igneous rock

Encyclopedia Article


Igneous rock occurs both above and below the earth's surface. Intrusive igneous rock forms under the earth's surface when magma cools and hardens. As a result of its slow cooling process, intrusive igneous rock, such as granite, forms large crystals, which can be seen with the naked eye. Extrusive igneous rock forms when magma erupts as lava onto the earth's surface and cools quickly, forming small crystals. Basalt is one of the most common forms of extrusive igneous rock.

In the U.S. Southwest, intrusive and extrusive igneous rock can be found. Intrusive igneous rock is revealed in many of the monumental buttes, spires, cliffs, and mountains that comprise some of the dramatic scenery of the Southwest landscape, especially as softer materials erode away from the more durable rock component beneath. Intrusive igneous rock can be found in the surface lava flows and cinder cones that surround the numerous fields of extinct volcanoes in the region, such as those that comprise portions of El Malpais National Monument in New Mexico.

Photo Credit: 

"El Capitan, Monument Valley, Utah, May 19, 2008" by Robert Cutts is licensed under CC BY.