Thumbnail for Chapter 18:
Thumbnail: “Death Valley Dunes” (CC-BY 4.0; Emily Haddad, own work)
What Is a Desert?
About 30% of the land surface of the world is arid or semi-arid. This includes much of the polar regions, where little precipitation occurs and so are considered polar deserts. Deserts are typically classified by the amount of precipitation that falls (fewer than 10 inches), by the temperature that prevails, or by geographical location. While temperature extremes are often associated with deserts, they do not define them. Deserts exhibit extreme temperatures because of the lack of moisture in the atmosphere, including the 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.
A person who studies the deserts and desert processes may have many titles including geomorphologist, sedimentologist or climate scientist. Like many other geoscientists, working with other disciplines is common, with a heavy influence from both math and technology. Many are employed by universities where they teach and/or do research, and state and federal agencies, including geological surveys, like the California Geological Survey or United State Geological Survey (USGS). Additional career pathways include environmental policy and legislation and consulting, via the private sector or state and federal agencies. Many of these career options require a college degree and postgraduate work. If this pathway is of interest to you, talk to your geology instructor for advice. We recommend completing as many math and science courses as possible. Also, visit National Parks, CA State Parks, museums, gem & mineral shows, or join a local rock and mineral club. Typically, natural history museums will have wonderful displays of rocks, including those from your local region. Here in California, there are a number of large collections, including the San Diego Natural History Museum, Natural History Museum of Los Angeles County, Santa Barbara Museum of Natural History, and Kimball Natural History Museum. Many colleges and universities also have their own collections/museums.
How Do Deserts Develop?
Deserts are not randomly located on the Earth’s surface, instead many deserts are gathered in distinct belts or regions. For example, sinking, dry air currents occurring at 30° North and South of the equator produce many of the most famous deserts including the Saharan Desert in Africa These areas are commonly referred to as subtropical deserts (Figure 18.2).
Subtropical deserts are the hottest deserts. In the US, the Chihuahuan, Sonoran, and Mojave are all examples of subtropical deserts (Figure 18.3). These deserts are often very hot and dry in the summer and cooler and dry in the winter. Rainfall does occur, but it short bursts, which may lead to flash flooding.
Dry areas located along a coast are referred to as coastal deserts. Coastal deserts typically occur in cool to warm areas along the west coasts of continents between 20° to 30° latitude and have cool winters and long, warm summers. Coastal winds blow in an easterly pattern (from the interior of the continent) and prevent moisture from moving onto the land. Examples include the Namib Desert in Africa (Figure 18.4) and the Atacama Desert in Chile, the driest place on Earth.
Other deserts can be found in the rain shadow created from prevailing winds blowing over mountain ranges (Figure 18.5). 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 mountain, 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. Most rain shadow deserts in the western United States, including the Great Basin Desert, are due to the Sierra Nevada and Cascade ranges (Figure 18.3).
Finally, polar deserts, like the vast areas of the Antarctic (Figure 18.6) and the Arctic, are created from sinking air that is too cold to hold much moisture. Although they are covered with ice and snow, these deserts have very low average annual precipitation. Consequently, Antarctica is Earth’s driest continent.
Weathering and Erosion Processes in Deserts
During our sedimentary section we discussed weathering and erosion in detail. Remember that water acts as a major agent of both physical and chemical weathering, and in desert climates water is scarce, which overall results in a much slower rate of weathering. When large amounts of precipitation fall over a short period of time, however, like during the rainy season, flash floods and mud and/or debris flows are likely to occur and are responsible for large amounts of rapid weathering and erosion (Figure 18.7).
As we learned about surface runoff in Chapter 9, the maximum particle size water can carry is tied to velocity, so high velocity flash floods play a major role in desert deposition. They are also a serious concern for desert travelers who must pay attention to regional weather. Surface runoff in deserts struggle to infiltrate the ground because the flow compacts the surface, plants are less common to slow flows, and soils in deserts can become hydrophobic. Water typically runs off as sheetwash to stream channels called arroyos or a dry wash that may be dry for part or most of the year. Dry ephemeral channels fill quickly, creating a mass of water and debris that charges down the channel, sometimes overflowing the banks of the arroyo. People entering or camping in such channels have been swept away by these sudden flash floods.
