- Distinguish three broad categories of deserts.
- Explain the location of deserts
- Identify and describe 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.
The location of climates on Earth’s surface are not random. Jungles, tundras, and deserts have scientific explanations for their locations. Approximately 30 percent of Earth’s terrestrial surface is desert. Deserts are defined as locations of low precipitation. While temperature extremes are often associated with deserts, they do not define them. The lack of moisture, including the lack of humidity and cloud cover, allow temperature extremes to occur. The sun’s energy is more absorbed by the Earth’s surface without cloud cover, and nighttime cooling is more drastic without cloud cover and humidity to absorb the emitted heat, so temperature extremes are common in deserts.
Deserts tend to occur at latitudes of around 30° and at the poles, both north and south, driven by circulation and prevailing wind patterns in the atmosphere. At approximately 30° north and south of the equator, sinking air produces trade wind deserts like the Sahara and the Outback of Australia . Rain shadow deserts are produced where prevailing winds with moist air dries as it is forced to rise over mountains.
The Western Interior Desert of North America and the Atacama Desert of Chile (the driest warm desert on earth) are examples of rain shadow deserts. Finally, polar deserts, such as the vast areas of the Antarctic and Arctic are covered by sinking cold air which is usually to cold to hold much moisture. Though covered with ice and snow, the average annual precipitation is very low, with Antarctica being Earth’s driest continent.
13.1 The Origin of Deserts
13.1.1 Atmospheric circulation
The engine that drives circulation in the atmosphere and oceans is solar energy which is determined by the average position of the sun over the earth’s surface. Direct light provides uneven heating depending on latitude and angle of incidence, with high solar energy in the tropics, and little or no energy at the poles. Atmospheric circulation and geographic location are the primary causal agents of deserts.
The figure shows the generalized circulation of the atmosphere. There are three generalized circlating cells of rising and sinking air, the Hadley Cell, the Ferrel or Midlatitude Cell, and the Polar Cell. Solar energy falling on the equatorial belt heats the air and causes it to rise. The rising air cools and its contained moisture falls back on the tropics as rain. The drier air then continues to spread toward the north and south where it collides with the Ferrel Cell and they sink back at about 30 degrees north and south latitudes. This sinking drier air creates belts of predominant high pressure along which desert conditions prevail in what are called the “horse latitudes.” These belts of predominantly high pressure have air that descends along these belts and flows either north to become the westerlies or south to become the trade winds. These circulation cells in the atmosphere rising in the tropics and polar regions and sinking in the horse latitudes produce the desert belts along the horse latitudes at approximately 30 degrees north and south of the equator . Note the arrows indicating general directions of winds in the latitude zones. The trade winds are predominant in the tropics and the westerlies in the mid-latitudes.
Other deserts have other atmospheric phenomenon to owe (at least part of) their origin, like the desert of Utah, Nevada, and surrounding areas, called the Great Basin Desert . This desert, while having some sinking air effects due to global circulation, is also a rain shadow desert produced as moist air from the Pacific rises by orthographic lifting over the Sierra Nevada (and other) Mountains and loses moisture from previous condensation and precipitation on the rainy side of the range(s).
One of the driest places on earth is the Atacama Desert of northern Chile . This is a strip along the west coast of South America, west of the Andes, lying north of 30 degrees south latitude, at the southern edge of the trade wind belt. Warm moist air moves west across the Amazon basin and rises over the Andes where it loses moisture, its precipitation falling on the rain forest side of the mountains. Once over the mountains, it descends onto the Atacama where it meets air cooled by the cold Peru (Humboldt) ocean current flowing north along the coast. This is considered to be the driest (non-polar) place on earth with locations in the Atacama having not received any precipitation for periods of years .
Referring again to the figure above, note that the polar regions are also predominantly high pressure areas of descending cold and dry air. Another circulation cell occurs there known as the Polar Cell . Here, air not only descends convectively because it is cold, but cold air can hold much less moisture than warm air, and thus the driest and coldest places on Earth are the polar deserts. Antarctica is not only currently the driest land on Earth today, but any land that occupies the poles in Earth history should always be dry.
