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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, and 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? 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.
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
caliche-rich 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 surface load, 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 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 elongated 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, 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, the hydrosphere (Chapters 14, 15, and 16),
and the biosphere (interwoven through many of the chapters). The
last part of this chapter concentrates on the biosphere. It discusses
human interactions with desert environments on the Great Plains of
North America and in the Sahel region of Africa. The process discussed
is 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.
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Key Terms
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