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Guide to Reading
Water is such an important factor in Earth’s surface processes that the last four chapters have focused on it. The author considered fresh surface water in Chapter 17, ocean water in Chapter 18, groundwater in Chapter 19, and atmospheric water in Chapter 20. The chapter after this will again deal with water, but in its frozen form: glaciers. Is there no end to water? In this chapter you find it, for the subject is deserts. However, in a sense water is still an issue, for the defining characteristic of all deserts is the lack of water.
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
Do note the geographic locations given to illustrate the desert types because, obviously, their geographic locations explain 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 phenomenon. In the case of deserts, plate tectonics movements have:
- Created the mountains that produce rain-shadow deserts by convergence, rifting, or collision.
- Sutured together small continents into large ones with interiors far from oceans and therefore dry.
- Moved land masses on shifting plates into the subtropics, where climatic conditions have turned them into desert regions.
The chapter continues with a discussion of how weathering and erosion processes are modified by desert conditions. Chemical weathering is made minimal and 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 not rare. They scour the land, produce dramatic steep-sided channels, and polish the canyon walls with their sand-laden waters. Wind, like water, is a fluid and, 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 possibly is the cause of desert pavement. Wind may abrade rock surfaces to produce smooth faces (facets) on pebble-sized particles called ventifacts or may carve mushroom-shaped columns (yardangs) by the differential erosion of rock strata. 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 logically deposition in desert environments is the next chapter 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. Overlapping alluvial fans may extend for miles along mountain fronts as elongate structures called bajadas. 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 and the Dead Sea along the Israel-Jordan border.
Desert landscapes are varied, and of course there is special vocabulary to describe all of them. Hamada, reg, and erg are terms used to designate very different aspects of the Sahara Desert region. Rocky desert areas change over time as scarp retreat forms pediments, mesas, buttes, chimneys, hoodoos, cuestas, hogbacks, dip slopes, inselbergs, and bornhardts. 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 (seif) dunes, as well as the anatomy of an individual dune (windward slope, lee slope, slip face, angle of repose, and ripples).
Modern scientific thought tends to emphasize the interrelationships between the sciences. Therefore, while this is a geology text and naturally concentrates on inorganic aspects of Earth study, it does point out interactions between the obvious realm of geology, the lithosphere (rocky Earth), and Earth’s other spheres, the atmosphere (Chapter 20), the hydrosphere (Chapters 17, 18, 19, and 22), and the biosphere (interwoven through many of the chapters). The last part of this chapter concentrates on the biosphere. It discusses many adaptations life forms have developed that help them survive harsh desert conditions, and many human interactions with desert environments. Since the times of early civilizations in Egypt and Mesopotamia, humans have lived in desert regions, affected them, and been affected by them. You read about ancient Egypt and Mesopotamia, Ayers Rock in Australia, the Great Plains of North America, the Sahel region of Africa, and the Aral Sea in Asia.
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 influence human society, human society influences geologic happenings, and these interactions may be harmful to some fragile Earth environments and life forms. |
