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Classroom presentations
to accompany
Understanding Earth, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 14
Wind and Deserts
Stanley Breeden/DRK
Deserts
Deserts are usually thought of as hot
and dry, but there are different ways to
define a desert:
• Annual rainfall (<25 cm)
• Less precipitation than the potential
for evaporation
Deserts can be cold if there is an
extremely small amount of precipitation.
Atmospheric
Circulation
Patterns
Fig. 14.1
Erosion and deserts
Wind is often thought to be the most
important agent of erosion in deserts.
However, even in deserts, most of the
work of erosion is done by water.
Because there is so little water in
deserts, erosion is very intermittent.
Erosion and deserts
Typically, when storms take place in
desert regions, dry stream courses fill
quickly with water.
With little vegetation to hold water,
flash floods can be brief, but violent.
Erosion and deserts
When rainfall is unusually heavy,
desert soil may become saturated
with water and begin to flow.
This is known as a debris flow.
Fig. 14.2
Fig. 14.3
Wind
Direction
Fig. 14.4
Tom Bean
Rate of
Sand
Movement
as a
Function
of Wind
Velocity
Fig. 14.5
Wind
• Transportation of material: Because
wind is much less dense than water,
it can transport only small particles,
mainly fine sand and silt (clay is
usually too cohesive).
• Particles move by either saltation
(sand) or suspension (dust).
Wind
Dust can be transported over great
distances. Skiers in the Alps
commonly
encounter a silty surface on the
snow.
The silt comes from the Sahara
desert
in Africa, over 1500 km away.
Wind
• Wind-borne material can become
extremely concentrated in air:
in 1 km3, there may be up to 1000
tons of dust.
• Sand grains carried by wind get a
frosted exterior (diagnostic of eolian
transport).
Dust Storm, 1937
Library of Congress
Frosted and Rounded Wind-blown Sand
Walter N. Mack
Fig. 14.6
Deflation
• The process of removing all of the
small (easily moved) particles.
• As this process proceeds, only
larger rocks are left. This is known
as “desert pavement”.
Deflation Hollow
Breck P. Kent
Fig. 14.7
Formation of Desert Pavement
Fig. 14.9b
Desert Pavement
David Muench
Fig. 14.8a
Ventifact
E.R.Degginger
Fig. 14.9
Yardangs in Iran
Comstock
Fig. 14.10
Linear Dunes in Saudi Arabia
Prevailing
Winds
ERIM
Fig. 14.11
Coastal Dunes in Peru
Loren McIntyre
Fig. 14.12
Formation
of a
Windshadow
Dune
Fig. 14.13
Dune Migration
Fig. 14.14
Dune
Migration
and the
Formation
of Cross
Bedding
Fig. 14.15
Compression of Streamlines over
Dune Increases Velocity
Fig. 14.16
Types of Dunes
Fig. 14.17
Pleistocene Loess
E.R.Degginger
Fig. 14.18
Loess in
China
Stephen C. Porter
Fig. 14.19
Where deserts are
• Tropic of Capricorn, Tropic of Cancer
• High pressure  subsiding air heats  loses
moisture
• Center of continent
• Rain shadow
• Interaction with ocean currents: e.g., Atacama
Desert (Peru and Chile). Air moves from above
cold ocean waters to warm land and expands,
absorbing moisture.
Major Deserts of the World
Fig. 14.20
Desert varnish
• Surface coating of Fe and Mn
oxides
• Can be used to date exposure
intervals.
Petroglyphs in Desert Varnish
Peter Kresan
Fig. 14.21
Streams and lakes in deserts
• Often streams in the desert dry
up before they reach the sea.
• Those that don’t dry up are
usually fed from a wetter area
(e.g., Colorado River).
• Interior drainages are common
in deserts — the two are linked.
Examples: Nevada, Tibetan
plateau
“Dry wash” in Flood
Peter Kresan
Fig. 14.22a
The Day After
Peter Kresan
Fig. 14.22b
Playa Lake
David Muench
Fig. 14.23
Typical
Landscape
Formed by
Desert
Weathering
Peter Kresan
Fig. 14.24
Playa lakes
• Formed in a closed basin.
• Water accumulates after rain; may
last days to months before
complete evaporation, leaving a
playa, a flat lake bed of clay, silt,
and evaporites.
Faulting
Fig. 14.25a
Deposition of Alluvial Fans
Fig. 14.25b
Erosional Retreat Forms Pediment
Fig. 14.25c
Pediment Expands with Continued
Erosion
Fig. 14.25d
Evolution of a Mesa
Rivers Breach Resistant Cap
Fig. 14.26a
Evolution of a Mesa
Continued Erosion
Fig. 14.26b
Evolution of a Mesa
Long-continued Erosion
Fig. 14.26c