Great Basin, Nevada -- Basin (foreground) and Range (background)
Photograph by Jenn Nylund, August 1997
Processes of compression and extension cause geological formations to bend or break, which alters the landforms on the surface. Most recently, the southwestern United States has undergone stress from extension. One result of this extension has been the creation of "normal faults", or fractures in the rocks, in which one block along the fault moves up, creating an elevated mountain range, and the adjacent block moves down, creating a relatively lower basin, which is filled with erosional debris from the adjacent ranges. Often the uplifted block also rotates on its long axis, which causes the block to slope (dip) away from the fault, creating a relatively steep face along the fault and a more gentle slope on the opposite side of the range. Most mountain ranges in the Basin and Range Province are "fault-block" ranges created along normal faults.
Tularosa Basin (White Sands) and San Andres Mountains from the Sacramento
Mountains
Osha Trail, Lincoln National Forest, near Cloudcroft, New Mexico
Photograph by Matt Withroder, March 2003
Perhaps the most dramatic region of faulting in the southwestern United States has been along the relatively narrow Rio Grande Rift, which extends from southern Colorado to El Paso, Texas. Eventually, the upper ancestral Rio Grande flowed south through basins in the rift to a lake near present-day El Paso. Later, the headwaters of the lower Rio Grande, which flowed into the Gulf of Mexico, cut upstream to capture the upper Rio Grande and create the present drainage. Much of the volcanic activity in New Mexico was associated with the Rio Grande Rift. In southern New Mexico, this activity was generally limited to basalt flows, such as Valley of Fires (described elsewhere on this website). In northern New Mexico, however, stratovolcanoes (such as Valle Caldera) and cinder cones (such as Capulin Mountain) created a second type of mountain. Numerous volcanoes also erupted in northern Arizona along the edge of the Colorado Plateau. See the webpage "Volcanic Features" in this Geology section for more information.
Valley of Fires Lava Flow (foreground, filling basin), Tularosa
Basin, New Mexico
Photograph by Mark Eberle, March 2003
In addition to fault-block ranges and volcanoes, a third type of mountain occurs in the southwestern United States. Earlier in the geological history of the region, rocks were stressed by forces of compression that caused folds rather than faults. An upward fold is called an anticline; a downward fold is called a syncline. On our trips, we see two mountain ranges that represent anticlines: the Santa Catalina Mountains east of Tucson and the Sacramento Mountains east of Alamogordo in southern New Mexico. The Sacramento Mountains on the east and the parallel San Andres Mountains on the west bracket the Tularosa Basin. Initially, the basin was the center of an anticline that later collapsed. Information on two geological features of the Tularosa Basin, White Sands and Valley of Fires, is available elsewhere in this Geology section of the website. Information on the montane ecosystems also is available elsewhere on this website ("Montane Communities").