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.

Fig Atmospheric Circulation Patterns

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 Tom Bean Wind Direction

Fig Rate of Sand Movement as a Function of Wind Velocity

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 km 3, 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 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 Breck P. Kent Deflation Hollow

Formation of Desert Pavement Fig. 14.9b

Fig. 14.8a David Muench Desert Pavement

Ventifact Fig E.R.Degginger

Yardangs in Iran Fig Comstock

Fig Linear Dunes in Saudi Arabia Prevailing Winds ERIM

Fig Coastal Dunes in Peru Loren McIntyre

Formation of a Wind- shadow Dune Fig

Dune Migration Fig

Fig Dune Migration and the Formation of Cross Bedding

Fig Compression of Streamlines over Dune Increases Velocity

Types of Dunes Fig

Fig Pleistocene Loess E.R.Degginger

Loess in China Fig Stephen 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 World Fig

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

Fig 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 a “Dry wash” in Flood Peter Kresan

Fig b The Day After Peter Kresan

Fig Playa Lake David Muench

Typical Landscape Formed by Desert Weathering Fig Peter 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.

Faulting Fig a

Deposition of Alluvial Fans Fig b

Erosional Retreat Forms Pediment Fig c

Pediment Expands with Continued Erosion Fig d

Evolution of a Mesa Rivers Breach Resistant Cap Fig a

Evolution of a Mesa Continued Erosion Fig b

Evolution of a Mesa Long-continued Erosion Fig c