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Chapter 17: Atmosphere, Winds, and
Deserts
Introduction: Wind as a Geologic Agent
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Mars is an arid, windy, and dusty planet,
extensively modified by wind action.
Wind is also an important agent of erosion and
sediment transport on Earth, bur its effects are
visible mainly in desert regions.
In the relatively moist part of the temperate and
tropical latitudes where a protective cover of
vegetation exist, wind is an ineffective geologic
agent.
Introduction: Wind as a Geologic Agent
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Wind has been important in shaping the
landscape at times when the continents were
drier and windier places than they are today.
Wind is the movement of air, principally
horizontal movement caused by:
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Heating by the Sun.
Rotation of Earth on its axis.
Planetary Wind System (1)
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The basic reason the atmosphere is always in
motion is that more of the Sun’s heat is received
per unit of land surface near the equator than
near the poles.
This unequal heating gives rise to convection
currents.
The heated air near the equator becomes lighter,
rises, and expands.
Planetary Wind System (2)
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High up, the air spreads outward in the direction
of both poles.
As it gradually cools, it becomes heavier, and
sinks.
This cool, descending air flows back toward the
equator completes a cycle of convection.
Planetary Wind System (3)
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The Coriolis effect is named for the nineteenthcentury French mathematician, GaspardGustave de Coriolis.
Earth and the atmosphere are rotating eastward
at a speed of 1670 km/h at the equator (at the
North Pole, the speed of rotation is zero).
The amount of the deflection is a function of the
speed of the moving air mass and the latitude.
Planetary Wind System (4)
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The Coriolis effect breaks up the simple flow of air
between the equator and the poles into belts:
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Between the equator and about 300 latitude are the Hadley cells:
the prevailing winds are northeasterly.
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These wind system are called the tradewinds.
A second cell of circulating air, called the Ferrel cell, lies
poleward of the Hadley cell. In these middle-latitude cells,
westerly winds prevail.
A third cell of circulating air, a polar cell, lies over each polar
region. These two cells are called the polar easterlies.
Planetary Wind System (5)
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Where cold air descends, dry conditions are
created at the land surface.
Other factors in explaining the global pattern of
airflow include:
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The topography of the land.
The distribution of oceans, continents, high mountains,
and plateaus.
The global pattern of airflow ultimately controls
the variety and pattern of Earth’s climates.
Figure 17.1
Movement of Sediment by Wind (1)
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Sediment size blown by the wind depends on
wind speed:
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In extraordinary wind storms, when wind speeds
locally reach 300 km/h or more, coarse rock particles
up to several centimeters in diameter can be lifted to
height of a meter or more.
In moist regions, where wind speed rarely exceeds 50
km/h, the largest particles of sediment that can be
suspended in the air stream are grains of sand.
Movement of Sediment by Wind (2)
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At lower wind speeds, sand moves along close to the
ground surface, and only finer grains of dust move in
suspension.
If a wind blows across a bed of sand, the grains begin
to move when the wind speed reaches about 4.5 m/s
(16 km/h).
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Figure 17.2
The resulting forward rolling motion of the sand is called
surface creep.
Movement of Sediment by Wind (3)
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With increasing wind speed, turbulence lifts moving
sand grains into the air: this is saltation.
Saltation accounts for at least three-quarters of the
sand transport in areas covered by sand dunes.
Sand movement increases rapidly with increasing
wind speed.
Sheets of well-sorted sand that have accumulated on
the land surface are unstable.
Movement of Sediment by Wind (4)
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Saltation moves the smaller, most easily transported
grains.
Sand grains too large to be moved are left behind.
The coarse grains form a series of small, linear ridges
of sand called sand ripples.
Figure 17.3
Windblown Dust (1)
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Fine particles of dust travel faster, longer, and
much farther before settling to the ground.
The dustiest places on Earth tend to coincide
with some of the world’s major desert regions.
Windblown Dust (2)
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Many types of terrain give rise to large
quantities of dust:
Dry lakes.
; Stream beds.
; Alluvial fans.
; Outwash plains of glacial streams.
; Regions underlain by deposits of windblown dust that have
lost their vegetation cover.
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Windblown Dust (3)
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As a result of frictional drag, the velocity of
moving air decreases sharply near the ground
surface.
Right at the surface lies a layer of relatively quiet
air less than 0.5 mm thick, within which airflow
is smooth and laminar rather than turbulent.
