Provide examples of desert landforms and discuss the processes that form them.

We usually think of deserts as hot, sandy places. For many desert regions, that image is accurate. But some deserts are cold and rocky, and the polar deserts are covered by ice sheets. What exactly is a desert? All deserts have severe moisture deficits for most or all of the year (see Section 8.4). Generally speaking, deserts occur where less than 25 cm (10 in) of precipitation falls annually. More humid regions can also be deserts if evapotranspiration rates are high and moisture loss exceeds precipitation.

Three main factors cause aridity: (1) the subtropical high-pressure zone found between about 20 and 30 degrees latitude north and south (see Section 4.3); (2) rain shadows on the leeward sides of major mountain ranges (see Section 3.3); and (3) inland locations at high latitudes or high elevations where the air is cold and its water vapor content is low. Between 20 and 30 degrees latitude, cold ocean currents may also contribute to aridity, as is the case for Antofagasta. Many deserts are caused by some combination of these three main factors, but the most geographically widespread cause of aridity is the influence of the subtropical high (Figure 18.2).

Because of their aridity, all deserts have sparse plant cover and low biomass. As a result, no other environment on Earth is as exposed to the erosive effects of flowing water and wind, save perhaps coastal areas, as we will see in Section 19.2. We explored life in the desert biome in Section 8.4. In this chapter, we focus on the processes of desert geomorphology.

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Physical weathering is more important than chemical weathering in deserts. Chemical weathering requires the presence of water, and water is only intermittently present in deserts. Chemical weathering therefore occurs very slowly in deserts compared with more humid regions. When water from rainfall or dew enters a rock, it gradually dissolves minerals within the rock. In sandstone, for example, calcite can hold quartz grains together. Quartz resists chemical weathering, but calcite does not. When calcite is dissolved through chemical weathering, the rock disintegrates and crumbles into quartz sand granes. These grains can then be carried away by wind and water and deposited to form quartz sand dunes. Additionally, weak joints in rock cause the rock to fracture. If rocks tumble or fall down steep slopes, they are broken into smaller fragments.

The exposed surfaces of many desert rocks are covered with rock varnish: a thin, weathered, and darkened surface formed by the activities of bacteria and the accumulation of windblown clay particles. Rock varnish is a type of biochemical weathering found on rock surfaces almost exclusively in arid and semiarid climates. In more humid climates, rain washes away the clay particles too fast for rock varnish to form.

Scratching the surface of a rock removes the thin patina of rock varnish and reveals the lighter-colored rock beneath. People around the world have written or drawn on rock varnish for thousands of years. Many prehistoric rock carvings, called petroglyphs, have been made by scratching away rock varnish from a rock’s surface. These images often record a history of animals and people that goes back many thousands of years, as discussed in Picture This.

The study of ancient environments such as those depicted in petroglyphs represents a line of scientific inquiry called paleoecology: the study of ancient ecosystems. Figure 18.3 explores the paleoecology of the Sahara further.

Aeolian (or eolian) means “of or relating to wind.” Just as sediments are transported in a stream, aeolian erosion moves particles through traction, saltation, and suspension. The surface load of wind consists of particles moving by traction, rolling, and saltation. The suspended load consists of fine sediments of silt and clay that are suspended in the atmosphere. These particles can travel distances ranging from a few meters to thousands of kilometers as dust storms (Figure 18.4).

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Dust storms can travel rapidly. The March 2011 dust storm shown in Figure 18.4 moved about 50 km/h (31 mph) as it crossed the Arabian Peninsula. Crunch the Numbers calculates approximately how long it took the dust storm to cross the Arabian Peninsula.

Saltating sand grains hop over the ground within the first meter (3.3 ft) or so of the ground surface, abrading and sandblasting rock surfaces they collide with and creating ventifacts. A ventifact is a wind-sculpted desert rock shaped by abrasion by saltating sand (Figure 18.5).

If enough fine-grained sediment is transported away from a region by the wind, the land surface will be lowered in the process of deflation. Deflation is the removal of sediments and lowering of the land surface by wind erosion.

Where the wind’s speed increases as it flows around an obstruction, such as a large boulder, a blowout depression may occur. A blowout depression is a bowl-shaped depression formed in sand where wind flow becomes turbulent. Blowout depressions often form around pedestal rocks. A pedestal rock is a remnant of a layer of erosion-resistant cap rock supported by a slender column of less resistant rock (Figure 18.6).

Yardangs are elongated ridges formed by deflation and sand abrasion in the desert. These ventifacts form where persistent winds move sand in the same direction for most of the year. They are generally two to three times longer than wide, and can be up to hundreds of meters high and several kilometers long (Figure 18.7).

