Geomorphology of dry climate areas

In dry climates, the great temperature contrasts, strong winds, rare but torrential rains, and limited but powerful water floods produce a diverse geomorphology as well as natural difficulties of existence for those who live in such areas.

Distribution of Dry Climate Areas

Dry climate geomorphology is the study of landforms in arid and semiarid regions. Arid lands of various kinds, usually termed deserts, amount to at least 25 percent of the world's land area outside the polar regions and up to forty-three percent by some definitions. They are generally considered to be those areas with less than twenty-five centimeters of precipitation per year or where the evaporation is twice that of precipitation. Semiarid regions, or steppes, are not quite as extensive as deserts but are still significant. They are defined as having precipitation between about twenty-five and fifty centimeters per year, which means evaporation is about equal to precipitation. The semiarid areas are generally peripheral to the desert areas.

The most extensive arid lands are associated with the two circumglobal belts of dry subtropical air that subside to the north and south from the rising equatorial air masses, heating and drying in the process. Examples include the Sahara and Kalahari deserts of Africa. A second type of dry area is found in continental interiors far from sources of moisture. The Gobi and Takla Makan Deserts of central Asia fall into this category.

A different and more local kind of desert is found on the lee side of mountain ranges. The mountains create a barrier to the flow of moist air, producing a rainshadow effect. As the air rises against the windward slope of a range, it cools and condenses into precipitation. This chain of events removes much of the moisture from the air mass. Then, as the air mass passes over the range and down the other side, it warms in descent and becomes even drier as it moves along. The deserts behind the coastal ranges of California are of this type.

A fourth category of deserts occurs along coastlines. They occur locally along west coasts, where upwelling cold seawater cools, passing marine air, thereby decreasing its ability to hold moisture. As the air encounters warm land, its limited moisture condenses and gives rise only to coastal fogs. Deserts of this type occur in Chile, Peru, and southwest Africa.

Temperature and Rainfall

The limited cloud cover in arid areas allows strong thermal contrasts from day to night through intense solar heating during the day and reradiation of heat back out into space at night. The resulting high-temperature gradients result in the strong winds and intensive movement of sediment so characteristic of dry climate geomorphology. No major geologic process, however, is restricted entirely to desert regions. Rather, the same processes operate with different intensities in moist and in arid landscapes. Thus, in a desert, the surface sediments, soils, and landforms show some distinctive differences from those elsewhere.

Although arid areas do not generally have rivers originating in their boundaries, rain occasionally occurs, and often, the storms are torrential. Most runoff that originates in desert areas never reaches the sea, as the water soon disappears through evaporation, or it soaks directly into the Earth. Rivers such as the Colorado or the Nile begin in high, humid mountains far from the lower deserts through which they flow. Such rivers carry so much water that they keep flowing to the sea, despite great losses where they cross a desert.

Flash Floods

In arid areas where vegetation and water are limited, soils are thin, and bedrock is, therefore, commonly exposed at the surface. Weathered fragments of rock tend to break off along natural fractures to leave the steep rugged cliffs that are so common in such regions. The plentiful rock fragments can be easily incorporated into flash floods and moved to lower elevations.

Studies show that typical torrential rainstorms in deserts are likely to be accompanied by flash floods that suddenly and swiftly transport large quantities of rock and soil debris. The debris is deposited as sediment that forms alluvial fans at the bases of mountain slopes and alluvial plains on the floors of wide valleys and basins. The debris-laden streams rapidly lose water into the porous and permeable sediment of the desert basins and end as dry washes or gullies.

Alluvial Fans and Pediments

Alluvial fans are one of the most characteristic landforms in dry regions. They are caused by the change from the steep, narrow river channels of upland areas to the lower gradients and the unconfined slopes of desert basins, where the water velocity is checked and the sediment load spread. The fan itself commonly shows a characteristic concave-upward slope profile. Alluvial fans are typically dominated on the upper slopes by coarse fragments and on the lower, gentler slopes by finer sands, silts, and clays. Debris flows—sudden rapid waves of wet, bouldery mud—sometimes rush out of mountains after torrential rains and add to the sedimentation on the fans.