Figure 17.5
Windblown Dust (4)
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Sand grains that protrude above this layer of
quiet air can be swept aloft by rising turbulent
eddies.
Dust constitutes the wind’s suspended load.
In most cases suspended sediment is deposited
fairly near its place of origin.
Windblown Dust (5)
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Figure 17.6
However, strong winds associated with large
dust storms are known to carry very fine dust
into the upper atmosphere, where it can be
transported thousands of kilometers.
Dust storms are most frequent in the vast arid
and semiarid regions of central Australia,
western China, Russian central Asia, the Middle
East, and North Africa.
Windblown Dust (6)
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In the United States, blowing dust is especially
common in the southern Great Plains and in the
desert regions of California and Arizona.
Deposition of Dust (1)
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Windborne dust can be deposited under several
conditions:
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Wind velocity and air turbulence decrease so that
particles can no longer remain in suspension.
The particles collide with rough or moist surfaces that
trap them.
The particles accumulate to form aggregates, which
then settle out because of their greater mass.
The particles are washed out of the air by rain.
Vegetated landscapes reduce wind velocity.
Deposition of Dust (2)
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Figure 17.7
Deposition occurs where a topographic obstacle
causes a reduction of wind velocity.
Deposits of dust are generally thick on the lee, or
downwind, side of obstacles.
Deposits of dust are thin or absent on the
windward, or upwind, side.
Figure 17.8
Detrimental Effects of Windblown
Sediment (1)
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Crops and other vegetation can be severely
damaged.
Engines of vehicles can be ruined.
Reduced visibility on roads can cause accidents.
Inhalation of dust can cause emphysema.
Quartz inhalation can lead to silicosis.
Detrimental Effects of Windblown
Sediment (2)
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Deadly germs, such as anthrax and tetanus, can be
transported in windblown dust.
In central China, a close correlation has been found
between deaths due to cancer of the esophagus and
the distribution of dust deposits.
Wind Erosion
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Erosion is important wherever winds are strong
and persistent.
Flowing air erodes in two ways:
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Deflation: the wind picks up and carries away sand
and dust.
Abrasion: wind-driven grains of sediment impact
rock.
Deflation (1)
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Nondesert sites where deflation occurs include:
Ocean beaches.
; Shores of large lakes.
; The floodplains of large glacial streams.
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Deflation may occur seasonally when farmland is
plowed.
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Especially severe during times of drought, when no
moisture is present to hold soil particles together.
Deflation (2)
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In the dry 1930s, deflation in parts of the western
United States amounted to 1 m or more within
only a few years.
Deflation hollows and basins are small saucer- or
trough-shaped hollows created by wind erosion.
Most are less than 2 km long and only a meter or
two deep.
Deflation (3)
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The immense Qattara Depression in the Libyan
desert of western Egypt lies more than 100 m
below sea level due to intense deflation.
The depth to which deflation can reach is limited
by water table.
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As deflation lowers the land, the surface soil becomes
moist, encouraging the growth of vegetation.
Deflation (4)
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When sand and dust are either blown away from
a deposit of alluvium or locally removed by sheet
erosion, stones too large to be moved become
concentrated at the surface.
Eventually, a continuous cover of stones forms a
desert pavement.
Figure 17.9
Figure 17.10
Abrasion
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Abrasion occurs when rock is scoured by
windborne grains of sediment.
Results include distinctive rock shapes called
ventifacts.
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A ventifact is any bedrock surface or stone that has
been abraded and shaped by windblown sediment.
The common landforms of some desert regions is
an elongate, streamlined, wind-eroded ridge
called a yardang.
Figure 17.11
Figure 17.12
Figure 17.13
Eolian Deposits
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Sediments deposited by wind are called eolian
deposits.
The major kinds of eolian deposits are:
Dunes.
; Loess.
; Dust in oceans.
; Glacial sediments.
; Volcanic ash deposits.
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Dunes (1)
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A dune is a hill or ridge of sand deposited by
winds.
A typical isolated dune:
Is asymmetrical.
; Has a gently sloping windward face (angle of repose: 33-34o).
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The lee face of an active dune is called the slip
face.
Dunes (2)
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Figure 17.14
The angle of the windward slope of a dune varies
with wind velocity and grain size but is always
much less than that of the slip face.
This asymmetry of form provides a means of
telling the direction of the wind that shaped the
dune.
Dune Types
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There are five different types of dunes:
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Barchan dune.
Transverse dune.
Linear dune.
Star dune.
Parabolic dune.
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Barchan dune.
