The Work of Wind and Deserts

Air Pressure Belts and Global Wind Patterns

• High and Low pressure areas: (how does your refrigerator work?; why are your lips chapped in winter?)
  • Rising air cools and loses moisture - rain in tropics (and 60° N and S)
  • Cool dry air descends, compresses, warms up, causes evaporation
    • Occurs near 30° North and South latitude (and poles)
• Coriolis Effect: to explain dominant wind directions

Distribution of Deserts

• Arid vs semiarid areas
• Belts at 30° north and south latitude
• Rain shadow desert - down wind from mountain range
• Continental interiors - great distance from source of moisture (oceans)
• Coastal deserts - cold ocean currents cool air which then moves ashore
• Polar deserts - descending air masses warm and cause evaporation

Characteristics of Deserts

• Temperature, Precipitation, Vegetation - great fluctuations; specialized
• Weathering and Soils - dominantly mechanical weathering; rock varnish
  • Angular particles, little chemical weathering
• Mass Wasting, Streams, Groundwater - cloud bursts; internal drainage
• Lack of through-flowing streams - Colorado and Nile Rivers are exceptions
• Wind

Desert Landforms

• Colorado Plateau: centered on the Four Corners area, high flat-lying beds of sedimentary rock
  • Plateaus to Mesas to Buttes
  • Monoclines to Hogbacks
• Basin and Range: Nevada and Utah - parallel isolated ranges of mountains bounded by faults (can consider them to be horst and graben structures - see Structure chapter)
  • Playas (salt pans, borax) and playa lakes (intermittent)
  • Alluvial fans (like deltas) and Bajadas (coalesced alluvial fans)

Sediment Transport by Wind

• Bed Load - remember the terms from flowing water chapter
  • Saltation, Sliding, Rolling
  • Surface Creep
• Suspended Load
  • Dust - and how we get it moving

Wind Erosion

• Abrasion
  • Ventifacts - sand blasted and sculpted rocks
• Deflation
  • Blowouts
  • Desert Pavements - self protecting once formed
    • Removal of fine grained particles by wind?
    • Wet/dry cycles causing swelling and contraction (analogous to freeze/thaw cycles in northern climes causing boulders to rise to surface over time - frost heaving)

Wind Deposits

• Dunes
  • Barchan - crescent, point downwind, limited sand, constant direction
  • Longitudinal - converging winds, limited sand, 3-100m high
  • Transverse - long ridges, perpendicular to wind, abundant sand - sand seas
  • Parabolic - strong onshore winds, blowouts
• Loess: very important as it makes excellent soil (both desert and glacial origins - what do these seemingly opposite environments have in common?)
  • Steep cliffs - angular nature of particles?
  • 1920 earthquake in China killed 100,000 living in cave homes dug in loess cliffs
• Rock varnish (and Petroglyphs): Iron and Manganese oxides deposited on surfaces - added from outside - may start with clays sticking to dew-wet rocks - probably always involves biological processes of bacteria