GEOG101: Physical Geography

Lecture 18: Wednesday, April 15, 2009

Topics
1. The Work of Wind
2. Overview of Desert Landscapes

The Work of Wind
• Eolian Erosion: Wind and surface water remove the fine grain sediments (silt and clay, leaving the coarse grain sediments in the surface --> Desert Pavement
• Eolian Abrasion: Winds that bring sands and other particles can polish the surficial landforms such as sandstone cliffs or slopes, forming features such as Yardang
• Eolian Transportation
      Suspension (mainly fine-grained dusts and clay); Saltation (the main transportation form); surface creep (slowly moving grains)
      Winds have the highest power within 0.8 to 2 m above the surface
      Winds form sand ripples and sand dunes of different shapes
• Eolian Depositional Landforms
      Sand dunes and sand "seas"
      Sand dunes migrate along the wind direction, forming a series of stacked slipfaces (the angle of repose--we have talked about this before);
      Different types of sand dunes (Fig. 15.10): Barchan (constant wind but insufficient sands); Transerve dunes (constant weak wind with enough sands); Parabolic (vegetation controlled, partially stabilized); Barchanoid ridge (effective winds and enough sands, the combination of Barchan type); Longitudal (winds from two different directions); Star (the giant dunes, formed by effective winds from different directions and abundant sands); Dome and Reversing dunes are between star and longitudal forms
• Loess Deposits
      Losses are composed of clays and other fine-grained particles
      Losses can be up to 300 m thick (e.g., north China). They can be good soils for some type of plants
      Losses are formed by wind-blown sediments, but the origin of these sediments are different: The North America losses are thouhgt to have been derived from fine-grained sediments of the Pleistocene glacial sediments, while the thick losses in north China are possibly from desert in central Asia

Overview of Desert Landscapes
• Desert Climates
      High insolation, low rainfall, high evaporation rates, etc.
• Desert Fluvial Processes
      Alluvian fan and braided river channels are common in desert, but less common in humid areas
      The apron of alluvian fan is visible in the topographic maps
      In the outter ring of the alluvian fan, sund dunes and playa lakes may be formed
      Alluvian fans are common in Death Valley (drive there and have a look)
• Desert Landscapes
      Desert Sculpture, buttes, pinnacles, mesas, etc; formed by abrasion and differential weathering
• Basin and Range Province
      Related to extensional tectonics: a series of horsts (ranges) and grabens (basins), forming alluvian fans and playa lakes along the hillside of the ranges and basins
• Desertification
      Global distribution of desert: 1) along the mid latitude high-pressure zone (15-35 degree in both hemisphere; e.g, Africa); 2) along the leeward side of the mountain areas formed by rain shadow (e.g., western US); 3) in areas far away from the maritime effects (e.g., central Asia);
      Desertification: expansion of desert. Overcultivation,overgrowth of cattles, human population growth, etc. This becomes a big problem currently and in the future, especially in central Asia countries