Deserts and dunes

Deserts are arid or semiarid regions defined as areas that receive an extremely small quantity of precipitation each year. They cover approximately one-third of the Earth's total surface area and are found on every continent. Deserts and semideserts are characterized by unique geological and ecological features that are not present in more humid environments. These features include dunes, landforms of loose, shifting mounds, or ridges made of sand or other sedimentation.

Desert Characteristics

Although the word “desert” immediately conjures an unmistakable image of a hot, sandy expanse starkly empty of life, much variation makes up desert environments. While many are hot, some are freezing cold. Some border the ocean, and others have annual rainy seasons. Three-fourths of desert surfaces are covered not by sand but rather by some other material, such as clay, rock, ice, or aridisol, a type of soil that contains little or no organic matter. (Aridisols are typically alkaline and mostly contain salts such as chlorides, sulfates, and carbonates.)

Multiple definitions of deserts and systems for classifying them exist. American scholar and field geographer Peveril Meigs developed one of the most widely used desert classification systems in the 1950s. Meigs sorted the Earth's dry regions based on the amount of precipitation each receives in a year. Regions with twelve or more months in a row with no rainfall are known as extremely arid; regions that receive less than 250 millimeters (10 inches) of rainfall per year are known as arid; regions that receive between 250 and 500 millimeters (10–20 inches) of rainfall per year are known as semiarid. Only extremely arid and arid regions qualify as true deserts; semiarid regions, often unforested grasslands that border deserts, are sometimes known as steppes. Another way of defining a desert draws on a system developed by Russian-born German climatologist Wladimir Köppen, who proposed that a desert be identified as a region that loses more water each year through evaporation than it gains through precipitation.

Despite being arid most of the year, many deserts have some surface water. Ponds, lakes, and streams may form temporarily during brief heavy rainstorms, causing flash floods. Some deserts, however, are also crossed by permanent bodies of water, such as Egypt's Nile River or the Rio Grande in the Chihuahuan Desert; both rivers have their sources in high mountainous areas and traverse the desert on their way to the coast. In other deserts, subsurface water exists in underground aquifers, or water reservoirs, that sometimes rise to the surface and create oases, or isolated regions of fertile land.

Major Deserts of the World

Most of the world's desert regions are located within two bands, or belts, each within 25 degrees of the equator. In these belts, dry trade winds cause cloud cover to disperse, letting sunlight quickly heat the Earth's surface. The Sahara Desert in North Africa is an example of a trade wind desert. The Sahara, which stretches from the Atlantic Ocean to the Red Sea, is the largest hot desert in the world. The central Sahara region, which occupies an area of about 9.1 million square kilometers (sq km), or about 3.5 million square miles, is the driest; its regions to the north and south receive more precipitation and are more vegetated. The central Sahara is recorded as receiving less than 25 millimeters (1 inch) of rainfall per year, although precipitation patterns are extremely erratic there. There may be some years when this region receives no rainfall at all, but it is also possible that brief, intense thunderstorms may occur.

Much of the central Sahara is covered in ergs, or vast, flat, sandy areas. However, the region is also marked by a variety of surface landforms, including large dunes (such as Chech dune and Raoui dune); flat, high, stone plateaus; wide plains covered in gravel; dry riverbeds or wadis; low-lying salt flats; and the occasional oasis. A few mountain ranges, such as the Ahaggar (also known as the Hoggar) and the Tibesti mountains, also occur in this area. The Sahara is extremely windy, where temperatures of above 50 degrees Celsius (122 degrees Fahrenheit) have been recorded; winter and nighttime temperatures, though, may dip to below freezing.

The equatorial belt region is also home to a type of desert known as a rain shadow desert. These dry areas form between two mountain ranges that each block moist air currents from being blown toward the valley they enclose. The Gobi Desert is a rain shadow desert kept arid by the vast mountain ranges of the Himalayas; it spans northern China and southern Mongolia (gobi is the Mongolian word for “desert”) and is bounded by, among other mountains, the Altai and Hangayn (or Khangai) to the north and the Yin and Qilian to the south.

Much of the Gobi, which occupies an area of about 1.3 million square kilometers (about 500,000 square miles), is not covered in sand but dry sedimentary rock such as chalk and gypsum. These rock plains were formed 65 million or more years ago, and the Gobi has been the source of dinosaurs and other fossils and the remains of Stone Age societies. The climate in this region is marked by bitterly cold winters and warm summers. The total yearly rainfall in the Gobi ranges from about 200 millimeters (about 8 inches) in the northeast, which receives summer monsoons, to less than 50 millimeters (about 2 inches) in the extremely arid west.

