Erosion and Deserts

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Classroom presentations to accompany Understanding Earth, 3rd edition : 

Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 14 Wind and Deserts

Slide 2: 

Wind and Deserts Stanley Breeden/DRK

Deserts : 

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 Atmospheric Circulation Patterns

Erosion and deserts : 

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 : 

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 : 

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.

Slide 8: 

Fig. 14.2

Slide 9: 

Fig. 14.3

Slide 10: 

Fig. 14.4 Tom Bean Wind Direction

Rate of Sand Movement as a Function of Wind Velocity : 

Fig. 14.5 Rate of Sand Movement as a Function of Wind Velocity

Wind : 

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 : 

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 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 : 

Dust Storm, 1937 Library of Congress

Frosted and Rounded Wind-blown Sand : 

Fig. 14.6 Frosted and Rounded Wind-blown Sand Walter N. Mack

Deflation : 

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 : 

Fig. 14.7 Breck P. Kent Deflation Hollow

Formation of Desert Pavement : 

Formation of Desert Pavement Fig. 14.9b

Slide 20: 

Fig. 14.8a David Muench Desert Pavement

Ventifact : 

Ventifact Fig. 14.9 E.R.Degginger

Yardangs in Iran : 

Yardangs in Iran Fig. 14.10 Comstock

Linear Dunes in Saudi Arabia : 

Fig. 14.11 Linear Dunes in Saudi Arabia Prevailing Winds ERIM

Slide 24: 

Fig. 14.12 Coastal Dunes in Peru Loren McIntyre

Formation of a Wind-shadow Dune : 

Formation of a Wind-shadow Dune Fig. 14.13

Dune Migration : 

Dune Migration Fig. 14.14

Dune Migration and the Formation of Cross Bedding : 

Fig. 14.15 Dune Migration and the Formation of Cross Bedding

Compression of Streamlines over Dune Increases Velocity : 

Fig. 14.16 Compression of Streamlines over Dune Increases Velocity

Types of Dunes : 

Types of Dunes Fig. 14.17

Pleistocene Loess : 

Fig. 14.18 Pleistocene Loess E.R.Degginger

Loess in China : 

Loess in China Fig. 14.19 Stephen C. Porter

Where deserts are : 

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 : 

Major Deserts of the World Fig. 14.20

Desert varnish : 

Desert varnish Surface coating of Fe and Mn oxides Can be used to date exposure intervals.

Slide 35: 

Fig. 14.21 Petroglyphs in Desert Varnish Peter Kresan

Streams and lakes in deserts : 

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 : 

Fig. 14.22a “Dry wash” in Flood Peter Kresan

The Day After : 

Fig. 14.22b The Day After Peter Kresan

Slide 39: 

Fig. 14.23 Playa Lake David Muench

Typical Landscape Formed by Desert Weathering : 

Typical Landscape Formed by Desert Weathering Fig. 14.24 Peter Kresan

Playa lakes : 

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 : 

Faulting Fig. 14.25a

Deposition of Alluvial Fans : 

Deposition of Alluvial Fans Fig. 14.25b

Erosional Retreat Forms Pediment : 

Erosional Retreat Forms Pediment Fig. 14.25c

Pediment Expands with Continued Erosion : 

Pediment Expands with Continued Erosion Fig. 14.25d

Evolution of a MesaRivers Breach Resistant Cap : 

Evolution of a MesaRivers Breach Resistant Cap Fig. 14.26a

Evolution of a MesaContinued Erosion : 

Evolution of a MesaContinued Erosion Fig. 14.26b

Evolution of a MesaLong-continued Erosion : 

Evolution of a MesaLong-continued Erosion Fig. 14.26c