Chapter 12 - DESERTS AND WINDS

Deserts are NOT deserted (a misconception). Desert data:

• Deserts are largely inhabited by humans.
• Only a third of deserts regions are covered with great dune fields most regions are covered with barren rock.
• Not all desert regions are hot!, rather many regions are in cold regions of high lattitude or high altitude plateaus.
• Deserts exist because of climate; climate changes through space and time.
• Regions that are now deserts (northern Sahara) were once forested.
• Regions like the American Midwest were deserts during the end of the Pleistocene.

DESERTIFICATION - the expansion of desert-like conditions into non-desert regions. -- Linked to climatic cycles, influenced by the activites of humans.

The "30 year cycle" of desertification in Africa is, in part, linked to cycles in hurricanes.

DRY CLIMATES

• ARID (deserts) - cover 10% of earth's surface; defined as land averaging less than 25 cm rain per year. Precipitation occurs as occurs as scattered rain or snow, or SEASONAL MONSOONS.

• Semiarid (STEPPES) - transition zone around deserts, typically grasslands or brush covered regions.

TYPES OF DESERTS

• RAIN SHADOW DESERTS (on the LEEWARD (down-wind) side of mountains). Wind rising over mountains cools and releases precipitation. Wind sinks on the down-wind side of the mountain warms up, and as a result, is dryer.

• LOW LATITUDE DESERTS (in the region of "sinking winds"). The direct sun on the equator region warms the air where it rises. Rising, moist air releases its precipitation as it rises in the equator region. The dry air moves toward the poles and cools and sinks in regions 15-30 degrees north/south latitude. These regions in the "sinking winds" are typically deserts.

• HIGH LATITUDE, FRIGID DESERTS - global polar regions rarely receive moisture from the warmer, tropical regions of the planet, and are, therefore, dryer.

SEDIMENT TRANSPORT IN DESERT REGIONS

• EPHEMERAL STREAMS (also called wash, arroyo, or wadi) -
  • related to arid conditions
  • lack of water keeps groundwater table deep in subsurface
  • lack of water prevents chemical weathering - results in little or no soil development!
  • CALICHE - "hard-pan" - calcite precipitates near surface making an inpenetrable crust, especially in dry creek bottoms.
  • "Desert Storms" -(6-12 inches typical fall in a rare desert thunderstorm after many months or even years of drought!)
  • With heavy rain in the desert water cannot penetrate the ground (partly due to caliche) and runs off surfaces as a "sheetwash."
  • Runoff gathers in gullies - piles up - and becomes a FLASH FLOOD.
  • flash floods can travel hundreds of miles, long after storm is gone.
  • In barren desert regions a single storm event cam causes incredible erosion! Flash floods can transport large quantities of sediments (because of both high capacity & competence).

  SEDIMENT TRANSPORT BY WIND

  • SALTATION (bed load) - sand is the most abundant sediment in most desert; when it is blown by the wind it stays low to the ground, it bounces along the ground. Sand piles up in drifts called DUNES.

  • SUSPENDED LOAD - silt is carried away by the wind, sometimes thousands of miles.

  • DEFLATION - wind removes fine-grained sediments, coarse grained materials accumulate on the surface.

  • DESERT PAVEMENT - surface covered with gravel (polished).

  • BLOWOUTS - "patches" of deflation in desert, down to the water table.

  • ABRASION - process of "sand blasting" rocks ventifacts - erosion on rock surfaces that display prevailing wind patterns.

  TYPES OF SEDIMENTARY DEPOSITS IN ARID REGIONS

  1) Wind blown deposits:
  • DUNES (sand)
  • blankets of silt
  • LOESS (from glacial outwash blowouts)

    2) Water deposits:

  • ALLUVIAL FANS (sand and gravel)
  • PLAYAS (ephemeral lakes) - (silt, clay, and salt)
  • CALICHE - (CALCITE precipitates from groundwater, cementing sand and silt)

  PROFILE OF A DUNE

  • Upwind side is side of erosion;
  • Downwind side is side of deposition; sand accumulates on the "slip face" on the downwind side of the dune. This causes the dune to migrate through time.
  • This migration results in the formation of CROSS BEDS - cross beds are common in sedimentary rocks formed from wind deposits.

  Many kinds of dunes (barchan, transverse, logitudinal, parabolic) - their shapes are influenced by wind patterns (daily, seasonal) and sediment supply.

  STAR DUNE - A high dune, usually the center of a "stationary" dune field (wind blows in different directions during different seasons, or piles up against the side of a mountain range.

  EVOLUTION OF A DESERT LANDSCAPE

  The features listed below are associated with desert landscapes:

  INTERIOR DRAINAGE - regions without rivers that flow to the sea.

  ALLUVIAL FAN - In mountainous desert regions sediments pile up at the mouth of a canyon on the valley floor.

  BAHADAS - An "apron" of alluvial fans that coalesce along the front of a mountain range.

  PLAYA - an ephemeral lake that forms during storms or wet seasons.

  PEDIMENT - "worn down mountains" - a rocky surface strewn with boulders and gravel.

  DUNE FIELDS - in "wind trap areas" at the downwind ends of long valleys.

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