Pedestal rocks and yardangs are formed where there is net erosion by wind. The suspended material is often transported over the oceans, where it settles and slowly accumulates as sediment on the seafloor (see Section 13.3). Windblown dust from deserts plays important roles in Earth’s physical systems, as Picture This explains.

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Sand Dunes

A sand dune is a hill or ridge formed by the accumulation of windblown sand. Only about 10% of deserts are covered by sand dunes. Where sand dunes do occur, they are prominent landforms. All sand dunes migrate in the direction of the prevailing wind, and they move across the desert floor more or less as a unit. Individual sand grains leave the windward side of the sand dune and accumulate at their angle of repose on the slip face on the leeward side of the dune. When viewed in cross section, sand dunes are composed of layers of deposited sand, creating a pattern called cross-bedding, as shown in Figure 18.8.

Three factors determine what kind of sand dune will form: (1) the amount of sand available, (2) the strength of the wind, and (3) whether the wind changes direction over the year. The most common types of sand dune formations are barchan dunes, star dunes, and transverse dunes. Barchan dunes are crescent-shaped sand dunes with the crescent tips pointing downwind. Star dunes are star-shaped sand dunes that form as the prevailing wind changes direction through the year. Transverse dunes are long, narrow sand dunes with the ridge crest perpendicular (or transverse) to the prevailing wind. As Figure 18.9 illustrates, wind direction plays an important role in sand dune formation.

Singing Sand

About 30 sand dune locations around the world have been observed “singing” or “booming.” As the grains of sand slide down the slip face, each grain vibrates when it encounters other grains. All together, billions of particles of sand vibrating at the same frequency can create an audible sound. The pitch of the sound depends on the size of the sand grains, their silica content, and how deep the sand dune is.

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Desertification and Stabilizing Dunes

As we have seen, desertification is occurring in many arid regions where growing human populations put increasing pressure on already fragile semiarid environments. Arid western China has a growing problem with desertification. As human activities convert formerly vegetated regions to desert (see The Human Sphere in Chapter 4), dust storms develop and sweep eastward into populated areas.

One means of addressing desertification is to stabilize the sand so that vegetation can be reestablished. Plant roots anchor the sand and stop its movement. Where sand dunes are threatening human settlements, fences and netting can be used to slow the sand’s movement so that vegetation can be planted (Figure 18.10).

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Deserts are by definition arid places, so it may seem odd that the most important agent of erosion in most desert areas is not wind, but flowing water. Deserts are particularly vulnerable to erosion by water because they have little vegetation. Plant roots anchor soil, and plants lessen the erosive effects of raindrops, sheet wash, and stream flow on the desert surface.

Fluvial Erosion

Many deserts receive summer thunderstorms and sudden flooding rains. The force of impact from raindrops dislodges particles. As water flows over the bare ground surface, these particles are picked up in the water and transported downslope. As tributaries merge, discharge increases, and streams gain erosive power. Desert streams are normally dry, but they run muddy brown from the suspended sediments they carry after thunderstorms.

Each time rainfall fills the channel of an ephemeral desert stream with water, sediments move downstream in the channel. Once the rains cease, water infiltrates the ground, and the channel goes dry again. The channel sediments will not move until the next time the channel fills with water. These ephemeral streams (see Section 16.1) go by many names, depending on the geographic locale. They are called arroyos, dry washes, or coulees in the United States. In the Middle East and northern Africa they are called wadis, in South Africa dongas, and in India they are nullahs.

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Fluvial Deposition

Where streams exit steep canyons, the stream gradient flattens out, stream velocity is reduced, and the kinetic energy of the stream is greatly reduced. As a result, the sediments carried by the stream are deposited, forming alluvial fans (see Section 16.3). Where alluvial fans emerge from adjacent valleys, they often coalesce to form a broad, sloping plain called a bajada (Figure 18.11).

Where ephemeral desert streams empty into an internal drainage basin (see Section 16.1), they form a temporary shallow lake called a playa lake. Standing water in the playa lake typically evaporates within days to weeks. As the water evaporates, the dissolved minerals in the water form evaporite deposits as they precipitate out of the water (see Section 13.3). These deposits form a flat, dry lake bed called a playa. Playas are among the flattest and smoothest of all natural landforms on Earth. The world’s fastest cars have set land speed records on the Bonneville Salt Flats, a playa in Utah. Picture This explores an unusual phenomenon that occurs in playas.

Some internal drainage basins in deserts develop saline lakes. A saline lake is a permanent body of salty water occupying a playa. Evaporation rates are high in these desert lakes, and as a result, dissolved minerals such as salt become concentrated in the water. Utah’s Great Salt Lake and the Dead Sea in the Middle East are good examples of saline lakes. At the end of this chapter, the Geographic Perspectives explores the environmental effects of human activities on saline lakes.