Climate Types -- figure 13.2

• Desert or arid
• Steppe or semiarid -- marginally more humid than deserts

Deserts form because of global weather patterns. Most deserts form at about 20 to 30 latitude. Where air low moisture content descends from high altitudes. Rain shadow deserts form where the prevailing wind crosses mountains. As air rises it rains, and then when it comes down off of the mountains, it is warmer and drier.

Regions

• Subtropics -- Coastal deserts form where cold oceanic currents lie on the downwind side of continents. Cold oceans keep the air pressure high and keep weather fronts from moving in. Examples are the Atacama and the Namib. -- figures 13.2 and 13.3
  • Low-latitude deserts
  • Near the Tropics of Cancer and Capricorn -- tropical highs - where the air is subsiding.
  • Areas of high pressure and sinking air that is compressed and warmed -- equitorial low - where the hot air rises.
• Middle-latitudes -- continental deserts are isolated from oceans by mountains or distance. Examples are the Sahara, Gobi and SW North America. Mountains can produce rain shadows. Clouds rise over mountains and drop their moisture. Deserts form on the downwind side. -- figures 13.2 and 13.4
  • Located in the deep interior of continents
  • High mountains in the path of the prevailing winds produce a rainshadow desert

Geologic Processes

• Weathering
  • Not as effective as in humid regions
  • Mechanical weathering produces unaltered rock and mineral fragments
  • Some chemical weathering in deserts does produce
    • Clay
    • Thin soils
    • Oxidized minerals
• Role of Water -- The material the water washes off the mountains can't be carried far and accumulates in alluvial fans at the edge of the mountains. The water flows through dry washes, or wadis, which have steep banks and gravelly bottoms. -- figure 13.5
  • Most streambeds are dry most of the time
  • Ephemeral Streams -- periodically carry large sediment load due to lack of vegetation
    • Carry water only during rainfall
    • Various names consist of:
      • Wash and Arroyo (in the western U. S.)
      • Wadi (in the Arabia and North Africa)
      • Donga (South America)
      • Nullah (India)
  • Rainfall in the Desert -- Rains in deserts are different. They ar rare, but rain usually falls in intense thunderstorms, resulting in flash floods. The intense rains lead to rapid erosion of steep areas. Desert mountains have thin or no soils. Because the rain falls all at once, it runs off, rather than sinking in to the groundwater.
    • Occurs as heavy showers
    • Runoff is large and flash floods typically occur due to lack of vegetation
    • Streams do not have an extensive system of tributaries and the drainage system is poorly integrated
    • most of the erosion is accomplished by running water

Basin and Range -- characterized by interior drainage -- figure 13.7

Landscape Evolution

• Block Faulting - Uplift of mountains
• Interior Drainage
  • Alluvial Fans -- cone of debris at the mouth of a canyon
  • Bajadas -- an apron of sediment produced by the coalescing of fans from adjacent canyons
  • Playas Lakes -- temporary shallow lakes
  • Playa -- the dried up lake bed
• Ongoing Erosion
  • Produces sediment that fills the basin
  • Diminishes local relief
  • Inselbergs -- isolated erosional remnants, late-stage desert landscape features -- p. 373
  • Mesas -- table-like hills or mountains, flanked by cliffs, The predominance of physical erosion means that softer rocks erode much more quickly than harder rocks,. Mesas form when a soft rock erodes from beneath a harder rock.

Wind in the Desert

• Transportation of sediments
  • Differs from running water by
    • Wind is less capable of picking up and moving coarse materials
    • Wind can spread sediment over large areas, not confined to channels
  • Mechanisms for transport
    • Bedload
      • Saltation -- skipping and bouncing along the surface. Wind has low competence but high capacity.
      • 20 to 25% of the sand is moved by sandstorms
    • Suspended load

Wind Erosion -- relatively insignificant erosional agent, water is more efficient

Mehcanisms

• Deflation
  • Lifting loose materials
  • blowouts -- shallow depressions -- figure 13.11
  • desert pavement -- surface of coarse pebbles and cobbles -- figure 13.12
• Abrasion -- sculpts rocks
  • venitfacts -- stones with flat faces -- figure 13.13
  • yardangs -- wind sculped ridges
• Limited vertical extent

Wind Deposits -- significant depositional landforms are created in some regions

Types of Deposits

• Dunes -- cross-bedded. Windblown sand creates giant piles that may be 100's of miles across called sand seas or ergs.
  • Mounds or Ridges of sand
  • Often assymetrically shaped -- figure 13.15
  • Windward slope is gentle inclined and the leeward slope is called the slip face. Sloping layers are called cross beds -- figures 13.14 and 13.15
  • Slow migration of dunes in the direction fo the wind movement, burying anything in its path.
  • Several types: -- figure 13.17
    • Barchan -- with horns that point down wind
    • Transverse -- perpendicular to the wind
    • Barchanoid -- scalloped rows of sand oriented at right angle to the wind
    • Longitudinal -- parallel to the wind
    • Parabolic -- horns that point upwind
    • Star Dunes -- have points and are formed where wind directions are variable
• Loess -- Often, the river ends in a dry lake, or playa, that accumulates salts. The wind picks up sand and dust from the desert floor. The sand bounces across the desert and the dust is blown high into the air. Windblown dust accumulates in sheets called loess. -- figure 13.20.
  • Blankets of windblown silt
  • sources are deserts and glacial outwash deposits
  • extensive deposits in China and central U. S.

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