Chapter 17: Dry Regions: The Geology of Deserts
Study Plan
The flowing shapes of the Little Egypt sand dunes in Utah's Escalante Canyon region from when strong, dry winds blow sand. In a desert, there is very little vegetation, because rain is so rare.
Credit: Jerry Sintz, Bureau of Land Management
Guide to Reading
Water is such an important factor in Earth's surface processes that the last three chapters have focused on it. The author considered fresh surface water in Chapter 14, ocean water in Chapter 15, groundwater in Chapter 16. The chapter after this will again deal with water, but in its frozen form: glaciers. Is there no end to water? This chapter does not focus on water, for the subject is deserts; however, water is still an issue, for the defining characteristic of all deserts is its absence.
The chapter begins by dispelling the popular notion that all deserts are hot and sandy places. It lists and describes the five classes of deserts and points out that their only common factor is their aridity (dryness). The five classes are:
- subtropical: in the hot, dry latitudes between 20 and 30?, both north and south
- rain shadow: on the landward side of coastal mountain ranges
- coastal: along coasts bordering cold ocean currents
- continental interior: deep within continents, far from major water sources
- polar: in the cold, dry polar regions, both north and south, note the geographic locations given to illustrate these desert types because, this will help you understand why the regions are deserts, and matching desert locations with desert types are frequently asked test questions.
Early in your reading you are reminded that plate tectonics activities, as usual, play a part in explaining geologic phenomena. In the case of deserts, plate tectonics movements have determined the latitude of land masses and their relationships to coasts and mountains.
The chapter continues with a discussion of how weathering and erosion processes are modified by desert conditions. Chemical weathering is slow, but it does happen and it creates hard deposits of calcrete in soils and some special desert features like desert varnish. Rainfall is minimal, so streams are intermittent (or ephemeral). Dry water channels and basins (washes, arroyos, wadis, and playas) are common. When water is present, it does a vigorous job of eroding the land and is a more important agent of erosion than the wind is. Flash floods are a significant factor; they scour the land, produce dramatic steep-sided channels, and polish the canyon walls with their sand-laden waters. Wind, like water, can carry its load of sediment suspended or as bedload, which it may bounce along the ground in the process of saltation. Wind isn't as powerful as water and can't move grains larger than coarse sand. Therefore it creates lag deposits and can cause of desert pavement. Wind may abrade rock surfaces to produce smooth faces (facets) on pebble-sized particles called ventifacts. Wind erosion can lower the land surface over large areas, a process called deflation, and can produce special circular depressed areas known as blowouts.
Material removed by erosion must be deposited somewhere, so deposition in desert environments is the next topic. Deposits may be of varying sizes and may have been transported by a variety of agents. Large angular rocks tumble downhill due to gravity and pile up in talus aprons. Dust-sized particles lifted and then dropped by wind are called loess deposits. Desert streams drop materials when their gradients lessen and produce triangular-shaped structures called alluvial fans. Streams carry various salts into desert lake basins (playas) where they are left as thick salt deposits when the water evaporates. Occasionally these interior basins (lakes with channel inlets but no channel outlets) are very large, like the Great Salt Lake of Utah.
Desert landscapes are varied, and of course there is special vocabulary to describe all of them. Rocky desert areas change over time as scarp retreat forms mesas, buttes, chimneys, hoodoos, cuestas, hogbacks, dip slopes, inselbergs, and pediments. Depending on the amount of sand present and the constancy and velocity of the wind, sandy deserts are filled with different-shaped sand dunes. You read about barchan, star-shaped, transverse, parabolic, and longitudinal dunes, as well as the anatomy of an individual dune (windward slope, lee slope, slip face, angle of repose, and ripples).
The chapter concludes with a discussion of the process called desertification, in which semi-arid regions are changed into true deserts, partially due to natural causes, partially due to human activities. It is a somber reminder that geologic happenings and human society influence each other, and these interactions may be harmful to some fragile Earth environments and life forms.
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