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 10 o and 30 o (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 (+130 o F) 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: First Stage: 1. First Stage: Playa lakes start to dry up; evaporation > precipitation Alluvial fans form -- fan-shaped sediment features Middle Stage: 2. Middle Stage: Lakes dry up -- playas form Alluvial fans dry up -- form bajadas of pediment Late Stage: 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 DesertCoriolis Effect SubtropicalRainshadow CoastalPolar Continental InteriorPlaya lake Alluvial fanBajadas PlayasInselberg Internal drainageEphemeral stream VentifactBlowout DeflationLoess RegolithDesert varnish BarchanBarchanoid ParabolicLongitudinal StellateTransverse

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 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.