Wind is another notable agent of weathering and erosion in deserts. Lower energy than water, wind transport can typically move sand, silt, and clay-sized sediment, but, just like water, maximum load size is dependent on velocity. Typically, silt and clay-sized material, like in dust storms or haboobs, can be carried miles up into the atmosphere and can travel across the globe. Dust from the Sahara Desert can reach the US and is found in sediment cores from the Atlantic Ocean. Dust from the Gobi Desert in China can reach the western United States and can be found in sediment cores in the Pacific Ocean.
Transported sand grains typically remain close to the surface and are moved by a process called saltation. Sand grains are lifted into the moving air, carried a short distance, then drop and impact the surface, dislodging other sand grains and continuing the cycle. These wind-blown saltating sand grains are consequently well-rounded and have frosted surfaces (Figure 18.8). In areas where sand accumulates to form sand dunes, clumps of vegetation often anchor sediment on the desert surface. However, saltation from winds may still be sufficient to move or remove materials not anchored by vegetation causing bowl-shaped depressions in the sand called blowouts. In many desert regions, the wind erodes by deflation, the removal of loose, fine-grained particles by the turbulent eddy action of the wind, and by abrasion.
The saltating sand sandblasts and carves features into the bedrock, including distinctly shaped rocks and boulders called yardangs and faceted rocks called ventifacts. Deflation is responsible for the formation of desert pavement, a surface covered with closely packed, interlocking angular or rounded rock fragments of pebble and cobble size (Figure 18.9). Commonly many of these desert surfaces will also have desert varnish (desert patina or rock rust), a thin dark brown layer of clay, iron and manganese oxides that form on stable surfaces within desert environments.
In deserts like the Mojave and Great Basin of California, streams drain the mountains through canyons that emerge into adjacent valleys. As the stream emerges from these narrow canyons, the sediment they carry spreads out and is deposited due to the lack of confinement, low slope angle, and slower velocities. The stream channel begins to fill with this conglomeratic material, and the stream must adjust and deflects around the fill. This process will continue to occur, causing the stream to be deflected back and forth, developing a braided system and constructing a fan shaped feature called an alluvial fan. Alluvial fans continue to grow and may eventually coalesce with neighboring fans to form an apron of alluvium along the mountain front called a bajada. Eventually mountains become buried in their own erosional debris and are referred to as inselbergs or “island mountains”.
Where the desert valley is an enclosed basin, streams entering it do not drain out but the water is removed by evaporation. This forms a flat dry lake bed called a playa. The playa lake that fills this flat area may cover a large area and be only a few inches deep, and that only after a heavy thunderstorm. Playa lakes and desert streams that fill and flow only after rainstorms are frequently intermittent or ephemeral.
The famous Racetrack Playa of Death Valley National Park perplexed scientists for years, as they attempted to explain the independent movement of cobbles and large boulders along flat surfaces (Figure 18.11). In 2014, several experimental and observational studies confirmed that thin layers of ice allow the stones to slip along the ground, with high winds providing propulsive energy.
Want to read more? Check out the article in PLOS ONE, an inclusive journal freely accessible to all. Norris RD, Norris JM, Lorenz RD, Ray J, Jackson B (2014) Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion. PLOS ONE 9(8): e105948. https://doi.org/10.1371/journal.pone.0105948
While deserts are defined by dryness, not sand, the popular conception of a typical desert is a sand-sea called an erg: a broad area of desert covered by a sheet of fine-grained sand often blown by aeolian (wind) forces into dunes. The best-known erg is the Empty Quarter (Rub’ al Khali) of Saudi Arabia, but other ergs exist including in parts of Death Valley National Park.
Sand builds up the crest of the dune and pours over the top until the leeward (downwind or slip) face of the dune reaches the angle of repose, the maximum angle which will support the sand pile. Dunes are unstable features and move as the sand erodes from the stoss side and continues to drop down the leeward side covering previous stoss and slip-face layers and creating cross-beds. Fossil ergs are represented by the Navajo Sandstone and Zion National Park of Utah (Figure 18.12).
Dunes are complex features formed by a combination of wind direction and sand supply, and in some cases interacting with vegetation. There are several types of dunes representing variable wind direction, sand supply, and vegetative anchoring.