However, not all the world's deserts have formed near the equator. In cold environments like at the Earth's poles, the air can no longer hold much moisture, and there is little or no precipitation; any water on the surface is not found in rivers or lakes but is locked in ice. The inland plateau of the Antarctic receives little snow, and it is considered the world's largest desert at more than 14.2 million square kilometers (about 5.5 million square miles). Although the rest of the continent is marked by such varied surface features as mountains, glaciers, and coral reefs, the Antarctic desert is relatively featureless. The snow is packed into flat, firm plains marked only by small dune-like formations known as sastrugi (or zastrugi).

Dune Formation Processes

A handful of key environmental factors must first be met for a dune to form. First, an abundant supply of dry sediment, usually sand, must exist. Second, there must be a way to transport that material, such as winds or moving water. These winds or water must be moving quickly enough to move the sediment but not so powerfully as to destroy the mounds that are formed. Finally, there must be a place for the accumulated sediment to be deposited. The sand that makes up a dune may come from the eolian processes that erode mineral particles from the surface of rocks or may be composed of biological materials like the calcium-carbonate-rich crumbled shells or skeletons of marine organisms.

In a dune formed by eolian, or wind-driven, processes, sand particles may be transported in several different ways. Extremely tiny or light particles may be held in place vertically while they move long distances horizontally; upward currents of air fully support their weight, while turbulent eddies of wind push them forward. This process is known as suspension. In saltation, particles move downwind continuously in hops, skips, and jumps. Each time a particle jumps, it moves a short distance only, but as it moves, it also hits other particles and triggers them to hop forward. In the process of creep, saltating particles that hit large, heavy grains may push those grains slowly downwind.

These processes cause sand particles to build up, though they do not necessarily create dunes. They may form sand sheets, or flat sandy plots of land, or ripples, which are low crests and troughs on the surface of a sandy surface (with the biggest particles at the crests). Ripples can grow into dunes under the continued action of the wind, which keeps moving particles up to the crest of the pile by saltation and creep until the slip face becomes steep enough to collapse in a tiny avalanche. This collapsing sand becomes stabilized at a particular angle, known as the angle of repose. As this cycle of build-up and collapse repeats, the dune inches forward in the direction of the wind.

The process by which underwater or riverbed dunes are formed through fluvial, or water-driven, processes is similar. However, fluvial processes can carry larger particles in suspension than eolian processes. In addition, saltation is less common underwater than in air, and suspension is more common. Finally, some particles are carried by rivers as dissolved sediments, which later recrystallize.

Types of Dunes

Geologists have identified at least five primary types of dunes—crescentic, linear, dome, parabolic, and star—although, many additional subtypes and variations exist. A dune's basic category depends on its geometric shape, which is determined by factors such as wind direction and strength, sediment type and quantity, and the presence or absence of vegetation.

Crescentic dunes resemble a crescent moon in shape. Narrow curved tips form at the edges of the dune, pointing downwind; the dune's concave slip face (a slip face is a slope on the opposite side of the prevailing wind) is between these tips. Crescentic dunes, also known as barchans, are Earth's most common dune shapes. Typically, they form in areas where the land is flat, and the wind blows consistently from a single direction. These fast-moving dunes have been known to travel as quickly as 100 meters (about 328 feet) per year across a desert.

Sand ridges that are either long and straight or sinuous—curving back and forth—are called linear or longitudinal dunes. Sinuous dunes are sometimes known as seifs, from the Arabic word for “sword.” (Ancient Eastern swords had curving blades.) Linear dunes often form when winds blow from two directions; each wind forms ridges parallel to the direction in which it blows. Rare dome dunes are roughly oval or circular and do not have a clear slip face; parabolic dunes are shaped like the letter U and often form in deserts near a coast. The long arms of such dunes point upwind and are stabilized by vegetation growing in the sand while the wind pushes forward the convex curve of a parabolic dune. Finally, star dunes are composed of a tall central mound from which several long arms—a minimum of three—radiate outward. Each arm has its own slip face and is formed by a wind that blows in a different direction. Star dunes are common in certain parts of the Sahara Desert.

These types of dunes may be further described as simple, compound, or complex. Simple dunes have the minimum number of slip faces required to fall into a particular category; they usually form when the prevailing wind or winds remain relatively stable in strength and direction. Compound dunes, which tend to be large, comprise two or more smaller dunes with the same geometric shape, forming one atop the next. Complex dunes are similar to compound dunes but are formed of multiple smaller dunes with different geometric shapes. For complex dunes to form, the strength and direction of the wind or winds must change after forming the first dune.