Erosion and sedimentation patterns are responsible for pediments, another of the characteristic landforms of arid zones. These are broad, gently sloping areas of bedrock, spread as aprons around the bases of the mountains. Measured cross sections of a typical pediment and its associated mountain show fairly steep bedrock mountain slopes that abruptly change to the gentler bedrock slopes of the pediment. Pediments are eroded by running water that also builds up a thin apron of sediment below as the water continues to erode back the slopes above.

A pediment meets the upper mountain slope, not at a curve but at a distinct angle. This angularity suggests that mountain slopes in the desert do not become gentler with time, as they would in a humid region where chemical weathering, soil formation, and downslope creeping movement of soil are dominant. Instead, the pediments seem to adopt an angle determined by the resistance of the bedrock and maintain that angle as they gradually retreat. In this way, retreat of the mountain slope should extend a pediment forward at its upslope edge. The growth of the pediment at the expense of the mountain may continue until the entire mountain has been consumed.

Playas and Badlands

In an arid region, water is rarely plentiful enough to flow for long into a basin to maintain a permanent lake on its floor. Instead, the ephemeral streams that flow down from the highlands after an exceptionally large rain can discharge enough water into the basin to produce a temporary shallow lake. These lakes, known as playas, may last a few days or weeks before the water evaporates or soaks into the ground. Many dry playas are white, gray, or tan because of precipitated salts and clays on the surface. The Spanish origin of the term playa (meaning “beach”) is a reflection of the observation by early explorers of southwestern North America that many old lakes had dried up and left only beaches and dry lake bottoms behind. These basins were once filled with water during the high-rainfall, or pluvial, periods during the last ice age when the climate was much wetter and cooler. Now dry, playa basins are scattered throughout the arid areas of the world.

Badlands in dry zones are extremely closely dissected landscapes on weak, impervious rocks or sediments largely devoid of vegetation. Badland topography is a wilderness of steep, smooth slopes, knife-edged or sharply rounded crests, and steep, narrow valleys. Such areas of bare ground erode through rain splash and slope wash in occasional intense rains.

Eolian Processes

Although water is the predominant eroding agent in dry areas, the wind effectively moves large quantities of sand and dust. Contrary to popular belief, however, most deserts are not covered with dunes. Nevertheless, landforms resulting from wind erosion or deposition predominate. Wind activity and the resulting landforms are commonly called eolian (after Aeolus, the Greek god of wind). Eolian processes in the world's dry regions are responsible for major erosional and depositional landforms as well as significant sedimentary deposits.

Wind erosion acts in two ways: by deflation, the lifting and removal of loose sand and dust particles from the Earth's surface, and by abrasion, the sandblasting action of windblown sand. Deflation causes depressions called blowouts in areas where loose sediment occurs that is of small enough grain size to be moved by wind. These deflation basins can become very large, even below sea level, as long as the underground water table is not reached and as long as large-sized gravels do not accumulate as lag deposits to protect the surface with desert pavements.

Yardangs and Ergs

Large landforms produced by wind abrasion are uncommon, but distinctive linear ridges called yardangs (from a Turkistani word meaning “ridge”) occur in some desert regions. Typical yardangs have the form of an inverted boat hull and commonly occur in groups, all aligned with the prevailing wind. This shape and orientation are streamlined by the wind to offer minimum resistance to the moving air. Yardangs develop best in soft sediment that is easily eroded but cohesive enough to retain steep slopes.

The most extensive areas of wind-transported sand are the ergs, or great sand seas, that occur in the world's major deserts. Ergs may cover more than half a million square kilometers—twice the size of the state of Nevada. About 99.8 percent of all windblown sand is in the ergs of the world. The largest are in Africa, Asia, and Australia. One-third of Saudi Arabia—about one million square kilometers—is covered with ergs in the vast “empty quarter” of the Rub’ al Khali.