- Barchan dunes form where sand supply is limited and there is a constant wind direction. Barchans migrate downwind and develop a crescent shape with wings on either side of a dune crest (Figure 18.13). Barchan dunes are mobile enough that they can overtake homes or even towns.
- Parabolic dunes (coastal dunes) also have a crescent shape; however, they form when vegetation anchors parts of the sand and the unanchored parts are removed and form a blowout (Figure 18.14).
- Longitudinal dunes form when two dominant wind directions average towards the direction of elongation. This bi-directional wind flow typically produces two slip faces (Figure 18.15).
- Transverse dunes form when abundant barchan dunes merge to form linear, slightly sinuous, dunes. They lie transverse, or across, the wind direction, with the wind blowing perpendicular to the ridge crest (Figure 18.15).
The Eureka Dunes (Figure 18.16), are the tallest dunes in California, and the second tallest in North America. They are likely a combination of both longitudinal and transverse dunes, with winds averaging east-west.
Star dunes form where the wind direction is variable, forming the distinct star shape. Sand supply can range from limited to quite abundant. Mobility is low, as they tend to not move in a dominant direction (Figure 18.17).
Climate change and other human activities, such as unsustainable farming practices, livestock overgrazing, and overuse of available water can degrade previously arable land, a process known as desertification. This is a serious problem worldwide and will continue to worsen with climate change. Figure 18.18 shows areas of the world and their vulnerability to desertification. Note the red and orange areas in the western United States. Mitigating the desertification process includes both societal steps and individual education on alternatives to harmful activities.
- Figure 18.1: “Panamint Valley” (Public Domain; Jesse Pluim/BLM via Flickr)
- Figure 18.2: Derivative of “World Map with Major Latitude Circles” (CC-BY-SA 3.0; Thesevenseas via Wikimedia Commons) by Chloe Branciforte
- Figure 18.3: Derivative of “Great Basin Map” (CC-BY-SA 3.0; Kmusser via Wikimedia Commons) by Chloe Branciforte
- Figure 18.4: "Namib Desert" (CC-BY 2.0; Sonse via Wikimedia Commons)
- Figure 18.5: Derivative of “Orographic Effect” (CC-BY-SA 2.0; Meg Stewart via Flickr) by Chloe Branciforte
- Figure 18.6: “Landsat Image Mosaic of Antarctica” (Public Domain; USGS, NASA, NSF, and the British Antarctic Survey)
- Figure 18.7: “Flood Damage Worse than Originally Realized” (Public Domain; NPS)
- Figure 18.8: “Desert Sand” ©Sepp via Sand Atlas, licensed for educational purposes or for non-commercial projects.
- Figure 18.9: “Desert Pavement Mojave” (Public Domain; Mark A. Wilson/Wilson44691 via Wikipedia)
- Figure 18.10: "Alluvial Fan in Death Valley" (Public Domain; Wilson44691 via Wikimedia Commons)
- Figure 18.11: Derivative of “Racetrack Playa, Death Valley National Park” (CC-BY-SA 2.0; Gregory "Slobirdr" Smith via Wikimedia Commons) and “Racetrack Playa in Death Valley National Park” (CC-BY-SA 4.0; Lgcharlot via Racetrack Playa, Death Valley National Park) by Chloe Branciforte
- Figure 18.12: "Cross-Bedded Sandstone" (CC-BY 2.0; James St. John via Flickr)
- Figure 18.13: Derivative of “Mesquite Sand Dunes” (CC-BY-SA 3.0; Daniel Mayer via Wikipedia) by Chloe Branciforte
- Figure 18.14: “Parabolic Dunes” (CC-BY 4.0; Chloe Branciforte via Google Earth, own work)
- Figure 18.15: Derivative of “Longitudinal and Transverse Dunes” (CC-BY 3.0; Po ke jung via Wikimedia Commons and Wikimedia Commons) by Chloe Branciforte
- Figure 18.16: “Eureka Dunes” (CC-BY 4.0; Chloe Branciforte via Google Earth, own work)
- Figure 18.17: “Panamint Star Dunes” (CC-BY 4.0; Chloe Branciforte via Google Earth, own work)
- Figure 18.8: “Global Desertification Vulnerability Map” (Public Domain; USDA-NRCS, Soil Survey Division)