13.3: Creating the Desert Environment

How Deserts Form

The Mojave and Colorado Desert Provinces are desert environments. Deserts are defined as locations of low precipitation; they exhibit extreme temperatures because of the lack of moisture in the atmosphere, including low humidity and scarce cloud cover. Without cloud cover, the Earth’s surface absorbs more of the Sun’s energy during the day and emits more heat at night, meaning that even though the days can be extremely hot, the nights can still be quite cold.

Deserts are not randomly located on the Earth’s surface. The Mojave and Colorado Desert Provinces, like many of the world’s deserts, are located at latitudes between 15° and 30°. These deserts occur at the interfaces of major wind belts and are regions where the rate of evaporation exceeds that of precipitation (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Major global climatic regions in relation to atmospheric convection cells and the Earth's continents. Rising air and low pressure creates rain and wet environments at 0° and 60° latitudes, while high pressure, sinking air creates drier conditions at 30° and 90° latitudes. As a result, regions in arid and semi-arid latitude zones have little or no vegetation "Climate map" by Paul Webb, is licensed under CC BY-SA 3.0. Access a detailed description of this map.

In these regions, prevailing wind circulation in the atmosphere causes sinking, dry air currents that create deserts like the African Sahara and Australian Outback. These deserts can readily be observed from space based on their notable lack of vegetation (Figure \(\PageIndex{2}\)). Similar effects are found in polar regions, as cool dry air falls to the Earth, although polar regions are cold and dry, rather than hot and dry.

Figure \(\PageIndex{2}\): Earth from NASA's DSCOVR satellite shows arid desert regions of the African Sahara and the deserts of the Arabian Peninsula as tan regions."Earth image" by Raziel Abulafia, is licensed under CC0.

The Mojave and Colorado Desert Provinces are also impacted by their locations in the rain shadows of nearby mountain ranges. Rain shadow deserts form in the leeward side of mountain ranges (Figure \(\PageIndex{3}\)). As the wind drives air up and over mountains, atmospheric moisture is released as snow or rain. Atmospheric pressure is lower at higher elevations, causing the moisture-laden air to cool. Cool air holds less moisture than hot air, and precipitation occurs as the wind rises up the mountain. After releasing its moisture on the windward side of the mountains, the dry air descends on the leeward or downwind side of the mountains to create an arid region with little precipitation called a rain shadow.

Figure \(\PageIndex{3}\): Diagram of a rain shadow and its effects. “Rain shadow effect” by Meg Stewart, is licensed under CC BY-SA 2.0.

Video: Rain Shadow

This brief video explains the formation of rain shadows in response to topographic barriers.

California’s desert regions in the eastern part of the state are created by the rain shadow effect superimposed on regional belts of high atmospheric pressure that are established by interactions between the Trade Winds and the Prevailing Westerlies. Throughout the state, as westerly moist air from the Pacific rises over the Sierra Nevada and Cascades Mountains and the Transverse Ranges, it cools and loses moisture as condensation and precipitation on the upwind or rainy side of the mountains. This rain shadow effect also controls the climate conditions in the Basin and Range and Modoc Plateau Provinces to the north, and the Mojave and Colorado Desert provinces to the south.

Desert Weathering

Weathering takes place in desert climates by the same means as other climates, only at a slower rate. Without abundant water in the arid environment, the chemical breakdown of rocks proceeds extremely slowly relative to equivalent rocks in humid climates. However, the mechanical breakdown of rock proceeds relatively quickly in the arid climate. In wet regions, the ground remains partially or fully saturated; as a result, plants covering the landscape bind the soil, and hence, prevent erosion. In dry climates, soil forms very slowly, and much of the bedrock remains exposed to erosion.

Box \(\PageIndex{1}\) Atmospheric Circulation and Deserts

Adapted from Introduction to Oceanography by Paul Webb.

Geographic location, atmospheric circulation, and the Earth’s rotation are the primary causal factors of deserts. Solar energy converted to heat is the engine that drives the circulation of air in the atmosphere and water in the oceans. The strength of the circulation is determined by how much energy is absorbed by the Earth’s surface, which in turn is dependent on the average position of the Sun relative to the Earth. In other words, the Earth is heated unevenly depending on latitude and angle of incidence. Latitude is a line circling the Earth parallel to the equator and is measured in degrees. The equator is 0° and the North and South Poles are 90° N and 90° S respectively (see the diagram of generalized atmospheric circulation on Earth). Angle of incidence is the angle made by a ray of sunlight shining on the Earth’s surface. Tropical zones are located near the equator, where the latitude and angle of incidence are close to 0°, and receive high amounts of solar energy. The poles, which have latitudes and angles of incidence approaching 90°, receive little or almost no energy.

