Classroom presentations Classroom presentations to accompany to accompany

Understanding EarthUnderstanding Earth, 3rd edition, 3rd edition

prepared by

Peter Copeland and William Dupré

University of Houston

Chapter 14Chapter 14Wind 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.

Fig. 14.1

Atmospheric Atmospheric Circulation Circulation PatternsPatterns

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

Fig. 14.4 Tom Bean

Wind Direction

Fig. 14.5

Rate of Rate of Sand Sand

Movement Movement as a as a

Function Function of Wind of Wind VelocityVelocity

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 greatdistances. Skiers in the Alps

commonlyencounter a silty surface on the

snow.The silt comes from the Sahara

desertin 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

Fig. 14.6

Frosted and Rounded Wind-blown Sand

Walter N. Mack

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”.

Fig. 14.7Breck P. Kent

Deflation HollowDeflation Hollow

Formation of Desert PavementFormation of Desert Pavement

Fig. 14.9b

Fig. 14.8aDavid Muench

Desert Pavement

Ventifact

Fig. 14.9E.R.Degginger

Yardangs in Iran

Fig. 14.10Comstock

Fig. 14.11

Linear Dunes in Saudi Arabia

PrevailingWinds

ERIM

Fig. 14.12

Coastal Dunes in Peru

Loren McIntyre

Formation Formation

of a of a Wind-Wind-

shadow shadow DuneDune

Fig. 14.13

Dune MigrationDune Migration

Fig. 14.14

Fig. 14.15

Dune Dune Migration Migration and the and the

Formation Formation of Cross of Cross BeddingBedding

Fig. 14.16

Compression of Streamlines over Compression of Streamlines over Dune Increases VelocityDune Increases Velocity

Types of DunesTypes of Dunes

Fig. 14.17

Fig. 14.18

Pleistocene Loess

E.R.Degginger

Loess in China

Fig. 14.19Stephen C. Porter

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 WorldMajor Deserts of the World

Fig. 14.20

Desert varnish

• Surface coating of Fe and Mn

oxides

• Can be used to date exposure

intervals.

Fig. 14.21

Petroglyphs in Desert Varnish

Peter Kresan

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

Fig. 14.22a

““Dry wash” in FloodDry wash” in Flood

Peter Kresan

Fig. 14.22b

The Day AfterThe Day After

Peter Kresan

Fig. 14.23

Playa LakePlaya Lake

David Muench

Typical Landscape Formed by

Desert Weathering

Fig. 14.24Peter Kresan

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.

FaultingFaulting

Fig. 14.25a

Deposition of Alluvial FansDeposition of Alluvial Fans

Fig. 14.25b

Erosional Retreat Forms PedimentErosional Retreat Forms Pediment

Fig. 14.25c

Pediment Expands with Continued Pediment Expands with Continued ErosionErosion

Fig. 14.25d

Evolution of a MesaEvolution of a MesaRivers Breach Resistant CapRivers Breach Resistant Cap

Fig. 14.26a

Evolution of a MesaEvolution of a MesaContinued ErosionContinued Erosion

Fig. 14.26b

Evolution of a MesaEvolution of a MesaLong-continued ErosionLong-continued Erosion

Fig. 14.26c