Most of Earth's mountain ranges have formed due to compression within or along tectonic plates and many others are volcanic in nature, having developed above a mantle plume or along subduction zones. A third type of orogeny, fault-block mountains, rise along faults where the crust is under tension; this stretching generally occurs between areas of uplift or due to friction with other tectonic plates (often distant from the rift zone). As the crust is stretched, fault lines (some new and others old) rupture perpendicular to the line of stress and one edge of the crust slips above the other; as a consequence, fault-block mountains have a steep face along the fault and a gentler slope on their other side.
In the U.S., fault-block mountains are most common across the Great Basin, in the desert Southwest and along the Rio Grande Valley. In the Great Basin, topographic waves of these fault-block ranges, running north to south and spaced east to west, rise above high, flat desert and scrublands between the Wasatch Plateau of Utah and the Sierra Nevada of California. Among our more well known fault-block ranges are the Tetons of Wyoming (their steep edge facing east) and the Sandia Mountains east of Albuquerque, New Mexico (their steep edge facing west).
Though difficult to appreciate during our brief life span, most of these fault-block ranges continue to rise as the rifting process persists. Earthquakes are common along and near these mountains and, eventually, those of the Southwest and Great Basin are expected to become islands as ocean waters invade from the Sea of Cortez.