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Transverse dune.
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Linear dune.
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Star dune.
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Parabolic dune.
Figure T17.1
Barchan Dune
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A crescent- shaped dune with pointing
downwind.
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Occurs on hard, flat desert floors in areas of constant
wind direction and limited sand.
Height 1 m to more than 30 m.
May migrate 25 m/year.
Figure 17.17
Transverse Dune
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A dune forming an asymmetrical ridge
transverse to dominant wind direction.
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Occurs in areas with abundant sand.
Can form by merging of individual barchans.
Linear Dune
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A long, relatively straight, ridge-shaped dune.
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Occurs in desert with limited sand supply where
winds are variable (bi-directional).
Slip faces change orientation as wind shifts direction.
Star Dune
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An isolated hill of sand having a base that
resembles a star in outline.
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Sinuous arms of dune converge to form central peak
as high as 300 m.
Tends to remain fixed in place in areas where wind
blows from all directions.
Parabolic Dune
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A dune shaped like a U or V, with the open end
facing upwind.
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Trailing arms.
Generally stabilized by vegetation.
Common in coastal dune fields.
Sand Seas
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Sand seas are vast tracts of shifting sand.
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Found in northern and western Africa, the Arabian
Peninsula,and the large desert of western China;
Contain a variety of dune forms.
Loess
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Loess is wind-laid dust consisting largely of silt.
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Important resource in countries where it is thick and
widespread because it provides rich agricultural lands.
Upper Mississippi Valley.
; The Columbia Plateau of Washington State.
; The loess Plateau region of central China.
; Much of eastern Europe.
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Characteristics of Loess (1)
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Loess has two characteristics that indicate that
it was deposited by the wind:
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It forms a rather uniform blanket over hills and
valleys alike.
It contains fossils of land plants and air-breathing
animals.
Characteristics of Loess (2)
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Typical characteristics of Loess:
Homogeneous.
; Lacks stratification.
; Can form vertical cliffs.
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Origin of Loess (1)
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2 principal sources of loess are:
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Deserts.
Floodplains of glacial meltwater.
The loess that covers central China was blown
from the great desert basins of central Asia.
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Part of the sediment may have come from the
breakdown of rock by frost action and glacial
processes in the high glaciated mountains of inner
Asia.
Origin of Loess (2)
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Glacial loess is widespread in the middle part of
North America, and in east-central Europe.
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The shapes and compositions of its particles resemble
the fine sediment produced by the grinding action of
glaciers.
Figure 17.18
Origin of Loess (3)
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Glacial loess is thickest downwind from former
large braided meltwater streams such as:
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The Mississippi.
The Missouri.
The Rhine.
The Danube.
Figure 17.19
Dust in Ocean Sediments and Glacier
Ice (1)
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Dust blown over the ocean forms an important
component of deep-sea sediments.
Deposits of eolian dust trend:
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Eastward across the North Pacific from China.
Westward across the subtropical North Atlantic from
Africa.
Westward into the Indian Ocean from Australia.
Figure 17.20
Dust in Ocean Sediments and Glacier
Ice (2)
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Fine particles of quartz deflated from Asian
deserts have been found in soils of the Hawaiian
Islands.
Windblown dust is also found in cores drilled
through polar ice sheets and low-latitude
mountain glaciers.
Dust in Ocean Sediments and Glacier
Ice (3)
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Large quantities of tephra can be ejected into the
atmosphere during explosive volcanic eruptions.
A distinctive igneous mineralogy and tiny
fragments of volcanic glass make tephra layers
easy to recognize.
Dust in Ocean Sediments and Glacier
Ice (4)
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Fine ash that reaches the stratosphere may circle
the Earth many times before it finally settles to
the ground.
Eruptions commonly form elongate plumes of
sediment that decrease in particle size and
thickness downwind from the source volcano.
Figure 17.21
Deserts
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Deserts are nearly devoid of vegetation if no
artificial water supply is used.
The term desert identifies for land where annual
rainfall is less that 250 mm, or in which the
potential evaporation rate exceeds the
precipitation rate.
Types and Origins of Deserts (1)
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Desert lands total about 25 percent of the
Earth’s land area.
Five types of desert are recognized:
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Figure 17.22
Subtropical.
Continental.
Rainshadow.
Coastal.
Polar.
Types and Origins of Deserts (2)
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The most extensive deserts are the subtropical
type, associated with the two circumglobal belts
of dry, descending air centered between latitudes
20o and 30o.
