12

• Desert


  Deserted or unoccupied
• Where water is available in deserts, plants and animals thrive
• Desert landscapes


  • Barren rocky outcrops with steep, angular slopes
  • Rocks tinted orange, red, gray, brown, and streaked with black
• Deserts are very different from the more humid places where most people live
• Arid regions are not dominated by a single geologic process, but rather the effects of tectonic forces, running water, and wind
• Dryness


  Water deficiency where yearly precipitation is less than the potential loss of water by evaporation
• Climatic types in water-deficient regions

  • Desert (arid)
  • Steppe (semiarid)
• Dry lands are concentrated in the subtropics and in the mid-latitudes
• Low-latitude deserts


  • Virtually unbroken desert environment stretching for more than 9300 km from the Atlantic coast of North Africa to the dry lands of northwestern India
  • Arid and semiarid areas in southern Africa, coastal Chile and Peru, and Australia
• Heated air in the equatorial low rises

  Air sinks in the subtropical highs, leading to clear skies, sunshine, and ongoing drought
• Middle-latitude deserts and steppes

  • Sheltered in the deep interiors of large landmasses, far removed from the ocean
  • Presence of high mountains across the paths of prevailing winds separates these areas from water-bearing maritime air masses
• Rainshadow desert


  Dry region on the leeward side of mountains where air has lost much of its moisture
• Middle-latitude deserts are more common in the Northern Hemisphere due to the greater extent of land area
• Weathering in deserts is dominated by mechanical processes due to the lack of moisture and organic acids
• Desert stream beds are usually dry, with water flowing only in response to specific rainfall events
• Desert floods


  • Arrive suddenly and subside quickly due to sparse vegetation cover and rapid runoff
  • Cause impressive erosional work during short-lived rain events
• Ephemeral streams


  Streams that carry water only in response to specific rainfall episodes
• Permanent streams in deserts often originate outside the desert, in well-watered mountains, to compensate for losses from evaporation and infiltration
• often in well-watered mountains. Here the water supply must be great to compensate for the losses occurring as the stream crosses the desert. For example, after the Nile leaves its headwaters in the lakes and mountains of central Africa, it traverses almost 3000 kilometers (1900 miles) of the Sahara without a single tributary. By contrast, in humid regions the discharge of a river increases as it flows downstream because tributaries and groundwater contribute additional water along the way.
• It should be emphasized that running water, although infrequent. nevertheless does most of the erosional work in deserts (FIGURE 12.8). This is contrary to a common belief that wind is the most important erosional agent sculpting desert landscapes. Although wind erosion is indeed more significant in dry areas than elsewhere, most desert landforms are carved by running water. As you will see shortly, the main role of wind is in the transportation and deposition of sediment, which creates and shapes the ridges and mounds we call dunes.
• Not all deserts are hot. Cold temperatures are experienced in middle-latitude deserts. For example, at Ulan Bator in Mongolia's Gobi Desert, the average high temperature on January days is only -19 °C (-2°F)!