13 Deserts

KEY CONCEPTS

By the end of this chapter, students should be able to:

• Explain the defining characteristic of a desert and distinguish between the three broad categories of deserts
• Explain how geographic features, latitude, atmospheric circulation, and Coriolis Effect influence where deserts are located
• List the primary desert weathering and erosion processes
• Identify desert landforms
• Explain how desert landforms are formed by erosion and deposition
• Describe the main types of sand dunes and the conditions that form them
• Identify the main features of the Basin and Range desert (United States)

Approximately 30% of the Earth’s terrestrial surface consists of deserts, which are defined as locations of low precipitation. 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.

Deserts are not randomly located on the Earth’s surface. Many deserts are located at latitudes of around 30° and the North and South Poles, created by prevailing wind circulation in the atmosphere. Sinking, dry air currents occurring at 30° north and south of the equator produce trade winds that create deserts like the African Sahara and Australian Outback.

Another type of desert is found in the rain shadow created from prevailing winds blowing over mountain ranges. 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.  Examples of rain-shadow deserts include the Western Interior Desert of North America and Atacama Desert of Chile, which is the Earth’s driest, warm desert.

Finally, polar deserts, such as vast areas of the Antarctic and Arctic, are created from sinking cold 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. As a result, Antarctica is Earth’s driest continent.

13.1 The Origin of Deserts

13.1.1 Atmospheric circulation

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.

The figure shows the generalized air circulation within the atmosphere. Three cells of circulating air span the space between the equator and poles, the Hadley Cell, Ferrel or Midlatitude Cell, and Polar Cell. In the Hadley Cell located over the tropics and closest to the equatorial belt, the sun heats the air and causes it to rise. The rising air eventually cools and releases its contained moisture as rain. The drier air spreads away from the equator and toward the north and south poles, where it collides with air inside the Ferrel Cell causing the air currents to sink back towards Earth at 30° latitude. This sinking drier air creates belts of predominantly high pressure at these latitudes 30° north and south of the equator, also called the horse latitudes, within which desert conditions prevail. The descending air flowing north and south creates prevailing winds called trade winds near the equator, and westerlies near the poles. Note the arrows indicating general directions of winds in the latitude zones.

Other deserts, like the Great Basin Desert that covers parts of Utah and Nevada, owe at least part of their origin to other atmospheric phenomena. The Great Basin Desert, while somewhat affected by sinking air effects from global circulation, is a rain-shadow desert. As moist air from the Pacific rises over the Sierra Nevada and other mountains, it cools and loses moisture as condensation and precipitation on the upwind or rainy side of the mountains.

One of the driest places on Earth is the Atacama Desert of northern Chile. The Atacama Desert occupies a strip of land along Chile’s coast just north of latitude 30°S, at the southern edge of the trade-wind belt. The desert lies west of the Andes Mountains, in the rain shadow created by prevailing trade winds blowing west. As warm moist air coming from the Amazon basin meets the eastern edge of the mountains, it rises, cools, and precipitates much of its water out as rain. Once over the mountains, the cool, dry air descends onto the Atacama desert and spreads out toward the ocean, where it is further cooled by the Peru (Humboldt) ocean current. This super-cooled air holds almost no moisture and some locations in the Atacama Desert have received zero precipitation for several years. This desert is the driest, non-polar location on Earth.

Notice in the figure that the polar regions are also areas of predominantly high pressure created by descending cold dry air, the Polar Cells. As with the other cells, cold air, which holds much less moisture than warm air, descends convectively to create polar deserts. This is why historically, land near the north and south poles has always been so dry.

13.1.2 Coriolis Effect

The Earth rotates toward the east. The speed of an object on the Earth depends on its distance from the Earth’s axis of rotation. Higher latitudes are a smaller distance from the Earth’s axis, and therefore do not travel as fast as lower latitudes that are closer to the equator. To conceptualize different speeds on a sphere, think of the example of a car wheel. The tire on the outside of the wheel spins faster than the hub at the center of the wheel for the same amount of rotation because it covers a longer radial distance the same amount of rotation. When a fluid like air or water moves from the equator to a higher latitude, the fluid maintains its momentum that it got from moving at a higher speed, so it will travel relatively faster than other fluids that are already at the higher latitudes.

