Transcript Deserts, Wind, and Dunes

Deserts, Wind, and Dunes
Dr. R. B. Schultz
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.
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.)
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
Deserts and Arid Regions
Stages of Development for Desert or Arid Landscape:
1. First Stage:
•Playa lakes start to dry up; evaporation > 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
Erosion by wind
•Deflation
•Deflation armor or desert pavement
•Water table control
•Ventifacts -- rock sculpted by work of wind (sandblasting)
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
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
Wind action
•Transport of sediment by wind
•Not efficient in moving great quantities of material
•Best sorter of particle size
•Suspended load
•Volcanic ash
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
Key Terminology
Desert
Coriolis Effect
Subtropical
Rainshadow
Coastal
Polar
Continental Interior
Playa lake
Alluvial fan
Bajadas
Playas
Inselberg
Internal drainage
Ephemeral stream
Ventifact
Blowout
Deflation
Loess
Regolith
Desert varnish
Barchan
Barchanoid
Parabolic
Longitudinal
Stellate
Transverse
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.
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.