Eolian processes, dryland landforms, and desertification
Many drylands have specific landforms that result primarily from processes associated with degradation, erosion by streams and flash floods developed after infrequent rains (fluvial processes), or erosion and deposition associated with wind (eolian processes).
Fluvial and eolian processes erode dryland landscapes, transport Earth materials, and deposit sediments.
Rocky surfaces, sparsely vegetated surfaces, sand dunes, and arroyos (steep-walled canyons with gravel floors) are typical of dryland landscapes. Therefore, humans living in drylands must adapt to these landforms, conditions, processes that have created the landforms, and the prospect of land degradation or even desertification.
Eolian Processes and Landforms
Water and ice are capable of moving large particles of sediment. The wind can move only smaller particles (Figures 1 and 2). Therefore, for this reason, the eolian (wind-related) landforms may be subtle or even invisible on a topographic map. (However, they may be more evident on aerial photographs that have a higher resolution.)
They may be superimposed on fluvial (stream-related) or glacial (ice- or glacier-related) features, particularly where recently exposed and unvegetated sediment occurs.
A lack of a dense vegetation cover is a prerequisite for significant wind erosion. This lack of vegetation can occur:
• On recently deposited sediment, such as floodplains and beaches.
• In areas where vegetation has been destroyed by fire, overgrazed, or removed by humans, or
• In true deserts, where the lack of water precludes substantial growth of vegetation.
When examining a topographic map, keep in mind that the green overprint represents only trees and shrubs. There could be an important soil-protecting grass cover present that is not indicated on the map. Your evaluation of the present climate of a topographic map the area should consider surface water features, groundwater features, and the geographic location of the area.
The most common wind-eroded landform visible on a topographic map is usually a blowout—a shallow depression developed where the wind has eroded and blown out the soil and fragmented rock (Figure 3C).
Blowouts may resemble sinkholes (depressions formed where caves have collapsed), or kettles (depressions formed where sediment-covered blocks of glacial ice have melted), but you can distinguish these different types of depressions in the context of other features observable on the map.
Unlike sinkholes and kettles, blowouts usually have an adjacent sand dune or dunes that formed where sand-sized grains were deposited after being removed from the blowout. Blowouts also range in size from a few meters to a few kilometers in diameter.
Sand transported by wind also erodes many rock surfaces by sandblasting them. Ventifacts are rocks that have flat or scoop-shaped surfaces that were
abraded in this manner (Figure 4).
When wind-blown (eolian processes) sediment accumulates, it forms sand dunes and silty loess deposits (see Figure 2). The process of dune and loess formation is shown in Figure 1. Some common types of dunes are illustrated in Figure 3 and described below:
• Barchan dunes are crescent-shaped. They occur where sand supply is limited and wind direction is fairly constant. Barchans generally form around shrubs or large rocks, which serve as minor barriers to sand transportation. The horns (tips) of barchans point downwind.
• Transverse dunes occur where sand supply is greater. They form as long ridges perpendicular to the prevailing wind direction. The crests of transverse dunes generally are linear to sinuous.
• Bachanoid ridge dunes form when barchan dunes are numerous and the horns of adjacent barchan dunes merge into transverse ridges. The crests of
barchanoid ridge dunes are chains of the short crescent-shaped segments that are the crests of individual barchan dunes. They can easily be distinguished from true transverse dunes that have long sinuous to very sinuous (like a snake) crests.
• Parabolic dunes somewhat resemble barchans.
However, their horns point in the opposite direction—upwind. Parabolic dunes always form adjacent to blowouts, oval depressions from which come the sandy sediments that form the parabolic dunes.
• Longitudinal (linear) dunes occur in some modern deserts where sand is abundant and crosswinds merge to form these high, elongated dunes. They can be quite large, up to 200 km long and up to 100 m high. The crests of longitudinal dunes generally are straight to slightly sinuous.
• Star dunes are isolated, star-shaped sand dunes that do not migrate.
Sharp-crested ridges run from each point of the star to a central peak that can be as high as 100 meters above the planar surface on which the dune formed. Star dunes form in locations where the wind blows from multiple directions at different times of the year.
Dunes tend to migrate slowly in the direction of the prevailing wind (Figures 1. and 3). However, revegetation of exposed areas, due to changes in climate or mitigation, may stabilize them.
Two characteristics of dryland precipitation combine to create some of the most characteristic dryland (including desert) landforms other than blowouts and dunes. First, rainfall in drylands is minimal. Second, when rainfall does occur, it generally is in the form of violent thunderstorms.
The high volume of water falling from such storms causes flash floods over dry ground. These floods develop suddenly, have high discharge, and last briefly. They carve steep-walled canyons, often floored with gravel that is deposited as the flow decreases and ends. Such steep-walled canyons with gravel floors commonly are called arroyos (or wadis, or dry washes).
Flash flooding in arid regions also erodes vertical cliffs along the edges of hills. When bedrock lies roughly horizontal, such erosion creates broad, flat-topped mesas bounded by cliffs. In time, the mesas can erode to small, stout, barrel-like rock columns, called buttes.
Therefore, In regions where Earth’s crust has been lengthened by tensional forces (pulled apart), mountain ranges and basins develop by block faulting—a type of regional rock deformation where Earth’s crust is broken into fault-bounded blocks of different elevations (Figure 5). The higher blocks are called horsts and the lower blocks are grabens (see Figure 5). The horst mountain ranges are eroded by running water and fluvial processes, which also transport the rock debris to adjacent graben basins (depressions where water and sediment accumulate). In a humid climate, these basins might collect water in permanent lakes.
In a desert, however, precipitation usually is insufficient to fill and maintain permanent lakes.
Another factor is that the climate is not constant through geologic time. Regions that now are deserts may have been more humid in the past. Landforms produced in the past, when the climate was different, still may be preserved on the present landscape.
These landforms are valuable clues to understanding environmental changes.
Landforms of arid mountainous deserts
These dryland landforms are illustrated in Figure 5:
• Alluvial fan—a fan-shaped, delta-like deposit of alluvium made at the mouth of a stream or arroyo, where it enters a graben, level plain, or basin.
• Bajada—(Spanish, “slope”) a continuous apron of coalescing alluvial fans below a mountain front.
• Mountain front—the sharp-angled intersection where the steep lower slope of a mountain range meets a pediment or alluvial fan and the slope of the land (and spacing of topographic contours) changes.
• Pediment—a gently inclined erosion surface in the upper part of the piedmont slope. It is carved into bedrock and generally has a thin veneer of alluvium.
• Playa—the shallow, almost flat, central part of a desert basin, in which water gathers after rain and evaporates to leave behind silt, clay, and evaporites (salts).
• Sand dune—a small hill, mound, or ridge (linear or sinuous) of windblown sand.
• Bolson—a basin into which water drains from the surrounding mountains to a central playa; a closed basin—one that has no outlet.
Adapted by AGI; NAGT R. M. Busch; Illustrated by D. Tasa