Examples:
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The Sahara.
The Kalahari.
The Rub-al-Khali (Saudi Arabia).
The Great Australian Desert.
Types and Origins of Deserts (3)
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Continental deserts are found in continental
interiors, far from sources of moisture.
Polar deserts have extremely low precipitation
due to the sinking of cold, dry air.
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The surface of a polar desert is often underlain by
abundant water, but nearly all in the form of ice.
Polar deserts are found in northern Greenland, artic
Canada, and in the ice-free valleys of Antarctica.
Types and Origins of Deserts (4)
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Where a mountain range creates a barrier to the
flow of moist air, it produces a rainshadow, on
the lee side of mountains.
Coastal deserts occur along the margins of
continents, where cold upwelling seawater cools
maritime air flowing onshore. This decreases the
air’s ability to hold moisture.
Desert Climate
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The arid climate of a hot desert results from the
combination of:
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High temperature.
Low precipitation.
High evaporation rate.
Other characteristics:
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Irregular rainfall.
Strong winds.
Surface Processes and Landforms in
Deserts (1)
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The regolith in a desert is thinner, less
continuous, and coarser in texture.
Much of the regolith is the product of
mechanical weathering.
Chemical weathering is greatly diminished
because of reduced soil moisture.
Surface Processes and Landforms in
Deserts (2)
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In deserts, because particles created by
mechanical weathering tend to be coarse, slopes
are generally steeper and more angular.
Among the most distinctive landforms in deserts
are:
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A butte (erosional remnant carved from resistant,
flat-lying rocks units).
A flat-topped mesa (wider landform of the same
origin).
Surface Processes and Landforms in
Deserts (3)
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In many desert areas, older deposits are darker
because of desert varnish (a thin, dark, shiny
coating, commonly manganese oxide).
Desert Streams and Associated
Landforms (1)
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Most streams that flow into deserts from
adjacent mountains never reach the sea.
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They soon disappear as the water evaporates or soaks
into the ground.
Exceptions are long rivers like the Nile.
A major rainstorm is likely to be accompanied
by a flash flood (a sudden, swift flood that can
transport large quantities of sediment).
Desert Streams and Associated
Landforms (2)
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The debris from flash floods forms fans at the
base of mountain slopes and on the floor of wide
valleys and basins.
Alluvial fans are common in arid and semiarid
lands, where they typically are composed of both
alluvium and debris-flow deposits.
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Entire cities have been built on alluvian fans or fan
complexes (for example, San Bernardino, California,
and Tehran, Iran).
Figure 17.25
Desert Streams and Associated
Landforms (3)
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Where a mountain front is straight and its
canyons are widely spaced, each fan will
encompass an arc of about 180o.
If canyons are closely spaced along the base of a
mountain range, coalescing adjacent fans form a
broad alluvial apron, or bajada.
Desert Streams and Associated
Landforms (4)
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Runoff in arid regions is rarely abundant enough
to sustain permanent lakes.
The floor of a desert basin may contain a dry
lakebed called a playa.
Runoff may be sufficient to form a temporary
playa lake that will last up to several weeks.
Desert Streams and Associated
Landforms (5)
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One of the most characteristic landforms of dry
regions is the pediment, a broad, relatively flat
surface, eroded across bedrock.
A pediment is a bedrock surface rather than a
thick alluvial fill.
The long profile of a pediment, like that of an
alluvial fan, is concave upward, becoming
progressively steeper toward a mountain front.
Figure 17.28
Desert Streams and Associated
Landforms (6)
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Steep-sided mountains, ridges, and isolated hills
that rise abruptly from adjoining plains like
rocky islands standing above the surface of a
broad flat sea are called inselbergs (German for
island mountain).
Inselbergs form in areas of relatively
homogeneous resistant rock that are surrounded
by rocks more susceptible to weathering.
Desertification (1)
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In the region south of the Sahara lies a belt of dry
grassland known as the Sahel (rainfall is normally only
100 to 300 mm).
In the early 1970s, the drought-prone Sahel experienced
the worst drought of the twentieth century.
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For several years in succession the annual rains failed to appear,
causing adjacent desert to spread southward (as much as 150
km).
The drought affected a population of at least 20 million people,
many of them seminomadic herders of cattle, camels, sheep, and
goats.
Desertification (2)
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Overgrazing during years of drought killed most of
the vegetation. Without vegetation, soil blows away
and the desert advances.
Figure 17.31
Figure 17.32
Figure 17.B02