Geomorphology of the Sonoran Desert

The face of the Sonoran Desert is complex. Not hidden behind a thick protective layer of vegetation, its surface is constantly exposed to forces of sun, wind and water. Climatic elements have combined with geologic forces over the millennia to create varied landforms. These landforms include a jumbled mixture of playas, sandy beaches, broad plains, mesas, arroyos, extensive sheets of lava, jagged cliffs, mountains (of volcanic, sedimentary, and granitic rock), alluvial fans, bajadas, "the most extensive desert dune system in North America" (IMADES), as well as deep river valleys and deltas.

In the bajadas of the Sonoran desert are found aquifers bearing precious groundwater that has been mined for the growth of cities such as Tucson and for irrigated agriculture not reached by surface water. (Figure 5: Alluvial Basins in Central Arizona, from Geohydrology of Arid Lands.)

Related Links

For more in-depth definition of Geomorphological subdivisions of intermontane areas visit the following web page. This location provides definitions and images for Piedmont Slope (alluvial fan, older alluvial deposit, bajada, wash, and undifferentiated sediment deposit) and Valley Floor (fluvial channel, fluvial floodplain, fluvial terrace, lacustrine terrace, reservoir and playa). [all these links to /rkd_dir/ are broken now]
http://www.geol.lsu.edu/rkd_dir/intermontaine.html.

• For specific definitions:
  Playa
  Inselberg
  Bajada

For excerpt from the Richard Scarborough's chapter "The Geologic Origin of the Sonoran Desert" in ASDM's book A Natural History of the Sonoran Desert go to
http://www.desertmuseum.org/asdmpress/natural_history/geologic_origin/landform.html

For an excellent illustration of "Typical Sonoran Desert landforms" go to
http://www.desertmuseum.org/asdmpress/natural_history/geologic_origin/geo_illustration2.html

The region, with the exception of most of the Baja California peninsula falls within the basin and range province of North America and is characterized by multiple, small mountain ranges lying in a predominantly northeasterly-southeasterly direction. These mountains are separated by basins or valley's filled with as much as 5000 feet of gravel, sand and clay that washed down from the mountains. The formation of the basin and range region was the result of the earth's crust being pulled apart "in a giant geo-taffy pull" by the movement of the Pacific Ocean tectonic plates' moving in a northwest direction (Scarborough 2000, p 75). In some subregions in Sonora, the mountain ranges have been eroded nearly to their roots and often take the appearance of inselbergs or isolated rock masses. These masses are separated by gently sloping bajadas where alluvial fans merge separate these mountains (West 1993). The ranges of the Sierra Madre Occidental that form the eastern border of the region and the ranges, a continuation of the line of mountains from the Aleutian Islands south, that form the spine of the Baja Peninsula also are oriented northwest to southeast (Johnson 1972).

Space shuttle photo looking south over the southwestern
United States and Baja California, Mexico

The same tectonic forces that created the basin and range province were responsible for the formation of the Gulf of California or Sea of Cortez. The Gulf lies along the San Andreas Fault which, in a tearing sideways action, ripped off part of the mainland Mexico (Scarborough 2000) and slowly carried it northwest approximately 250 miles (Johnson 1972) to form what is now the Baja Peninsula. The land to the west of the fault is still moving northwest at an average of a few inches each year (Scarborough 2000.)

Because of the remarkable diversity of landform and vegetation within the region it is easier to understand and discuss the region when it is divided into subregions. Based on Shreve (1951) and McGinnies (1976) these regions are as follows:

• Lower Colorado Valley (which includes the Grand Desierto)
• Arizona Uplands
• Plains of Sonora
• Foothills of Sonora
• Central Gulf Coast
• Vizcaíno Region
• Magdalena Region

Note: Some of these regions, such as the Foothills of Sonora or the Vizcaíno and Magdalena Region are no longer included in the discussion of the Sonoran Desert by organizations such as the Arizona-Sonoran Desert Museum and The Nature Conservancy.

Related Links

• Grand Desierto, Sonoran Desert - from Geomorphology from Space, NASA. 1986.
• Central Region of Baja California Peninsula - Smithsonian Institution National Museum of Natural History. Excellent description and discussion.

"The desert is traversed by many mountain ranges, some of them long, some short, some low, and some rising upward ten thousand feet. They are always circling you with a ragged horizon, dark-hued, bare-faced, barren - just as truly desert as the sands which washed down from them."

(--John Van Dyke. The Desert: Further Studies in Natural Appearances.