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13: Deserts

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    Learning Objectives

    • Distinguish three broad categories of deserts.
    • Explain the location of deserts
    • Identify and describe desert landforms.
    • Explain how desert landforms are formed by erosion and deposition.
    • Describe the main types of sand dunes and the conditions that form them.

    Approximately 30 percent of Earth’s terrestrial surface is desert. Deserts are defined as locations of low precipitation. While temperature extremes are often associated with deserts, they do not define them. The lack of moisture, including the lack of humidity and cloud cover, allow temperature extremes to occur. The sun’s energy is more absorbed by the Earth’s surface without cloud cover, and nighttime cooling is more drastic without cloud cover and humidity to absorb the emitted heat, so temperature extremes are common in deserts.

    • 13.1: Prelude to Deserts
      Deserts tend to occur at latitudes of around 30° and at the poles, both north and south, driven by circulation and prevailing wind patterns in the atmosphere. At approximately 30° north and south of the equator, sinking air produces trade wind deserts like the Sahara and the Outback of Australia. Rainshadow deserts are produced where prevailing winds with moist air dries as it is forced to rise over mountains.
    • 13.2: The Origin of Deserts
      The engine that drives circulation in the atmosphere and oceans is solar energy which is determined by the average position of the sun over the earth’s surface. Direct light provides uneven heating depending on latitude and angle of incidence, with high solar energy in the tropics, and little or no energy at the poles. Atmospheric circulation and geographic location are the primary causal agents of deserts.
    • 13.3: Desert Weathering and Erosion
      Weathering takes place in desert climates by the same means as other climates, only at a slower rate. This is besides the higher temperatures, which typically spur faster weathering. Water is the main agent of weathering, and lack of water slows weathering. Precipitation occurs in deserts, only less than in other climatic regions. Chemical weathering proceeds more slowly in deserts compared to more humid climates because of the lack of water.
    • 13.4: Desert Landforms
      In deserts like those of the American Southwest, streams draining mountains flow through canyons and emerge into adjacent valleys. As the stream emerges from the narrow canyon and spreads out, and with a lower slope angle and slower speeds and no longer constrained by the canyon walls, it drops its coarser load. As the channel fills with this conglomeratic material, the stream is deflected around it.
    • 13.5: The Great Basin and the Basin and Range
      The Great Basin is the largest area of interior drainage in North America, meaning there is no outlet to the ocean and all precipitation remains in the basin or is evaporated. It covers western Utah, most of Nevada, and extends into eastern California, southern Oregon, and southern Idaho. Streams in the Great Basin gather runoff and groundwater discharge and deliver it to lakes and playas within the basin.

    Thumbnail: Sand dunes in the Rub' al Khali ("Empty quarter") in the United Arab Emirates. (CC BY-SA 3.0 Unported; Nepenthes via Wikipedia)

    This page titled 13: Deserts is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher (OpenGeology) .

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