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Classroom presentations to accompany Understanding Earth , 3rd edition. prepared by Peter Copeland and William Dupré University of Houston. Chapter 14 Wind and Deserts. Wind and Deserts. Stanley Breeden/DRK. Deserts are usually thought of as hot and dry, but there are different ways to - PowerPoint PPT Presentation
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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
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.
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. 14.5
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).
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. 14.15
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
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.
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
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.
Evolution of a MesaEvolution of a MesaRivers Breach Resistant CapRivers Breach Resistant Cap
Fig. 14.26a