Inset Box Figure \(\PageIndex{1}\): Major global climatic regions in relation to atmospheric convection cells. Rising air and low pressure creates rain and wet environments at 0° and 60° latitudes, while high pressure, sinking air creates drier conditions at 30° and 90° latitudes. “Earth Global Circulation - en” by Kaidor, is licensed under CC BY-SA 3.0. Access a detailed description of this figure.

Generalized air circulation within the Earth’s atmosphere consists of three cells of circulating air that span the space between the equator and poles in both hemispheres: the Hadley Cell, the Ferrel or Mid-latitude Cell, and the Polar Cell (Inset Box Figure \(\PageIndex{1}\)). In the Hadley Cell, located over the tropics and closest to the equatorial belt, the sun heats the air, causing it to rise. The rising air cools and releases its contained moisture as tropical rain. The rising dried air spreads away from the equator and toward the north and south poles, where it collides with dry air in the Ferrel Cell. The combined dry air sinks back to the Earth at 30° latitude. This sinking drier air creates belts of predominantly high pressure at approximately 30° north and south of the equator, called the horse latitudes. Arid zones between 15° and 30° north and south of the equator thus exist within which desert conditions predominate. The descending air flowing north and south in the Hadley and Ferrel cells also creates prevailing winds called trade winds near the equator, and westerlies in the temperate zone.

Physical forces in the desert that break down rocks include the daily heating and cooling of rocks on the surface, expansion of plant roots in cracks, the freezing and melting of ice in cracks, and exposure to wind and precipitation (particularly during storms). When rainfall occurs, particularly long-slow drenching rain, the mountain slopes become saturated. The added weight induces rock falls, landslides, and other forms of mass movement of material down slope. Even though rain falls infrequently in the desert, when it does rain, large quantities of sediment move down slope and into canyons entrained in flash-flood waters or as debris flows (Figure \(\PageIndex{4}\)). A debris flow is a moving mass of rock fragments, mud, soil, and enough water to keep the mass fluid. They typically travel at great speeds down steep canyons, but may move slowly across a gentle surface of an alluvial fan. They can flow for great distances until the water they contain dissipates or separates from the alluvial material.

Figure \(\PageIndex{4}\): Poorly sorted debris flow sediments. “Debris flow channel, Ladakh, NW Indian Himalaya” by DanHobley, is licensed under CC BY-SA 3.0.

The churning action of flood waters or sediment-choked debris flows pulverize rocks into fragments (gravel, sand, silt, and clay). As rocks break into smaller and smaller fragments, the surface area of the resulting sediment volume increases. This increase in surface area accelerates the chemical changes that convert feldspar and other minerals in granite into clays. Soluble components in the rock dissolve and are carried away. With increasing distance away from mountain sediment source areas, the average size of rock fragments steadily diminishes and particles become increasingly round in shape.

One unique weathering product in deserts is desert varnish. Also known as desert patina or rock rust, it is made of thin dark brown layers of clay minerals and iron and manganese oxides that form on very stable surfaces within arid environments (Figure \(\PageIndex{5}\)). The exact way this material forms is still unknown, though biologic mechanisms have been proposed.

Figure \(\PageIndex{5}\): Petroglyphs carved into dark desert varnish. This work by Bob Cates, is licensed under CC-BY-NC-ND 2.0.

References

1. Harden, D. R. (2004). California Geology. Pearson Prentice Hall.
2. Johnson, C., Affolter, M. D., Inkenbrandt, P., & Mosher, C. (2023). An Introduction to Geology. Salt Lake Community College.https://slcc.pressbooks.pub/introgeology/
3. Our Dynamic Desert. (2009, December 18). Our Dynamic Desert. Retrieved August 31, 2023, from https://pubs.usgs.gov/of/2004/1007/intro.html
4. Palmer, A. R., & Halley, R. B. (n.d.). Physical stratigraphy and trilobite biostratigraphy of the Carrara Formation (Lower and Middle Cambrian) in the southern Great Basin. USGS Professional Paper, 1047, 139. 10.3133 / pp1047
5. Stoffer, P. (2004). Desert Landforms and Surface Processes in the Mojave National Preserve and Vicinity [Open-File Report 2004-1007]. USGS.
6. Walker, A. S. (1996). Deserts: Geology and Resources. USGS.