Chapter 16: Deserts and Wind

Fig. 16.9

OBJECTIVES• Explain that deserts originate from a variety of processes

and therefore form in a variety of settings.

• Identify the surface processes that lead to weathering and erosion in deserts.

• Describe the role wind plays in sculpting the desert landscape and the features it creates.

OBJECTIVES• Explain how human activities such as cultivation and

overgrazing pose a serious environmental threat in many parts of the world, resulting in the creation and expansion of deserts.

• Show the influence of plate tectonics on the distribution of deserts using the evidence for ancient deserts in the rock record.

• Use photographs of Mars to show that desert landforms like those on Earth are also present on other worlds.

• Arid and semiarid conditions affect more land areas than any other climate.

• Dry and dusty soil is readily picked up and transported by the wind.

• Running water remains the dominant agent of desert erosion and deposition.

Deserts and Winds: An Overview

Global Distribution of Deserts: Note that most deserts and the semiarid regions that surround them lie between 20 and 30 degrees latitude to the north and south of the equator.

Fig. 16.1

Global Pattern of Atmospheric Circulation: Subtropical deserts form beneath the subtropical highs between about 20 and 30 degrees of the equator. Color changes from red to blue track decreasing temperature from the equator to the poles. Red and blue arrows show circulation of warm and cool air, respectively. (H = high pressure; L = low pressure.)

Fig. 16.2

• Deserts are barren land areas receiving minimal rainfall.

The Origins of Deserts

The Sierra Nevada mountains create a cloud-free rain shadow on their leeward side.

Fig. 16.3b

• Types of Deserts

• Subtropical Deserts: result from from a global pattern of atmospheric circulation that removes moisture from the air at the equator

• Rain-Shadow Deserts: the dry regions on the far sides (from the prevailing wind) of certain mountain ranges

• Coastal Deserts: form when the cool, dry offshore air becomes heated as it blows onshore and evaporates all surface moisture

• Continental Interior Deserts: located so far from the sea that any rain they might have received falls before reaching them

• Polar Deserts: form beneath polar high-pressure belts

The Origins of Deserts

The Origins of Deserts

Rain Shadows and Deserts in the Pacific Southwest: Moist air from the Pacific Ocean loses its moisture as it moves eastward across the mountains.

Fig. 16.3a

• Mechanical Processes• Mechanical weathering and mass wasting produce

angular clasts of unaltered rock fragments.• Desert air lacks moisture, causing repeated expansion

and contraction with daily temperature fluctuations.

• Desert varnish is formed by a combination of chemical and physical weathering over extended periods of time.

Weathering and Erosion in Deserts

Fig. 16.5a

• Running water is the principal agent of erosion and deposition in deserts.

• Desert rainfall typically• Flows rapidly• Is not slowed by vegetation• Is full of sediment load

• Intermittent streams• Flow only occasionally• Carve dry valleys in landscape

Weathering and Erosion in Deserts

Fig. 16.6

• Sediment carried in running water can lead to • Alluvial fans (pictured)• Bajada: a continuous apron of sediment at the foot of

mountains

Weathering and Erosion in Deserts

Fig. 16.7

Weathering and Erosion in Deserts• Types of Erosional Remnants

• Inselbergs• Isolated remnants of

mountains• Pediments

• Gently sloping, sediment-strewn bedrock erosion surface

• Mesas• Broad, flat-topped remnants

bounded on all sides by steep cliffs

• Buttes• Pillar-like remnants of mesas

Fig. 16.8

• Playa Lakes• Usually shallow• Dry up over a period of a few days or weeks• Leave behind a layer of mud that shrinks and develops

mud cracks as the lake dries up• The dry, salt-encrusted lake bed formed is known as a

playa

• Oases• Springs of water in desert regions• Most common in valleys within mountainous regions or

in deep faults penetrating bedrock

Weathering and Erosion in Deserts

• Wind transports material by• Moving it along the ground as bed load• By carrying it in the air as a suspended load

• Saltation occurs during windblown sediment transport, as in streambeds.

Weathering and Erosion in Deserts

Fig. 16.11a

• Features of Wind Erosion• Deflation

• Lowering of the ground surface due to wholesale removal of loose material by the wind

• Blowouts• Shallow depressions in the ground

surface produced by deflation• Desert Pavement

• Veneer of pebbles and boulders eventually produced by wind erosion

Weathering and Erosion in Deserts

Fig. 16.16

• Features of Wind Deposition• Dunes

• Mound- or ridge-shaped deposits of windblown sand• Form wherever an obstacle in the wind’s path slows

the wind, allowing windblown sand to accumulate

Weathering and Erosion in Deserts

Multifaceted ventifacts are produced when pebbles and boulders exposed on the desert floor are subjected to windblasting by saltating sand from several prevailing wind directions.

Fig. 16.15

Weathering and Erosion in Deserts

Fig. 16.17

• This dune, seen in cross section, is asymmetric with a gently sloping windward side and a steeper leeward face, the slope of which is governed by the angle of repose for dry sand (about 34 degrees).

• Movement of asymmetric dunes produces cross-bedding as sand grains that migrate up the leeward slope by saltation are deposited on the windward face.

Weathering and Erosion in Deserts• Dune Types

• Barchan dunes• Transverse dunes• Longitudinal• Parabolic• Barachanoid• Star

Fig. 16.19

Desertification: Natural and Human Induced

• Desertification: the expansion of deserts into formerly fertile regions

• Causes of Desertification• Climate change• Drought• Mismanagement of land

• Agriculture• Logging• Overgrazing• Overpopulation

Desertification: Natural and Human Induced

• The Sahel • Forms a transition zone between the Sahara Desert and

regions of savannah and rain forest to the south• A semiarid region that has experienced rapid desertification

over the past 30 years• Desertification caused by

a number of factors• Drought• Overcultivation• Overgrazing

Fig. 16.22

Desertification: Natural and Human Induced

Sahel Rainfall: In the western Sahel, 20 years of above-average rainfall has been followed by 40 years of drought (data from the Nationals Oceanic and Atmospheric Administration).

Fig. 16.23

Deserts and Plate Tectonics• The characteristic features of deposits associated with modern

deserts help geologists recognize similar features preserved in the rock record.

• Wegener used the existence of ancient deserts in regions that now enjoy temperate climates in support of his hypothesis of continental drift.

• Plate tectonics plays a controlling influence on the distribution of deserts by controlling the movement of the continents.

• Mars has a desert climate, with dust storms and desert landforms such as dunes.

Deserts on Mars

Fig. 16.28

SUMMARY• Dry climates produce five types of deserts: subtropical, rain-

shadow, coastal, continental interior, and polar.

• The global distribution of deserts is related to atmospheric circulation.

• The principle agent of erosion and deposition in deserts is running water.

• Wind transports material in the desert and causes erosion.

SUMMARY• Desertification is a serious problem and can be caused by

mismanagement of land, climate change, overpopulation, and drought.

• Plate tectonics influences the distribution of deserts because it affects the latitude of continental masses.

• The planet Mars is a desert environment with dust storms many recognizable desert landforms.