If the Earth did not rotate, a unified mass of air would rise at the equator and sink at the poles in each hemisphere, creating two circulation cells separated at the equator. However, each hemisphere has three cells because of the Coriolis Effect, which influences how masses of air (and water) move on a rotating sphere. The Coriolis Effect deflects moving air masses, breaking them up into separate cells, within which the air flows in different directions.

For example, in the Hadley Cell located in the northern hemisphere, the air currents flowing south towards the equator are deflected to the west (or right if you are facing the equator) by the Earth’s rotation. This deflected air generates the prevailing trade winds that European sailors used to cross the Atlantic Ocean and reach South America and the Caribbean Islands. This air movement is mirrored in the Hadley Cell in the southern hemisphere; the lower altitude air current flows north and west, creating trade winds that blow to the left while facing the equator.

In the northern Mid-Latitude Cell, cell circulation sends surface air currents from the horse latitudes (latitude 30°) toward the North Pole, and the Coriolis Effect deflects them to east, or to the left when facing the equator, producing the westerlies wind zone. In the southern hemisphere Mid-Latitude Cell, the surface air flows south and east, and westerlies blow to the right.

Another Coriolis-generated deflection produces the Polar Cells. Warm air rising from the equator (latitude 0°) that would otherwise descend at each pole (latitude 90°) as it cools, instead splits off and sinks at latitude 60°.

Any object moving on Earth is impacted by the Coriolis Effect. For example, artillerymen must take its effects on ballistic trajectories into account when making long-distance targeting calculations. Geologists are particularly interested in how its effect on air and oceanic currents creates deserts in designated zones around the Earth. For example, the Coriolis Effect creates large rotating ocean currents, called gyres, which push cold water along the North and South American continents, creating cooler, drier climates along their western coastal regions. The Coriolis Effect causes the ocean gyres to turn clockwise in the northern hemisphere and counterclockwise in the southern. It also creates high-altitude, polar jet streams that can push desert zones much closer to the equator, down to latitude 30° from the usual 60°.

Explanation of Coriolis Effect.

 

13.2 Desert weathering and erosion

Weathering takes place in desert climates by the same means as other climates, only at a slower rate. This is besides the higher temperatures, which typically spur faster weathering. Water is the main agent of weathering, and lack of water slows weathering. Precipitation occurs in deserts, only less than in other climatic regions. Chemical weathering proceeds more slowly in deserts compared to more humid climates because of the lack of water. Even mechanical weathering is slowed, because of a lack of runoff and even a lack of moisture to perform ice wedging. However, when precipitation does occur, often in the form of flash floods, a large amount of mechanical weathering can happen quite quickly.

One unique weathering product of deserts is desert varnish. Also known as desert patina or rock rust, they are thin dark brown layers of clays and iron and manganese oxides that form on very stable surfaces within arid environments. The exact cause of the material is still unknown, though cosmogenic and biologic mechanisms have been proposed.

While water is still the dominant agent of erosion in most desert environments, wind is a notable agent of weathering and erosion in many deserts. This includes suspended sediment traveling in haboobs, or dust storms, that frequent deserts. Deposits of windblown dust are called loess. Loess deposits cover wide areas of the midwestern United States, much of it from dust that melted out of the ice sheets during the last ice age. Lower energy than water, wind transport nevertheless moves sand, silt, and dust. As noted in chapter 11, the load carried by a fluid (like air) is distributed among bedload and suspended load. As with water, in wind these components depend on wind velocity. 

Sand size material moves by a process called saltation in which sand grains are lifted into the moving air and carried a short distance where they drop and splash into the surface dislodging other sand grains which are then carried a short distance and splash dislodging still others [zotpressInText item=”{J8N5GRPZ}” format=”%num%” brackets=”yes”].

Since saltating sand grains are constantly impacting other sand grains, wind blown sand grains are commonly pretty well rounded with frosted surfaces. Saltation is a cascading effect of sand movement creating a zone of wind blown sand up to a meter or so above the ground. This zone of saltating sand is a powerful erosive agent in which bedrock features are effectively sandblasted. The fine-grained suspended load is effectively removed from sand and the surface carrying silt and dust in haboobs. Wind is thus an effective sorting agent separating sand and dust sized (≤70 µm) particles. When wind velocity is high enough to slide or roll materials along the surface, the process is called creep.

