Chapter
12
Deserts and Wind
Deserts and Wind begins with a discussion of the common misconceptions about dry lands. Following an examination
of the distribution and causes of dry regions, geologic processes in arid climates, the evolution of the dry Basin and
Range region of the United States, and the work of wind are investigated. The chapter closes with an analysis of wind
deposits.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Explain the distribution and origin of dry regions on Earth.
Briefly discuss the major geologic processes operating in arid climates.
Discuss the evolution and features of the Basin and Range Province.
Briefly explain the mechanisms of transporting sediment by wind.
Discuss the processes involved in wind erosion.
Compare and contrast the features produced by wind erosion.
Compare and contrast the various types of sand dunes.
Discuss the origin and geologic occurrence of loess.
Chapter Outline___________________________________________________________________
I.
DISTRIBUTION AND CAUSES OF DRY LANDS
A. Dry regions cover 30 percent of Earth's land surface
B. Two climatic types are commonly recognized
1. Desert, or arid, and
2. Steppe, or semiarid
C. Drylands are concentrated in two regions
1. Subtropics
a. Low-latitude deserts
b. In the vicinities of the Tropics of Cancer and Capricorn
c. Areas of high pressure and sinking air that is compressed and warmed
2. Middle-latitudes
II. GEOLOGIC PROCESSES IN ARID CLIMATES
A. Weathering
1. Not as effective as in humid regions
2. Mechanical weathering produces unaltered rock and mineral fragments
3. Some chemical weathering in deserts does produce
a. Clay
b. Thin soils
c. Oxidized minerals
B. Role of water in arid climates
1. Practically all streambeds are dry most of the time
a. Located in the deep interiors of continents
b. High mountains in the path of the prevailing winds produce a rainshadow desert
2. Desert streams are said to be ephemeral
a. Carry water only during periods of rainfall
b. Different names are used for desert streams in various regions
1. e.g., Wash and arroyo (dry western United States)
2. e.g., Wadi (Arabia and North Africa)
3. e.g., Donga (South America)
4. e.g., Nullah (India)
3. Desert rainfall
a. Rain often occurs as heavy shower
b. Because desert vegetative cover is sparse, runoff is largely unhindered and flash floods are
common
c. Poorly integrated drainage system and streams lack an extensive system of tributaries
4. Most of the erosional work in a desert is done by running water
III. BASIN AND RANGE: EVOLUTION OF A DESERT LANDSCAPE
A. Characterized by interior drainage
B. Landscape evolution in the Basin and Range region
1. Uplift of mountains - block faulting
2. Interior drainage into basins produces
a. Alluvial fans
b. Bajadas
c. Playas, and
d. Playa lakes
3. Ongoing erosion of the mountain mass
a. Produces sediment that fills the basin
b. Diminishes local relief
c. Produce isolated erosional remnants called inselbergs
IV. WIND IN THE DESERT
A. Transportation of sediment by wind
1. Differs from that of running water in two ways
a. Wind is less capable of picking up and transporting coarse materials
b. Wind is not confined to channels and can spread sediment over large areas
2. Mechanisms of transport
a. Bedload
1. Saltation - skipping and bouncing along the surface
2. About 20 to 25 percent of the sand transported in a sandstorm is moved this
way
b. Suspended load
B. Wind erosion
1. Wind is a relatively insignificant erosional agent, with most erosion in a desert performed by
intermittent running water
2. Mechanisms of wind erosion
a. Deflation
1. Lifting of loose material
2. Deflation produces
a. Blowouts - shallow depressions
b. Desert pavement - a surface of coarse pebbles and cobbles
b. Abrasion
1. Produces ventifacts - interestingly shaped stones
2. Limited in its vertical extent
C. Wind deposits
1. Significant depositional landforms are created by wind in some regions
2. Two types of wind deposits
a. Dunes
1. Mounds or ridges of sand
2. Often asymmetrically shaped
a. Windward slope is gently inclined
b. Leeward slope is called the slip face
1. Steep - about 34 degrees, the angle of repose for loose
sand
2. Cross beds - sloping layers of sand that form on the slip face
3. Slow migration in the direction of air movement
4. Types of sand dunes
a. Barchan dunes
b. Transverse dunes
c. Longitudinal dunes
d. Parabolic dunes
e. Star dunes
b. Loess
1. Blankets of windblown silt
2. Two primary sources
a. Deserts
b. Glacial outwash deposits
3. Extensive deposits occur in
a. Western and northern China
1. Sediments blown from the desert basins
2. Accumulations of 30 meters are common
b. Central United States
1. Correlates with the rich soils of the Midwest
2. An indirect product of glaciation