Simple Dunes

Individual dunes in an erg form and move as many different mounds or ridges of sand. Generally, dunes form where an obstacle distorts the flow of air. On encountering an obstruction, wind sweeps over and around it but leaves a pocket of slower-moving air directly behind the obstacle. As sand is blown up or around the obstruction and into the protected wind shadow, its velocity is reduced, and deposition occurs. Once a dune is formed, it acts as a barrier itself, further disrupting the flow of the wind and causing sustained deposition downwind. Continued erosion on the windward side and redeposition on the leeward side can produce rates of dune migration of as great as 25 meters per year. As a dune grows larger, the sand grains saltate up the low-angle windward slope to the top, then slip down into the wind shadow. The slip face keeps a more or less constant angle of repose and creates the high-angle cross-bedding so typical of dunes. Dunes can range in size from a few meters high to as much as 250 meters high.

In an attempt to make some order out of immense diversity, geologists have lumped dunes into several major types, though there are gradations among them and many irregular shapes that are hard to fit into any scheme. The basic classification of dunes is threefoldsimple, compound, and complex draa dunes.

Simple dunes consist of arrangements of different dune forms relating to one or more wind directions and different amounts of available sand. Simple dunes may be convex or concave, and they may be transverse or longitudinal to the wind.

Compound and Complex Draa Dunes

Compound dunes are combinations of simpler forms without a change of scale or size of the individual component dunes. The solitary crescent-shaped dune, the barchan, which moves over a flat surface of pebbles or bedrock, is the product of a limited sand supply and winds of moderate velocity that blow in a constant direction. Typically, they are small isolated dunes from 1 to 50 meters high. The tips of a barchan point downwind, and sand grains are swept around them and up and over the crest. The steep slip face occurs inside the crescent. Transverse dunes typically develop where there is an abundant supply of sand and a constant wind direction. These dunes develop a wavelike form, with sinuous ridges and troughs that extend perpendicular to the prevailing wind.

Longitudinal, or seif (an Arabic word meaning “sword”), dunes are long, parallel ridges of sand; that is, they elongate in a direction parallel to the resultant of several slightly different wind directions. These dunes develop where strong prevailing winds converge and blow over an area having a limited supply of sand. The grains are shepherded into a long ridge by the blowing of the wind, first in one direction across and along the dune and then from the opposite side of the dune. Many longitudinal dunes are less than 4 meters high, but they can extend downwind for several kilometers. In large deserts, they can be more than 100 meters high and 120 kilometers long; they can be spaced 0.5 to 3 kilometers apart.

Parabolic, or blowout, dunes develop out of preexisting, partly vegetated and thereby stabilized dunes. In these dune forms, first a deflation depression develops, and the sand is piled on the downwind edge of the oval depression. As the shallow deflation hollow enlarges, the sand piles up to form a crescent-shaped ridge. In map view, a parabolic dune is similar to a barchan, but the tips of the parabolic dune point upwind around the deflation hollow from which they originate. Where greatly elongated, parabolic dunes have been called hairpin dunes, again with the extended tips pointing upwind.

Complex draa dunes have dimensions of hundreds of meters, and spacings of kilometers; because of their great bulk, they appear to be relatively static. They are thought to be manifestations of atmospheric turbulence on a distinctly larger scale than that manifested by simple dunes, involving interaction between the regional winds and massive, relatively unchanging sand forms. Barchanoid, transverse, longitudinal, and peaked forms all occur, but in masses that can be more than one hundred times larger than the simple forms. Draa dune fields are characteristic of thick sand accumulations in extremely arid zonal deserts, notably the Sahara and the deserts in Saudi Arabia.

