University of Colorado GEOLOGY 1010 Class Note 14

Wind and Deserts

The geographic location of deserts is controlled by descending air masses. This may happen on the leeward side (rain shadow) of a mountain range or at latitudes 30 degrees N and 30 degrees S.

Rising air masses expand, cool, and precipitate moisture. Descending air masses compress, warm, dry, and evaporate moisture.

Deserts tend to have sparse vegetation due to low moisture, and high relief due to short dura tion catastrophic rain with no vegetation to hold soils.

Desert regions with sedimentary bedrock develop characteristic landforms including plateaus, mesas, and buttes.

• A plateau is a broad, level, elevated region commonly bounded partly by cliffs.
• A mesa is smaller, flat-topped hill bounded by cliffs.
• A butte is a narrow hill of resistant rock bounded by cliffs.

Here in the Mojave Desert. in SE California, there is very sharp line separating the zone of erosion above with vegetattion-free granite outcrop from the low-relief, vegetated, zone of deposition below.

Desert regions with igneous or metamorphic bedrock also develop characteristic landforms with

• Zones of erosion which have little or no soil or vegetation, high relief and near total outcrop of bedrock.
• Zones of deposition which have low relief, some soil development, and sparse vegetation.

The upper portion of the area of deposition is usually an alluvial fan. If the drainage basin is closed (no outlet to the ocean) the lower portion will be a playa lake, a lake that is dry for most of the year.

Moving water is the dominant erosional agent in deserts.

Wind may also be a significant agent of erosion, and may cause characteristic land forms and surfaces.

Removal of sand and silt sized particles by wind action is called deflation. The result of deflation is a pebble and cobble surface called a desert pavement.

Angular clay, silt, and sand sized particles deposited by wind is called loess. The lack of prominent size-sorting indicates that the material has not moved far from its source. The typical source for loess deposits in North America is glacial outwash from the retreat of the last ice sheet.

In arid regions the silt and clay size particles are typically removed with time leaving well-sorted sand deposits, commonly in the form of dunes.

Dunes are typical of desert regions where sorting is by wind, and also of coastal regions where sorting is by wave action in water.

The dunes form different characteristic shapes depending on the amount of sand and the amount of moisture and the strength of the wind.

• A barchan is a small crescent shaped dune with the limbs downwind that is typical of arid re gions with limited supply of sand.

• If more sand is available, the barchans may coalesce into transverse dunes which are narrow linear dunes that form at right angles to the prevailing wind direction.

• In more humid coastal regions, typically with some vegetation, a parabolic dune may form which is a crescent-shaped dune with the limbs pointed upwind.

• In arid regions with exposed bedrock, little surface relief, and variable wind directions, longitudinal dunes may form. These are parallel to the prevailing wind and may be as much as 200m high and 100 km long.

Class Note 15

Class Note 13

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