Section 4 - Unit 4: Aeolian Processes
The Work of the Wind
The term aeolian is used when referring to the work/action of the wind, and discussion of aeolian processes is often combined with arid landscapes. Why do you think this is?
As you know, wind is not confined to a particular climate regime, such as arid regions, Nevertheless, the work of the wind (in terms of erosional and depositional landforms) is often most dramatic in arid climate regimes. Would you like to make a guess as to why this is so?
The ability of wind to move material is small compared to the agents of water and ice, and this is because air is less dense and less viscous. But over time, it can do enormous work. The largest and smallest particles are the most difficult for wind to move; the largest because of weight (too heavy), and the smallest because of the internal cohesion caused by electrostatic forces.
As you know, wind is not confined to a particular climate regime, such as arid regions, Nevertheless, the work of the wind (in terms of erosional and depositional landforms) is often most dramatic in arid climate regimes. Would you like to make a guess as to why this is so?
The ability of wind to move material is small compared to the agents of water and ice, and this is because air is less dense and less viscous. But over time, it can do enormous work. The largest and smallest particles are the most difficult for wind to move; the largest because of weight (too heavy), and the smallest because of the internal cohesion caused by electrostatic forces.
Transport of Particles by Wind
3 Ways Wind Transports Material: Suspension, Saltation, and Creep
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The power of wind to erode depends on particle size, wind strength, and whether the particles are able to be picked up. Wind transports small particles, such as silt, over great distances, even halfway across a continent or an entire ocean basin. Particles may be suspended for days. Wind more easily picks up particles on ground that has been disturbed, such as a construction site, plowed field, or a sand dune. Just like flowing water, wind transports particles as both bed load and suspended load. For wind, bed load is made of sand-sized particles, many of which are picked up and bounced along the ground (medium and coarse grained sand), referred to as saltation; or rolled or pushed (coarse sand and gravel), known as surface creep or just creep. The suspended load is very small particles of silt and and clay, transported via suspension.
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I could watch the following YouTube video for hours... you don't have to, but I had to share!
Degradation by Wind
Reg in the foreground;
Erg in the background.
Ventifact: A rock sculpted by wind erosion in the Altiplano region of Bolivia.
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WIND EROSION
Wind degrades in two ways: Deflation: removal and lifting of loose or non-cohesive particles by the wind; and Abrasion: grinding of rock surfaces by the “sandblasting” action of particles entrained in the wind. Think about how sand blasting is used to clean surfaces of buildings, bridges, etc. with air and sand. DEFLATION In humid areas, water and vegetation bind the soil so it is harder for wind to pick up. In arid regions, small particles are selectively picked up and transported. As the smaller particles are removed, the ground surface gets lower and rockier, causing deflation. What is left is desert pavement, a surface covered by gravel sized particles that are not easily moved by wind. Because the Arabian Peninsula is largely arid, with many aeolian landscapes, scientists use the arabic word for stone when referring to desert pavement: Reg. Reg covers the majority of desert areas! It is a tight covering of coarse gravel, pebbles, cobbles and stones from which all sand and dust have been removed. ABRASION Particles that are picked up and moved by wind do the work of abrasion. As a grain strikes another grain or surface it erodes that surface. Abrasion by wind may pit, groove and/or polish natural or human-made surfaces, such as rocks and buildings. Stones that have become polished and faceted due to abrasion by sand particles are called ventifacts (artifacts of the wind). On a large scale, if rocks form elongated formations parallel with the most effective wind direction, these are called yardangs: elongated rock structures formed by abrasion.
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Aggradation by Wind
Dune field with slip face (left side)
Sandstone showing aeolian cross-bedding.
Transverse dune
Longitudinal dune
Star dune
Barchan dune
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WIND DEPOSITION
When people think of deserts they often think of the rolling sand dunes from Lawrence of Arabia or other popular films that take place in deserts, However, only 10% of deserts are covered in sand, the rest is predominantly desert pavement. A dune is wind-sculpted accumulation of sand and other loose material. Sand Seas, or Dune Fields are called Ergs, the arabic name for dune field. Dune sands are usually uniform in size and shape. Particles are sand-sized, because sand particles are too heavy for the wind to transport by suspension. They are rounded, since rounded grains roll more easily than angular grains. For sand dunes to form there must be an abundant supply of sand and steady winds. A strong wind slows down, often over some type of obstacle, such as a rock or some vegetation and drops its sand. As the wind moves up and over the obstacle, it increases in speed. It carries the sand grains up the gently sloping, upwind side of the dune by saltation. As the wind passes over the dune, its speed decreases. Sand cascades down the crest, forming the slip face of the dune. The slip face is steep because it is at the angle of repose for dry sand, about 34 degrees. Wind deposits dune sands layer by layer. If the wind changes directions, cross beds form. Cross beds are named for the way each layer is formed at an angle to the ground. Sand dunes can be “live”,which means they’re moving; or “fixed”, which means they are not moving because: a. the sand has blown against a structure inhibiting further movement, or b. vegetation has grown over the dune and is thus anchoring it in place. DUNES The types of sand dune that forms depends on the amount of sand available, the character and direction of the wind, and the type of ground the sand is moving over. When winds originate from a predominant direction, dunes will form a characteristic shape with a windward and a leeward side, and successive slip faces are created as the dune migrates. I would like you to know the following four types of dunes:
LOESS Windblown silt and clay deposited layer on layer over a large area are loess, which comes from the German word loose. Loess deposits form downwind of glacial outwash or deserts, where fine particles are available. Loess deposits make very fertile soils in many regions of the world. It forms a thick blanket over the land that is usually 10s of meters thick and can be over 100 m thick, such as in China. Loess has an internal cohesion that allows it great vertical durability, though it can be easily eroded. You’ll often see tall vertical cliffs as erosional features, and humans have made their homes in these cliffs by digging cavities into them. Loess covers about 10% of the Earth’s surface, and about 30% of the coterminous U.S. (Great Plains). 
Loess landscape in China
FLUVIAL FEATURES IN ARID AREAS
When we covered fluvial processes before, we talked about intermittent and ephemeral streams that occur in arid and semi arid areas. These stream beds are called a wash, or arroyo - a dry stream bed in the desert. Occasionally, deserts get precipitation events. When these events happen a lot of run-off occurs due to the imperviousness of desert soil and sparse vegetation. These washes can fill with a torrent creating a flash flood - sudden torrents of water that occur in desert and semi-arid washes. Don’t camp in a wash!!!! 
Flash flood in the Gobi Desert
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Terms
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Prepare for your Unit QuizTo check for understanding, you will take a short quiz on Unit 4. Read this page on aeolian processes. You will have two chances to take the quiz on Moodle, and I will record your highest score. Review the following to prepare for the quiz:
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Photo and Image Copyright:
"Arbol de Piedra" by El Guanche - originally posted to Flickr as Arbol de Piedra. Licensed under CC BY 2.0 via Commons.
By Po ke jung (Own work) [CC BY 3.0], via Wikimedia Commons
"Star-dune". Licensed under CC BY-SA 2.5 via Wikimedia Commons.
"Barchan dune" by Po ke jung - Own work. Licensed under CC BY 3.0 via Wikimedia Commons.
"GobiFlood" by Qfl247 (talk) - I (Qfl247 (talk)) created this work entirely by myself.. Licensed under GFDL via Commons.