Desertification is defined as the degradation of formerly fertile land—the decrease in the ability of what was once productive land to support the continued growth of vegetation. Desertification can be attributed to various complex environmental factors, including the loss of nutrients and moisture in the soil, increased soil salinization, and increased wind and water erosion. Throughout Earth's history, desert regions have formed, enlarged, and shrunk in size because of natural processes that had nothing to do with human activities. Desertification, however, is accelerating because of anthropogenic causes.

Over 70 percent of the world's dryland regions are experiencing either moderate or severe desertification. This phenomenon is caused by droughts and changes in temperature, cloud cover, and wind patterns from global climate change, overgrazing, deforestation, irrigation, and other effects of unsustainable human land use. For example, livestock grazing on already semiarid land can remove vegetation that would otherwise prevent soil erosion by wind or water, and repeated cultivation of crops on the same land may deplete the nutrients in the soil or make it harder for it to absorb water. Proximity to an existing desert is not necessary for desertification to occur.

In the twenty-first century, desertification continues to be a significant environmental issue affecting agriculture production, damaging global economies, and threatening the Earth’s biodiversity. The effects of anthropogenic climate change, such as rising temperatures and changes to precipitation, increase desertification globally. This land loss allows more carbon dioxide to be admitted into the atmosphere, further exacerbating climate change. Importance must be paid to this symbiotic relationship to mitigate the effects of global climate change. 

Principal Terms

aridisol: a type of soil that contains little or no organic matter; typically alkaline and composed mostly of salts such as chlorides, sulfates, and carbonates

deflation: the removal of particles of clay, dust, sand, and rock from dry soil by strong winds

desert pavement: a surface covered by a thin layer of small, round, stones that are closely packed together; the residual deposits left behind after the process of deflation

desert varnish: a smooth, dark, hard, shiny coating that forms on exposed rock in desert environments; usually, the film is composed of manganese or iron oxides

eolian: of a deposit arising from or carried by the action of the wind

erg: any vast, flat area of desert covered in sand; applied especially to the sand dunes of the Sahara Desert

fluvial: of a deposit arising from, carried by, or occurring in a river

playa: a flat, dry lakebed usually characterized by clay or salt deposits, where rainwater accumulates temporarily but eventually evaporates or moves underground; also called a pan or a dry lake

rain shadow: a dry region that forms on the side of a mountainous area protected from the prevailing winds and, therefore, the moisture they carry

saltation: a bouncing, jumping motion in which small, hard particles such as sand move over an uneven surface as a result of the flow of air or water

slip face: the sloping side of a dune that faces away from the prevailing wind; also called lee slope

wash: a narrow channel or streambed that is dry through most of the year but that can become partly or totally submerged during flash floods that occur during sudden heavy rains; also called wadi

Bibliography

Abraham, Athol D., and Anthony J. Parsons. Geomorphology of Desert Environments. New York: Springer, 2009.

Aleshire, Peter. Deserts. New York: Chelsea House, 2008.

“Desertification and Climate Change - Ecosystem Restoration at Project Wadi Attir.” The Sustainability Laboratory, sustainabilitylabs.org/ecosystem-restoration/desertification. Accessed 27 July 2024.

Doran, Peter T., W. Berry Lyons, and Diane M. McKnight, editors. Life in Antarctic Deserts and Other Cold, Dry Environments. New York: Oxford University Press, 2010.

Eskandari Dameneh, Hadi, et al. "Desertification of Iran in the Early Twenty-first Century: Assessment Using Climate and Vegetation Indices." Scientific Reports, vol. 11, no. 1, 2021, pp. 1-18, doi.org/10.1038/s41598-021-99636-8. Accessed 27 July 2024.

Laity, Julie. Deserts and Desert Environments. Hoboken, N.J.: Wiley-Blackwell, 2008.

Mather, Anne. “Arid Environments.” Environmental Sedimentology, edited by Chris Perry and Kevin Taylor. Malden, Mass.: Blackwell, 2007.

Maun, Anwar M. The Biology of Coastal Sand Dunes. New York: Oxford University Press, 2009.

McSweeney, Robert. “Explainer: 'Desertification' and the Role of Climate Change.” Climate-Diplomacy, 9 Aug. 2019, climate-diplomacy.org/magazine/environment/explainer-desertification-and-role-climate-change. Accessed 27 July 2024.

Pye, Kenneth, and Haim Tsoar. Aeolian Sand and Sand Dunes. New York: Springer, 2008.

“Types of Dunes.” USGS Publications Warehouse, 29 Oct. 1997, pubs.usgs.gov/gip/deserts/dunes. Accessed 27 July 2024.

Verstraete, Michel M., Robert J. Scholes, and Mark Stafford Smith. “Climate Change and Desertification: Looking at an Old Problem Through New Lenses.” Frontiers in Ecology and the Environment, vol. 7, no. 8, Oct. 2009, pp. 421-428.