Dust Storms and Loess

Dust storms are a major process in deserts. They can transport thousands of tons of fine sediment high in the atmosphere for hundreds of kilometers. These processes have long been known as sandstorms, but, in fact, far more dust is carried aloft than is sand close to the ground. Great dust storms can reach altitudes of more than 2,500 meters above the desert floor and advance at speeds of up to 200 meters per second. Perhaps 500 million tons of windblown dust is carried out of deserts each year, about the same amount carried annually by the Mississippi River. Large quantities blow out into the oceans from Australia or Africa, and some even move across the Atlantic Ocean from the Sahara to the east coast of South America.

Windblown dust is deposited as loess (a German word meaning “loose”) and is defined as wind deposited silt and clay; loess accumulates slowly and ultimately blankets large areas, commonly masking preexisting landforms. Such deposits may cover as much as one-tenth of the world's land surface and are particularly widespread in semiarid regions along the edges of the world's great deserts. Large deposits can reach 300 meters in thickness. Loess is a quite distinctive sedimentary deposit. Most loess is massive and lacks layering, apparently because grains of different sizes settled progressively from the air and were deposited at random. Where exposed, loess commonly stands in steep cliffs because the molecular attraction between the very fine grains is enough to make the particles quite cohesive. One of the primary differences between loess deposits and sand-dune deposits is that sand continues to be mobile even while it is on the ground, whereas loess dust is cohesive and stable once deposited. Loess also tends to settle out on semiarid grasslands or occasionally on woodland, which are not active areas of further wind erosion. Once deposited, therefore, loess tends to remain in place and can continue to accumulate to great thicknesses over time; loess makes very fertile soil.

Study of Processes and Phenomena

Running water and wind are the primary processes of dry climates, but significant weathering, gravity-driven slope failure, mass wasting, and even cold climate processes occur. With this wide range of landform-controlling factors, the study of the relevant processes involves a nearly complete range of geological and geographical techniques. Aside from plentiful climate measurements of different types, sediment movement is monitored, weathering characteristics described and measured, and landform types and distributions mapped.

To develop an understanding of the historical dimension of landform development in dry climates, considerable effort is expended by scientists to develop long-term chronologies. This goal is achieved through measurement of landform morphologies that show progressive erosion and deposition development through time, elucidation of the nature of varied sedimentary layers through stratigraphic analysis, and radiometric and relative-age dating techniques to fix the chronologies.

Significance for Human Populations

Dry climate geomorphology is very important because so much of the world's population lives in such environments and must contend with the extremes of climate and geomorphic processes there. Agriculture is particularly difficult in such arid situations. In the United States, much of the West's dry agricultural grassland and shrubland is characterized by a special suite of arid landforms. Some of the dry desert basins of the Southwest are important sources of exotic salts, and their different terrains have special significance to the military and space agencies. For example, playa salt deposits, formed by repeated filling and evaporation of the lakes, can be tens of meters thick and a valuable source of industrial chemicals. Borax, the active ingredient in some scouring powders, is one example. Dry playas are also extraordinarily flat surfaces and have been used for a long time as huge landing fields for rockets and spacecraft. In addition, the largely cloud-free and windy dry lands are important sites for solar and wind power generation.

Principal Terms

borax: a sodium borate mineral that is an ore of boron and occurs as surface crust or large crystals in the muds of alkaline lakes; borax is used in glass, ceramics, cleansing agents, water softeners, and other industrial applications

desert (rock) varnish: a thin, dark, hard, shiny or glazed iridescent (red, brown, black) film, coating, stain, or polish on rocks that is composed largely of iron and manganese oxides and silica formed by weathering of dust films and by microbial action

differential weathering: physical and chemical weathering that occurs at irregular or different rates, caused by variations in composition and resistance of a rock or by differences in intensity of weathering, and usually resulting in an uneven surface where more resistant material stands higher or protrudes above less resistant parts

pluvial period: an episode of time during which rains were abundant, especially during the last ice age, from a few million to about ten thousand years ago

saltation: a mode of sediment transport in which the particles are moved progressively forward in a series of short intermittent leaps, jumps, hops, or bounces from a bottom surface

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