13.1.2 Coriolis Effect
In a non-rotating Earth, air would rise at the equator, sink at the poles, creating one circulation cell. However, as noted above, Earth has three cells. Why? As objects move on a rotating sphere, an effect called the Coriolis Effect occurs which causes a deflection in the motion. In the northern hemisphere, this deflection is to the right; in the southern hemisphere it is to the left. This has two consequences on masses of air (and water) moving on the earth. The lower air in the Hadley Cells moves toward the equator over the earth’s surface. This air is deflected to the right in the northern hemisphere and to the left in the southern hemisphere creating the trade winds that carried European explorers to South America and the Caribbean. The midlatitude cells move surface air north toward the pole in the northern hemisphere (and south in the southern hemisphere) from the horse latitudes which is deflected again to the right (or left in the southern) producing the zone of westerlies. High above the Earth, the rising air from the equator would attach to the sinking air at the poles, but again, is deflected, causing instead sinking air at 30° and rising at 60°. This splits the circulation into three cells instead of one.
To understand the Coriolis Effect, first consider motion along the meridians (the lines connecting the poles and running north-south). The earth rotates toward the east, i.e. everything on Earth moves at an eastward speed depending on its latitude. At a given latitude, objects possess a certain momentum of that motion depending on the length of the radius from its latitude to the rotational axis of the Earth. In the northern hemisphere, if the object is moving north, it has greater momentum toward the east than other objects at the new more northerly latitude. If it moves south, it has less momentum than other objects at that new more southerly latitude. It therefore tends to move to the right compared to fixed locations at that new latitude. The opposite happens in the southern hemisphere.
Now consider motion in an east-west direction again thinking of the momentum imparted by the radius from the Earth’s rotational axis. The centripetal effect of Earth’s rotation causes objects on the earth to tend to be forced outward perpendicular to the rotational axis, the centripetal effect or “force.” Since gravity holds things on the earth’s surface (the gravitational force points toward the Earth’s center, perpendicular to the surface), they do not actually fly outward of course. But considering the components of force involved acting on a mass at or above the Earth’s surface, the centripetal component is perpendicular to the Earth’s spin axis. The component of gravity is perpendicular to the earth’s surface pointing toward the Earth’s center. If the object is moving eastward, the speed of the object adds to the earth’s rotational speed and the centripetal effect is enhanced, thus the net effect of gravity and the centripetal component parallel to the surface causes deflection to the right (left in the southern hemisphere). If the object is moving west, its speed subtracts from the rotational speed and reduces the centripetal effect. Deflection is again to the right (left in the southern hemisphere). At any direction of motion, the meridional and the centripetal effects combine, thus no matter which direction an object moves on the rotating earth, there is a tendency for deflection to the right in the northern hemisphere (left in the southern).
The objects of greatest interest in geoscience that are affected by the Coriolis Effect on earth are air and water masses. Since wind patterns, especially prevailing patterns, cause ocean currents, then water masses feel it as well. In reality, any object moving on the earth experiences it. For example, the Coriolis Effect must be taken into account by artillerymen calculating the trajectory of artillery shells for accuracy in hitting targets over long distances.
The Coriolis effect creates large sub-circular rotating currents called gyres in the oceans, turning clockwise in the northern hemisphere and counterclockwise in the southern under the Coriolis Effect. These currents bring cold water along the west coasts of both North and South America contributing to the drier climates of the Atacama and Central and Southern California. The Coriolis Effect acting on both the atmosphere and ocean is a major contributor to climate and weather on the earth.
An application of the Coriolis Effect can be seen on the TV weather report. High pressure systems are typically shown by a large “H” and indicate dry conditions, and low pressure systems by a large “L” indicating clouds or precipitation. Air flows outward from a high and because of the Coriolis Effect, it rotates clockwise (to the right). It flows inward to a low and again turns to the right, rotating counter clockwise. Of course weather reports in the Southern Hemisphere show the opposite. Another interesting realization from the Coriolis Effect and the Zone of Westerlies is that weather systems tend to move from west to east across both North America and the southern part of South America. The high pressures and low pressures that exist due to uneven heating of the atmosphere and the Coriolis Effect create the high and low pressures on the weather map. The chaotic nature of the atmosphere (and fast moving flows like the Jet Stream) make these high and low pressures constantly and consistently move. This is important, because at 30 degrees, without this movement, low pressure would never exist! This means rain would never arrive. Even in the driest parts of this zone, like the Atacama, it rains on occasion. High pressure normally exists here, but just not all the time. These air movements, both prevailing and sporadic, are thus important in understanding climate and its geological implications.
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 .
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.
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.
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.
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.