Chapter 13 Deserts and Winds

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Chapter 13 Deserts and Winds
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Desert and Its distribution

• Defined as an area where the annual precipitation
  is less than 25 cm (10 in).
• Most of the areas are located near the 30
  latitude ? 15 (Figure 13.3)
• Some located in rain shadow
• Arid climates are function of precipitation,
  evaporation, i.e. rate of evaporation gt the rate
  of precipitation

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Desert Distribution
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Cause of desert

• Air sink at 30.
• As air sinks it gets compressed.
• As air gets compressed it warms up and able to
  hold more water

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Rainshadow Deserts
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Rainshadow DesertDeath Valley, CA
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Characteristics of Desert

• Lack of flow through stream, thus intermittent
  flow, ephemeral
• Having internal drainage system due to local base
  level, playa lakes
• Rain often occurs as heavy showers, since no
  desert vegetative cover, runoff is largely
  unhindered and cause flash floods
• High daily temperature difference, thus physical
  weathering is predominant

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Playa LakeDeep Springs Lake, CA
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Dry channel After heavy rain
Desert River Channel
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Desert Topography in SW US

• In Colorado plateau
• Mesa and butte common due to differential erosion

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Desert Topography in SW US

• Normal fault very common to form basin and range

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Basin and Range Evolution of a Desert Landscape

• Uplift of mountains block faulting
• Interior drainage into basins produces
• Alluvial fans
• Playa lakes and Playas
• Ongoing erosion of the mountain mass
• Produces sediment that fills the basin
• Diminishes local relief
• Produce isolated erosional remnants called
  inselbergs

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Evolution of Desert Landscape
Early Stage
Middle Stage
Late Stage
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Desert Landscape Features

• Alluvial fans cone of debris at the mouth of a
  canyon, deposited during sporadic heavy rain
• Playa lakes shallow lake near center of basin
  forming during rare abundant rainfall
• Playas dry flat lake bed remaining after water
  has infiltrated and evaporated
• Inselbergs bedrock knobs projecting above
  sediment-filled basin, isolated erosional
  remanent

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Alluvial FanDeath Valley, CA
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Racetrack PlayaDeath Valley, CA
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Salt PolygonsDeath Valley, CA
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Inselbergs southern California
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Wind in the Desert

• Transportation of sediment by wind
• Differs from running water transport
• Wind is less capable of picking up and
  transporting coarse materials
• Wind is not confined to channels and can spread
  sediment over large areas
• Mechanisms of transport
• Bedload sand grains carried by wind
• Saltation skipping and bouncing along surface
• 20 to 25 of the sand transported this way
• Suspended load primarily silt grains w/ some
  clay transported in a cloud of dust

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Dust Storm Southeastern, CAExample of suspended
load
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Wind in the Desert

• Wind erosion
• Wind is a fairly insignificant erosional agent
• Most desert erosion caused by intermittent
  running water
• Mechanisms of wind erosion
• Deflation lifting of loose material, produces
  blowouts (shallow depressions) and desert
  pavement (a surface of coarse pebbles and
  cobbles)
• Abrasion particles bump and grind
• Limited in vertical extent (surface features)

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Formation of Desert Pavement
Wind removed fine particles, leaves coarse
particle
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Desert Pavement Caused by Deflation
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Formation of a Desert Blowout
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Soil Mound Prior to Blowout Formation
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Death Valley, CA Caused by Abrasion
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Wind Deposits

• Significant depositional landforms are created by
  wind in some regions
• Two primary types of wind deposits
• Loess
• Dunes

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Loess

• Blankets of windblown silt (suspended load)
• Deposited over 1000s of years
• Maintains vertical cliff at road cuts or streams
• Extensive deposits occur in China and the central
  United States
• Thick deposits of 30 100 m

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Loess DepositMississippi River, southern IL
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Dunes

• Mounds or ridges of sand
• Often asymmetrically shaped
• Windward slope is gently inclined and the leeward
  slope is called the slip face
• Slow migration of dunes in the direction of wind
  movement
• Several types of sand dunes including barchan,
  transverse, longitudinal, parabolic and star dunes

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Slip Face of DuneWhite Sands National Monument,
NM
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Sand DunesDeath Valley, CA
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Formation of Sand Dunes
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Cross Beds-Navajo SandstoneZion National Park, UT
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Sand Dune Types
Barchan
Longitudinal
Parabolic
Transverse
Star
Barchanoid
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Sand Dunes western United States