One extreme version of sediment movement was shrouded in mystery for years: Sliding stones. Also called sailing stones and sliding rocks, these are large moving boulders along flat surfaces in deserts, leaving trails. This includes the famous example of the Racetrack Playa in Death Valley National Park, California. For years, scientists and enthusiasts attempted to explain their movement, with little definitive results. In recent years, several experimental and observational studies have confirmed that thin layers of ice allow the stones to move with high winds providing propulsive energy. These studies include measurements of actual movement, as well as re-creations of the conditions, with resulting movement in the lab.

The zone of saltating sand is an effective agent of erosion through sand abrasion. A bedrock outcrop which has such a sandblasted shape is called a yardang. Rocks and boulders lying on the surface may be blasted and polished by saltating sand. When predominant wind directions shift, multiple sandblasted and polished faces may appear. Such polished desert rocks are called ventifacts.

In places with sand dunes, clumps of vegetation often anchor sediment that has accumulated on the desert surface. Yet, saltation from winds may be sufficient to move or remove materials not anchored by vegetation. This causes a bowl-shaped depression in the sand called a blowout.

13.3 Desert landforms

In deserts like those of the American Southwest, streams draining mountains flow through canyons and emerge into adjacent valleys. As the stream emerges from the narrow canyon and spreads out, and with a lower slope angle and slower speeds and no longer constrained by the canyon walls, it drops its coarser load. As the channel fills with this conglomeratic material, the stream is deflected around it. This process causes the stream to be deflected back and forth, developing a system of radial distributaries and constructing a fan shaped feature call an alluvial fan, similar to a delta made by a river entering a body of water. 

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.

As the mountains erode away and the debris accumulates first in alluvial fans, then bajadas, the mountains eventually are buried in their own erosional debris. Such residual buried mountain remnants are called inselbergs, “island mountains,” as first described by the German geologist Wilhelm Bornhardt.

Where the desert valley is an enclosed basin, i.e. streams entering it do not drain out but the water is removed by evaporation, a dry lake bed is formed called a playa.

Playas are among the flattest of all landforms. Such a lake 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 flow only after rainstorms are called intermittent  or ephemeral. Drainage basins of ephemeral streams gather water from large areas and ephemeral channels may suddenly fill with water from storms many miles away and not even visible at that location plus, lack of organic matter and soil structure in arid regions inhibits infiltration and adds to runoff.  

Such high-volume ephemeral lows may be non-channelized and move as sheet flows. Such flash floods are a major factor in desert deposition and a serious concern for desert travelers who need to pay attention to regional weather. Water is less able to infiltrate because the flow compacts the surface, plants are less common to slow flows, and soils in deserts can become more hydrophobic. Water typically runs off as sheetwash to stream channels called arroyos or a dry wash that may be dry part or most of the year. Dry ephemeral channels can  fill quickly, creating  a mass of water and debris that charges down the channel, possibly even overflowing the banks of the arroyo.  People entering such channels or camping by them have been swept away by sudden flash floods.

13.3.1 Sand

While deserts are defined by dryness, not sand, the popular conception of a typical desert is a sand sea called an erg. An erg is a broad area of desert covered by a sheet of fine-grained sand often blown by aeolian forces (wind) into dunes. Probably the best known erg is the Empty Quarter (Rub’ al Khali) of Saudi Arabia, but other ergs exist in Colorado (Great Sand Dunes National Park), Utah (Little Sahara Recreation Area), New Mexico (White Sands National Monument), and California (parts of Death Valley National Park). It is not only deserts that form dunes; the high supply of sand can form ergs anywhere, even as far north as 60° in Saskatchewan at the Athabasca Sand Dunes Provincial Park. Coastal ergs on the shores of lakes and oceans also do exist, and can be found in places like Oregon, Michigan, and Indiana.

The way dunes form creates an internal feature called cross bedding. As wind blows up the windward side of the dune, it carries sand to the dune crest depositing layers of sand parallel to the windward (or “stoss”) side. The 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 the cross beds. Mostly, these are reworked over and over again, but occasionally, the features are preserved in a depression, then lithified. Shifting wind directions and abundant sand sources create chaotic patterns of cross beds like those seen in the fossil ergs represented by the Navajo Sandstone and Zion National Park of Utah. 

