Arid Topography

Arid-Region Physical Geography

The terrain in arid climate regions has a distinctive appearance that can be attributed to the nature of the precipitation that falls in those landscapes. In addition to being low in average annual amount, precipitation in desert regions tends be sporadic in time and place. When storms do occur, they are often intense, delivering a substantial amount of precipitation over a short time. Although arid lands are typically vegetated, desert plants are specialized to cope with the moisture stress and do not form a continuous cover across the ground surface. With barren ground exposed between individual plants, loose rock and soil particles are easily moved downslope during sudden cloudbursts. As a result, unlike humid regions that have plenty of available moisture, a continuous coverage of vegetation, and a thick surface mantle of fine-grained (small particle size) soil covering the underlying rock, arid landscapes are more sparsely vegetated and are often visually dominated by outcrops of solid rock or loose, coarsegrained (large particle size) sediments. Far from being a wasteland, however, arid environments provide an austere and beautiful, yet fragile, home for a wide variety of specially adapted flora and fauna. In addition, because landforms and rock formations are not hidden under thick vegetation and soils, arid lands offer some of the most spectacular scenery on Earth.

Structural Setting

Volcanic and tectonic processes originate inside the Earth. Over geologic time, these processes have subjected Earth's surface to local and regional accumulations of volcanic rock and to various stresses, leading to the formation of mountains, basins, ridges, valleys, and plateaus. In many arid regions, these elements of geologic structure are well exposed and easy to identify. They form the large-scale relief features that the geomorphic processes originating on Earth's surface modify by breaking down rock (weathering), by eroding weathered rock materials from highlands, and by depositing those materials in lowlands.

Two generalized contrasting styles of aridregion topography are recognized, although they are best considered end members of a range of possible types. Mountain and basin deserts are those that retain considerable relief in the form of tectonically generated alternating uplands and depressions (see photo). This topography is created by a succession of uplifted and down-dropped fault blocks or large-scale upfolds and downfolds. [Page 111]The second general topographic style is marked by wide expanses of low relief, which is typical of large structural plateaus or tectonically stable continental shield locations. The Basin and Range physiographic province of Western North America exemplifies the mountain and basin desert setting; the Sahara in North Africa represents the shield and platform category. Most desert landforms are found in each of these structural settings, but some geomorphic features are especially well developed, or occur in distinctive associations, in mountain and basin or shield and platform deserts. Other differences exist within various mountain and basin deserts depending on whether or not the relief-building tectonic processes are still active.

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