deserts - Deserts EENS 1110 Physical Geology Tulane...

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This page last updated on 13-Apr-2011 EENS 1110 Physical Geology Tulane University Prof. Stephen A. Nelson Deserts Deserts Deserts are areas where rainfall is less than 250 mm (10 in.)/year, or where evaporation exceeds precipitation. Thus, deserts are areas that we think of as arid. They may be hot or cold. They are characterized by specialized ecosystems and low human populations. Because of their dryness, unique geologic processes operate in deserts. Origin of Deserts Deserts originate by several different mechanisms that result in five types of deserts. 1. Subtropical deserts 2. Rain shadow deserts 3. Coastal deserts 4. Continental interior deserts 5. Polar deserts. Subtropical Deserts - the general atmospheric circulation brings dry, subtropical air into mid- latitudes. Examples: Sahara of Northern Africa, Kalahari of Southern Africa, and the Great Australian Desert. Deserts 4/13/2011 Page 1 of 9
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Rainshadow Deserts - Areas where mountainous regions cause air to rise and condense, dropping its moisture as it passes over the mountains. Examples: Deserts east of the Sierra Nevada Mountains, California & Nevada, East of the Cascades of Oregon and Washington, and East of the Andes Mountains in South America. Coastal Deserts - Areas where cold upwelling seawater cools the air and decreases its ability to hold moisture. Examples : Atacama Desert of coastal Peru, Namib Desert of coastal South Africa. Continental Interior Deserts - Areas in the continental interiors, far from source of moisture where hot summers and cold winters prevail. Example: Gobi, Mongolia Polar Deserts - Cold polar regions where cold dry air prevails and moisture available remains frozen throughout the entire year. Examples: Northern Greenland, and ice-free areas of Antarctica. We will concentrate on the first four types of deserts, the one's which occur in hot arid climates. Surface Processes in Deserts The same geologic processes operate in deserts as in other more humid climates. The difference is the intensity to which the processes act. Weathering and Mass Movements z Deserts have little soil because moisture is so low and the rate of chemical weathering is slow. Recall that chemical weathering is responsible for the formation of soils Bedrock commonly occurs at the surface. Exposed rock surfaces develop desert varnish a dark reddish brown surface coating of of iron and manganese oxides. This forms very slowly by bacterial activity, dust, and water. z Little plant life develops because of lack of soils and water. Plants tend to hold soil and fine-grained rock fragments in place so without plants, erosional processes can remove the thin desert soils. z Desert soils are usually colored like the bedrock nearby. Trace elements in the soils bring out wide color variations.
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deserts - Deserts EENS 1110 Physical Geology Tulane...

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