In the Mesozoic, Utah was covered by a series of ergs, thickest in Southern Utah. Perhaps the best known of these sandstone formations is the Navajo Sandstone. The Navajo forms the dramatic cliffs and spires in Zion National Park and covers a large part of the Colorado Plateau. It is exposed beneath the Entrada Sandstone in Arches National Park, a later series of sand dunes in which the conditions of the lithified rock allowed the formation of arches.

As the cements that hold the grains together in these modern sand cliffs disintegrate and the freed grains gather at the base of the cliffs and move down the washes, sand grains may be recycled and redeposited. These great sand ergs may represent ancient quartz sands recycled many times, just passing now through another cycle. One example of this is Coral Pink Sand Dunes State Park in Southwestern Utah, which is sand that is eroded from the Navajo Sandstone forming new dunes.

Dune Types

Dunes are complex features formed by a combination of wind direction and sand supply, in some cases interacting with vegetation. There are several types of dunes representing variable of wind direction, sand supply and vegetative anchoring. Barchan dunes or crescent dunes form where sand supply is limited and there is a fairly constant wind direction. Barchans move downwind and develop a crescent shape with wings on either side of a dune crest.  Barchans are known to actually move over homes, even towns.

Longitudinal dunes or linear dunes form where sand supply is greater and the wind is variable around a dominant direction, in a back-and-forth manner.  They may form ridges tens of meters high lined up with the predominant wind directions.

Parabolic dunes form where vegetation anchors parts of the sand and unanchored parts blowout.  Parabolic dune shape is similar to barchan dunes but usually reversed, and it is determined more by the anchoring vegetation than a strict parabolic form.

Star dunes form where the wind direction is variable in all directions.  Sand supply can range from limited to quite abundant.  It is the variation in wind direction that forms the star.

 

 

13.4 The Great Basin and the Basin and Range

The Great Basin is the largest area of interior drainage in North America, meaning there is no outlet to the ocean and all precipitation remains in the basin or is evaporated. It covers western Utah, most of Nevada, and extends into eastern California, southern Oregon, and southern Idaho. Streams in the Great Basin gather runoff and groundwater discharge and deliver it to lakes and playas within the basin. A subregion within the Great Basin is the Basin and Range which extends from the Wasatch Front in Utah west across Nevada to the Sierra Nevada Mountains of California. The basins and ranges referred to in the name are horsts and grabens, formed by normal fault blocks from crustal extension, as discussed in chapter 2 and chapter 9. The lithosphere of the entire area has stretched by a factor of about 2, meaning from end to end, the distance has doubled over the past 30 million years or so. This has created the bowl-like shape of the region, which creates an overall internal drainage, and countless sub-drainages in individual basins. Each of these are lined by alluvial sediments leading into playa or lacustrine depositional environments. Even without the arid conditions, there would be these types of deposits, with lacustrine becoming more common in place of playa. This most recently occurred with pluvial lakes that formed during the last glacial maximum (see chapter 14.4.3).

The desert of the Basin and Range extends from about 35° to near 40° and has a rain shadow effect created by westerly winds from the Pacific rising and cooling over the Sierras, depleted of moisture by precipitation on the western side. The result is relatively dry air descending across Nevada and western Utah. A journey from the Wasatch Front southwest to the Pacific Ocean will show stages of desert landscape evolution from the young fault blocks of Utah with sharp peaks and alluvial fans at the mouths of canyons, through older landscapes in Southern Nevada with bajadas along the mountain fronts, to the oldest landscapes in the Mojave Desert of California with subdued inselbergs sticking up through a sea of old bajadas. These landscapes illustrate the evolutionary stages of desert landscape development.

13.4.1 Desertification

Previously arable and usable land may be turned into desert by climate change and the activities of humans, such as poor farming practices, livestock overgrazing, and overuse of available water.  This is a process called desertification and it is a serious problem worldwide. Plants and soil types that are non-arid specifically help groundwater infiltration and water retention. Adding aridity to an area converts these soils and plants to be less effective in retaining water, and via a positive feedback loop (meaning that the processes feed on themselves promoting an increasing spiral). This only increases the aridity and spreads the desert further. The figure shows areas of the world and their vulnerability to desertification.  Note the red and orange areas in western and midwestern United States. The Dust Bowl of the 1930s is a classic example of human caused desertification.  Sometimes there is a conflict between what is known to prevent desertification and what an individual farmer feels he needs to do to make a living. Mitigating the desertification process includes both societal steps and individual education on alternatives.

References