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The Qaidam Depression in China is the highest desert in the world. This Landsat image illustrates a salt lake and evaporite basins in the depression. |
The sand and rock of China's Turpan Depression resemble closely those in the view of the Martian surface shown in "Types of Deserts." |
The arrow points to shiny black desert varnish on these rocks of Egypt's southwest desert (photograph by Carol Breed). |
A dark, shiny stain, called desert varnish or rock varnish, is often found on surfaces of some desert rocks that have been exposed at the surface for a long period of time. Manganese, iron oxides, hydroxides, and clay minerals form most varnishes and provide the shine.
Deflation basins, called blowouts, are hollows formed by the removal of particles by wind. Blowouts are generally small, but may be up to several kilometers in diameter.
Wind-driven grains abrade landforms. Grinding by particles carried in the wind creates grooves or small depressions. Ventifacts are rocks which have been cut, and sometimes polished, by the abrasive action of wind.
Sculpted landforms, called yardangs, are up to tens of meters high and kilometers long and are forms that have been
streamlined by desert winds. The famous sphinx at Giza in Egypt may be a modified yardang.
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| Yardangs of the Lut desert of Iran. View from Landsat (A), from high-altitude photograph (B), and from low-altitude photograph (C). These yardangs are among the largest on Earth, with almost 100 meters of relief (low-altitude photograph by J.T. Daniels, aerial photograph by U.S. Air Force). |
Small particles may be held in the atmosphere in suspension. Upward currents of air support the weight of suspended particles and hold them indefinitely in the surrounding air. Typical winds near the Earth's surface suspend particles less than 0.2 millimeters in diameter and scatter them aloft as dust or haze.
Saltation moves small particles in the direction of the wind in a series of short hops or skips. |
Eolian turbidity currents are better known as dust storms. Air over deserts is cooled significantly when rain passes through it. This cooler and denser air sinks toward the desert surface. When it reaches the ground, the air is deflected forward and sweeps up surface debris in its turbulence as a dust storm.
Crops, people, villages, and possibly even climates are affected by dust storms. Some dust storms are intercontinental, a few may circle the globe, and occasionally they may engulf entire planets. When the Mariner 9 spacecraft arrived at Mars in 1971, the entire planet was enshrouded in global dust.
Dust storm along the Mohave River near Daggett, California, October 24, 1919 (photograph by D. G. Thompson). |
Small whirlwinds, called dust devils, are common in arid lands and are thought to be related to very intense local heating of the air that results in instabilities of the air mass. Dust devils may be as much as one kilometer high.
Sand sheets are flat, gently undulating sandy plots of sand surfaced by grains that may be too large for saltation. They form approximately 40 percent of eolian depositional surfaces. The Selima Sand Sheet, which occupies 60,000 square kilometers in southern Egypt and northern Sudan, is one of the Earth's largest sand sheets. The Selima is absolutely flat in some places; in others, active dunes move over its surface.
Wind blowing on a sand surface ripples the surface into crests and troughs whose long axes are perpendicular to the wind direction. The average length of jumps during saltation corresponds to the wavelength, or distance between adjacent crests, of the ripples. In ripples, the coarsest materials collect at the crests. This distinguishes small ripples from dunes, where the coarsest materials are generally in the troughs.
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| Wind-blown sand moves up the gentle upwind side of the dune by saltation or creep. Sand accumulates at the brink, the top of the slipface. When the buildup of sand at the brink exceeds the angle of repose, a small avalanche of grains slides down the slipface. Grain by grain, the dune moves downwind. |
Sand grains move up the dune's gentle upwind slope by saltation and creep. When particles at the brink of the dune exceed the angle of repose, they spill over in a tiny landslide or avalanche that reforms the slipface. As the avalanching continues, the dune moves in the direction of the wind.
Some of the most significant experimental measurements on eolian sand movement were performed by Ralph Bagnold, a British
engineer who worked in Egypt prior to World War II. Bagnold investigated the physics of particles moving through the
atmosphere and deposited by wind. He recognized two basic dune types, the crescentic dune, which he called "barchan," and
the linear dune, which he called longitudinal or "sief" (Arabic for "sword").
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| Sand dunes in Death Valley, California (photograph by Richard Frear). |
