Bed load and suspended load. Winds in the desert are often extreme and unrestricted by trees and vegetation. Wind can be an effective erosion and transportation agent if it is strong and blows across fine‐grained sediment such as sand, silt, and clay. A wind's
bed load consists of the heavier grains (usually sand) that hop and skip along the ground by saltation. These rarely rise more than 1 meter (3 feet) into the air as they are transported. The
suspended load is the finer‐grained clay and silt fraction that is actually carried long distances in the wind.
Wind velocity. The velocity of wind is a result of air pressure differences due to heating and cooling. Desert winds are the result of temperatures that commonly fluctuate from 7 degrees centigrade at night to 43 degrees centigrade or more during the day (45−110 degrees Fahrenheit) and can travel at speeds up to 70 miles per hour.
Dust storms. Depending on the amount of fine‐grained material that is available and the speed of the wind, dust storms that blot out the sun can result. Particles can be carried thousands of feet upward into the atmosphere and for hundreds of miles laterally. Dust storms stripped the fertile soil from the overfarmed and drought‐stricken fields of the “Dust Bowl” in the United States in the 1930s. A small but regular component of land‐derived sediment is deposited in the ocean. Volcanic ash from famous eruptions such as Krakatoa was carried around the world by winds for two years. Abrasive windblown sand carves rocks and boulders into unusual shapes called ventifacts, which have flat, wind‐abraded surfaces.
Deflation. Deflation is the removal of sediment from a land surface by wind. It can lower the surface of land significantly by forming a bowl‐like depression called a blowout. Blowouts can be over 60 kilometers in diameter and over 40 meters deep. Another result of deflation is thought to be desert pavement, a large surface of the desert floor that is covered by pebbles and stones that resemble rounded paving stones. Some geologists believe that the wind removes the fine‐grained material from the surface until only coarser material remains; others suggest that the pebbles move up through the finegrained sediment by thermal expansion and contraction (much like frost heaving). Desert pavement is likely the result of a combination of both processes.
Silt and clay that is deposited by wind is called loess. Typically very porous, it forms downwind from the source and blankets hills or accumulates in depressions. It is typically cemented by calcite. Loess can reach thicknesses of 100 meters (300 feet). The fertile soils of the midwestern and Pacific northwestern United States include loess.
Sand dunes. Sinuous heaps of unconsolidated sand called sand dunes are the classic feature of the great deserts of the world. Dunes are deposited by winds in desert regions or along sandy coastlines and beaches. Dune material varies in composition and includes sand‐size grains of quartz, feldspar, calcite, gypsum, and rock fragments that are well sorted and well rounded.
A dune's shape is constantly changing according to the wind direction. The steeper, downwind slope is called the slip face. The loose sand maintains angles up to about 35 degrees on the slip face, creating cross‐bedded layers. Dunes migrate in the direction of the prevailing winds about 12 meters (40 feet) per year, a result of the wind continually eroding the gentle slope and redepositing the sand on the slip face. The surface is typically marked by a series of sand ripples.
Figure illustrates various sand dunes in planar (flat) view from above. One of the largest and most dramatic dunes is the longitudinal dune, or seif. A large ridge of sand that parallels the wind direction, it can be over 100 meters high and over 100 kilometers long. Barchan dunes are widely separated, crescent‐shaped dunes that form in areas of sparse sand. Often found on bedrock, the ends of the crescents point downwind. Transverse dunes are a series of long ridges that form perpendicular to the wind. They typically occur in coastal areas. A barchanoid dune, an intermediate variety between barchan and transverse dunes, is formed of scalloped rows of sand perpendicular to the wind. It resembles a series of side‐by‐side barchan dunes. A parabolic dune usually forms around a blowout in vegetated areas—the dune is deeply curved and the tips point into the wind. Star dunes are isolated hills of sand formed by variable winds in the Sahara and Arabian Deserts. The bases of these dunes resemble multipointed stars.
Figure 1
Types of Dunes