Wind and Deserts

        Deserts and ocean shorelines may seem like greatly different environments, but both have the important role of wind as an erosive agent in common. The work and effects of wind is best exemplified by the desert regime.

        There is no generally accepted definition of a desert. Deserts can at least be characterized by its lack of moisture, and are general synonymous with arid climates. The distribution of arid regions or deserts is determined by climate and topography, such that arid terrains may be subdivided into tropical deserts and topographic deserts. The controlling factors on deserts involve:
 

1. A hot and dry climate is typically most influential in desert formation been latitudes of 5 and 30 degrees N and S of the equator. Heat enhances evaporation, e.g., parts of the southwestern Unites States have evaporation rates of exposed water surfaces exceeding 100 inches annually. Dryness is reflected by rainfalls less than 10 inches per year, with most precipitation may be occurring in two rains or in a single month. Rain has little influence the rest of the year. Deserts and adjacent semi-arid regions (steppes), which may have rainfall between 10 and 20 inches per year, are characterized by a non-continuous vegetation cover. Such climatic regions are generally characterized by high winds due to atmospheric convection, where hot air rises and is replaced by descending cooler air.
2. Topographic controls on desert formation also reflect a deficiency in rainfall. Topographic deserts are located near the center of continents, where climates are hot in summer and cold and dry in winter. These regions are typically far from ocean moisture sources, and most often are cut off from rain-bearing winds by high mountains. The desert climate of large sections of Utah, Nevada, Arizona and Colorado reflects the influence of the Sierra Nevada mountains of eastern California which cut off rain-bearing winds blowing inland from the Pacific ocean. A similar situation exists in Argentina in the shelter of the Andes.

        Humid lands have virtually continuous vegetation cover, but the lack of moisture in deserts ensures that vegetation is absent or sparse. Thus, wind can directly affect the land surface aided by other geological processes that include:

1. weathering - role of mechanical weathering is greater than chemical weathering, so that there is little soil development and abundant exposed bedrock.
2. mass wasting - short, heavy rainfalls may produce mudflows and rockfalls
3. running water - with little rainfall, evaporation and infiltration give rise to interior drainage and an intermittent water supply that is susceptible to flash floods.
4. ground water - the water table lies far below the surface, so there is no supply of subsurface water to streams.

        Wind velocities increase rapidly with height above the surface. Like water, most air moves in turbulent flow regimes. Wind velocities increase at a greater rate than water velocities and the maximum velocities attained by wind are also greater.
        The general movement of wind is forward, but within this movement, air is moving upward, downward and side-to-side. In the zone about 1 meter above the ground, the average velocity of upward motion is approximately 1/5th that of average forward velocity.
        Along the ground surface, a thin but definite zone, where air moves very little or not at all, is present. The depth of this zone depends on the size of particles that cover the surface. The average depth of this zone of non-movement ("dead-air") is about 1/30th the average diameter of surface grains. For example, the zone would be 1 millimeter thick on a surface of evenly distributed pebbles with an average diameter of 30 mm.
        The average velocity of upward air motion and the depth of the dead-air zone have a bearing on the ability of wind to transport sediment. Material blown by wind usually falls into two groups:

1. sand grains - material 0.06-0.3 mm diameter
2. dust - material <0.06 mm diameter

        Careful observations show that sand grains move forward in a series of jumps, by a process known as saltation. Wind saltation differs from water saltation in that and eddy of water can actually lift individual particles into the main current. Wind by itself can not pick up sand particles from the ground. Sand particles are thrown into the air only under impact of other particles. When wind reaches a critical velocity, grains of sand begin to roll forward along the ground surface. When one rolling grain collides with another, the impact may lift either particle into the air. Once aloft, these particles are subjected to the forces of:

1. gravity, pulling them down, and,
2. horizontal wind velocity.

        Dust particles are small enough to be lifted aloft by currents of turbulent air and carried in suspension. Laboratory experiments, however, show that particles less than 0.3 mm in diameter can not be picked up by wind. Where a surface consists of only silt and clay, there are few irregularities and there is no particle movement because individual grains lie within the zone of dead air. Some agent other than wind must set the particles in motion and lift then into the zone of turbulence. Irregularities in the surface or the presence of sand grains may create sufficient turbulence, so that finer material can be readily transported. Vertical downdrafts of chilled air during thunderstorms may locally strike the ground with velocities of 40 to 80 km/hour. Under such conditions, fine particles may be swept upwards hundreds or thousands of meters into the air. During the dust bowl of 1935, one storm at Wichita, Kansas, produced a cloud that extended 12,000 feet above the ground and contained 166,000 tons/cubic mile of suspended material.

