13.S: Summary

Summary

Approximately 30% of Earth’s surface is arid lands, the location of which is determined by latitude, atmospheric circulation, and terrain. The arid belts between 15^o and 30^o north and south latitudes are produced by descending air masses associated with major cells in the atmosphere and include the major deserts like the Sahara in Africa and the Middle East. Rain shadow deserts lie behind mountain ranges or long land expanses in zones of prevailing winds like the deserts of western North America, the Atacama of South America, and the Gobi of Asia. Dry descending air also creates the polar deserts at the poles.

Major atmospheric circulation involves the Hadley cells, midlatitude or Ferrel cells, and the polar cells in each hemisphere. Warmed and rising air in the Hadley cells rains back on the tropics and moves toward the poles as drier air. That air meets the drier equatorward moving air of the Ferrel cells. This dry air descends in the arid zones, called the horse latitudes, to produce the arid belts in each hemisphere. Rotation of the Earth creates the Coriolis effect that deflects these moving air masses to produce zones of prevailing winds, the trade winds in the subtropics and the westerlies in the midlatitudes. A combination of latitude, rain shadow, and cold adjacent ocean currents causes the Atacama Desert of northern Chile, the driest desert on Earth.

Weathering in deserts takes place just as in other climes only slower because of less water. Desert varnish is a weathering product unique to desert environments. As in more humid climes, water is the main agent of erosion although wind is a notable agent. Large dust storms called haboobs transport large amounts of sediment that may accumulate in sand seas called ergs or finer-grained loess deposits. Sand transport occurs mainly by saltation in which grain to grain impact causes frosting of grain surfaces. Sandblasting by persistent winds produces stones with polished surfaces called ventifacts and sculpted bedrock features called yardangs.

Landforms produced in desert environments include alluvial fans, bajadas, inselbergs, and playas. Windblown sand can accumulate as dunes. The forms of dunes, like barchans, parabolic, longitudinal, and star dunes, relate to the abundance of sand supply and wind direction as well as presence of vegetation. The internal structure of dunes shows cross bedding. Fossil dunes in an ancient desert leave cross bedding in places like Zion and Arches National Parks in Utah showing shifting wind directions in these ancient environments. Ephemeral streams in desert regions may carry water only after storms and pose risk of flash floods.

The Great Basin of North America is an enclosed basin with no drainage to the ocean. The only exit for precipitation there is by evaporation. Travels in the Great Basin show stages of development of desert landscapes from playas and alluvial fans, to bajadas, to inselbergs which are eroded mountains buried in their own erosional debris.

Poor land management can result in dying vegetation and loss of soil moisture producing an accelerating process of desertification in which once productive land is degraded into unproductive desert. This is a serious worldwide problem.