Deserts, Wind, and Dunes

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Deserts, Wind, and Dunes

• Dr. R. B. Schultz

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• Deserts and Wind Action (Aeolian Processes)
•  
• Desert -- arid land with less than 250 mm (10
  inches) precipitation per year or in which
  evaporation exceeds precipitation. No
  significant vegetation exists. It need NOT be
  hot and dry.
• Includes 25 of the world's land area outside
  the Polar Regions.
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• Depends on the circulation of the earth's
  atmosphere
• Coriolis effect -- deflection process of winds
  around earth
• Easterlies, westerlies, trade winds
• Most arid land lies between latitudes 10o and 30o
  (N. S.) 

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There are several types of deserts 1.
Subtropical Those associated with the
circum-global belts of dry, subtropical air
(Sahara, Kalahari, Great Australian) 2.
Rainshadow Lee side of mountain ranges in the
rain shadow (Sonoran). 3. Continental interiors
with low rainfall (Gobi, Takla Makan in Asia) 4.
Coastal deserts where upwelling cold seawater
cools the air, lowering its ability to hold
moisture (Peru, Chile, SW Africa) 5. Polar
deserts (N. Greenland, ice-free areas in
Antarctica)   Desert climate and vegetation 1.
High temperature (130oF) 2. Low precipitation
(20-50 mm) 3. High evaporation rate (up to 250
mm/yr) 4. Frequent strong winds
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Deserts and Arid Regions
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• Stages of Development for Desert or Arid
  Landscape
• 1.    First Stage
• Playa lakes start to dry up evaporation gt
  precipitation
• Alluvial fans form -- fan-shaped sediment
  features
• 2.   Middle Stage
• Lakes dry up -- playas form
• Alluvial fans dry up -- form bajadas of pediment
• 3.   Late Stage
• Isolated remnants of resistant rock remain --
  inselbergs
• Drainage is internal -- never out of region
  except major
• storm events
• Intermittent or ephemeral streams drain region

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• Erosion by wind
• Deflation
• Deflation armor or desert pavement
• Water table control
• Ventifacts -- rock sculpted by work of wind
  (sandblasting)
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• Wind-transported sediments
• 1. Sand ripples
• 2. Dunes
• a. Barchan -- crescent-shaped points downwind
  much sand
• b. Transverse -- perpendicular to wind much sand
• c. Linear (longitudinal) -- parallel to
  strong winds limited sand
• d. Parabolic -- crescent points upwind
  much sand vegetation
• present
• e. Stellate (star-shaped) -- shifting
  wind direction much sand
• 3. Loess -- rock flour usually glacially derived

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• Geologic processes in deserts
• 1. Weathering and mass wasting
• 2. Mostly mechanical weathering
• 3. Thin regolith (material overlying bedrock)
• 4. Desert varnish -- veneer coating of manganese
  oxides
• 5. Streams flash floods, debris flows
• 6. Playa lakes, pediments, bajadas, inselbergs
• 7. Groundwater
• 8. Wind
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• Wind action
• Transport of sediment by wind
• Not efficient in moving great quantities of
  material
• Best sorter of particle size
• Suspended load
• Volcanic ash
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Desert landforms 1. Fans and bajadas 2.
Pediments 3. Inselbergs   Droughts and
desertification 1. Droughts and soil erosion 2.
Stabilization techniques   Key terms Deflation
-- the process of removing sediment by wind
erosion Blowout -- area deflated by wind
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Key Terminology
Desert Coriolis Effect Subtropical Rainshadow
Coastal Polar Continental Interior Playa
lake Alluvial fan Bajadas Playas Inselberg In
ternal drainage Ephemeral stream Ventifact Blow
out Deflation Loess Regolith Desert
varnish Barchan Barchanoid Parabolic Longitudi
nal Stellate Transverse
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Pertinent Web Sites
Arid Lands Links This Central Michigan
University site maintained by Dr. Mark Francek
lists several good links to Web sites containing
information relative to topics discussed in the
chapter. Convention to Combat Desertification
Reports of the Convention to Combat
Desertification held in Rome, Italy in the Fall
1997. Deserts and Wind Links (Houghton Mifflin)
Links to several desert and wind sites,
including class lecture notes, arranged by
topic. Desertification and Land Degradation
(CIESIN) Links to several sites related to
desertification and land degradation. Desert
Links (NAGT) An extensive listing of desert
links arranged by topic from the National
Association of Geology Teachers (NAGT). Desert
Research Institute The home page of the Desert
Research Institute in Nevada features information
on arid areas and links to related WWW
sites. Deserts Geology and Resources (USGS) An
excellent site that presents information about
desert landforms, climates, etc. from the United
States Geological Survey (USGS). Eolian Landforms
of the Central Andes A study of wind produced
(eolian) landforms of the central Andes conducted
at Cornell University. Global Hydrology and
Climate Center (NASA) The Global Hydrology and
Climate Center (GHCC) is a joint venture between
government and academia to study the global water
cycle and its affect on climate. Great Sand Dunes
National Monument, Colorado A brief explanation
of the geology and origin of the sand at Great
Sand Dunes National Monument, Colorado.
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Hydrologic Cycle, Arid and Semi-Arid Lands This
location at the University of Arizona offers
several links to related sites. Lost City of
Arabia (NOVA Online) A NOVA documentary on
locating the lost city of Ubar, Arabia using
remote sensing techniques. National Parks State
Listing Links to national parks, national
monuments, etc. TopoZone The TopoZone bills
itself as the Web's center for recreational and
professional topographic map users. Here you can
interactively view topographic maps from the
entire United States. Wind Erosion Wind erosion
research and information from the United States
Department of Agriculture, Agriculture Research
Service Wind Erosion Laboratory at Kansas State
University. World Drylands Exhibit An
electronic exhibit about the world's drylands.