    Wind erosion accomplished by two processes:

• Abrasion - saltating grains of sand driven by wind are highly effective abrasives in eroding rock surfaces. Some evidence of the effects of saltating grains provided by fence posts and telephone poles abraded near ground level, and by bedrock cliffs with a small notch along their base. Abrasion produces the following characteristics in eroded materials:

1. ventifacts, the most common products of wind erosion, are rock fragments found wherever wind blows sand grains against rock surfaces. Fragment surfaces are characterized by a relatively high gloss or sheen, and by facets, pits, gouges and ridges. Fragment faces may display a variable number of facets that meet with sharp ridges
2. frosted sand grains appear just like frosted glass, due to impact, with resulting pitted surface.

• Deflation - this process involves the picking up and removal of loose rock material by the wind, and creates several recognizable landscape features. These include:

1. blowouts are small basins scooped out in soft unconsolidated deposits. These basins range from a few meters to several kilometers in diameter. They are common in the high plains of eastern New Mexico and western Texas, where bedrock is loosely cemented by calcium carbonate.
2. desert pavement represents surfaces where deflation removes all sand and finer grains, leaving behind larger particles of pebble- or cobble-size.

        Whenever wind loses velocity, it loses the ability to transport sand and dust, and must deposit them. Various types of landscape features are formed depending on the size of the particles, the presence or absence of vegetation, the constancy of wind direction, and the amount of material available for movement by wind. The deposits include:
 

• Sand deposits - these are generally are ridges or mounds of sand, called dunes, that may be symmetric or asymmetric. They possess the following characteristics:

• external form - have a sharp crest and one slope at the angle of repose (about 30 degrees)
• size - 100 to 500 feet high is very large. Some dune in Iran (seifs) rise more than 700 feet above their bases and are 3/4 mile wide. Individual seifs may be 60 miles long
• migration - dunes move downwind by shifting of sands at rates of up to 50 feet/year
• internal features - several distinct bedforms include:

1. ripples - flat surface of sand is unstable in wind, because wind is unidirectional, grains are preferentially dropped, and undulation results
2. cross-bedding - as it migrates, sand shifts its position on a dune. It bounces up the (gentle) windward slope and rolls down the (steeper) leeward side where there is a pocket of low energy (wind shadow).
3. sorting - wind has a very strong sorting ability

two kinds of dunes may form:
 
• transverse dunes - wavelike ridge with long dimension normal to wind direction are of two types:
• barchan dunes - arcuate to crescent-shaped type with horns pointing downwind, form where wind in uni-directional, there is limited sand, and no vegetation on the hard flat desert floor. Heights range from 1 meter to more than 30 meters
• U-shaped or parabolic dunes - form under similar conditions except that vegetation is abundant
• longitudinal dunes - ridge-shaped dunes with long dimension parallel to wind direction. They form where wind has a constant direction and the supply of sand is small. Dunes may be as much as 100 m high and 100 km long. The slip faces vary as the wind shifts direction. The wind may be laterally restricted, with deposition occurring by moving up a gully onto a plain where velocity is less due to no lateral restriction.

• Silt and Clay deposits - fine-grained wind-related deposits include

• loess- silt-sized material. Such deposits, which may range from a few cm to over 100 meters in thickness, are responsible for modern fertile soils of Iowa, Missouri, and Illinois, where the loess occurs as cover on glacial sediments of the last ice age. Such deposits typically decrease in thickness in direction of wind movement. The most extensive loess deposits occur in the Yellow River area of China, where a 200 foot thickness of fine material has been blown out of stream valleys in the great desert basin of central Asia.
• bentonite - clay-sized material that represents altered volcanic ash deposited in water.

    Larger scale features developed as a result of or are enhanced by wind action are:

1. playa - dry lake bed formed by evaporation from temporary (few hours to several months), shallow accumulations of excess water (playa lake) following infrequent and intense rainstorms. Playas are characterized by mudcracks and precipitated salt crystal, forming salt pans
2. alluvial fans - sediments deposited downslope of pediments, typically as aprons at the mouth of canyons or as a piedmont plain. Alluvial fans coalesce to form a bajada, a broad alluvial apron with an undulating surface
3. pediments - sloping low-relief surfaces adjacent to mountains resulting from erosion and retreat of the mountain front. Most covered by thin veneer of debris, alluvial fans, or bajadas.
4. inselbergs -isolated, steep-sided erosional remnants of bedrock (characterized by greater resistance to weathering than surrounding mountains) that rise above flat desert plains. Inselberg is a German word meaning "island mountain".
5. mesas - broad, flat-topped erosional remnants bounded on all sides by steep slopes. Mesas consist of relatively easily weathered sedimentary rocks capped by nearly horizontal and more resistant rock layer.
6. buttes - isolated pillar-like structures resulting from continued weathering and erosion of mesas
7. badlands - areas of closely spaced ravines with little or no vegetation.