earth's surface, especially the upper oceanic crust, basalt is a dark-colored igneous rock. It can be fine-grained and compact, although as it usually reaches the earth's surface via volcanic activity and lava flows, hardening into rough, crystallized, pumiceous formations. Many areas within the U.S. Southwest are scattered with extinct volcanoes, and basalt is therefore abundant throughout the region.