Chapter 16c/15 Eolian (Wind) Processes
Geosystems, 4CE, pp. 516-527 (Geosystems, 3CE, pp. 453-481)
“Anyone who listens to my teaching and obeys me is wise, like a person who builds a house on solid rock. Though the rain comes in torrents and the floodwaters rise and the winds beat against that house, it won’t collapse, because it is built on rock. But anyone who hears my teaching and ignores it is foolish, like a person who builds a house on sand. When the rains and floods come and the winds beat against that house, it will fall with a mighty crash.”
Matthew 7:24-27 NLT
**There is a video version of this lecture here: https://youtu.be/YJXi3MIPuE8.
**The exam is based on the content in these notes, so please print them off to study from.
I. Eolian Processes
As discussed previously, the main erosive agents are water, wind, and ice. Wind-related processes are called eolian/aeolian processes (after Aeolus, the Greek ruler of the winds).
Because wind is so much less dense than either water or ice, it does not move as much material as the other agents of erosion. However over long periods of time, wind can accomplish significant erosion.
Wind speed is the critical factor for how much erosion wind accomplishes (See Figure 16.27, 4CE, p.518, “Sand movement and wind velocity,” (Figure 15.2, 3CE, p. 455)).
For a good introduction to Aeolian Processes, check out the U.S. Geological Survey.
A. Eolian Erosion
There are two major ways that wind causes erosion:
- Deflation
Blowout – sand/soil erosion, Jasper, AB
This refers to the removal/lifting of loose particles by wind. The finest particles are simply blown away by the force of the wind. This leaves behind a landscape of pebbles and rocks that resembles a cobblestone street, thus it is called a desert pavement (See Figure 16.30, “Desert pavement,” 4CE, p. 520 (Figure 15.3; 3CE, p. 456)).
Blowout depressions are indentations or depressions caused by intense wind activity that removes loose particles by deflation. Note that only about 10% of deserts are actually covered by sand … the rest are usually bare pavement with the sand concentrated in localised areas of dunes. The Hollywood “desert” … endless miles of sand punctuated by occasional palm-lined oases populated by exotic sheiks is more fiction than fact! Most deserts are rock.
Rocky or pavement deserts are often called reg deserts.
Abrasion
This refers to the “sand-blasting” action of particles already suspended in the air. Abrasion can result in rocks that appear:
- smoothed and polished,
- grooved, or
- pitted
depending upon the size of the airborne particles and the resistance of the rock being eroded..
Most rocks that have been eroded by eolian abrasion are aerodynamically shaped, parallel to the dominant wind direction.
Long ridges of rock, streamlined by abrasion and deflation, are called yardangs
Hoodoos, Writing-on-Stone Provincial Park, AB
Individual spires of rock are commonly called hoodoos (wikipedia site)
B. Eolian Transportation
See Figure 16.25, 4CE, p.517 “How the wind moves sand,” (Figure 15.6; 3CE, p. 458).
Wind is capable of transporting large amounts of fine particles.
Extremely fine sediment is carried in the air by suspension (it never touches the ground).
Most sediment is bounced along the ground by saltation.
Sediment too large to be lifted or bounced may be rolled or slid along as surface creep (or traction). Other sediment, either bounced or in suspension, is particularly effective in causing surface creep.
C. Eolian Landforms
- Ripples
Ripples are small undulations in sandy surfaces which form in crests and troughs, positioned transversely (at a right angle) to the direction of the wind (See Figure 15.7, “Sand ripples,” 3CE, p. 458).
Larger deposits of sand grains form dunes, defined as wind-sculpted, transient ridges or hills of sand.
As noted earlier, most desert sand is concentrated in dunes, which occupy only about 10% of desert land areas. Extensive areas of sand dunes in a desert is often called an erg desert (after the Arabic word for “dune field”), or a sand sea. See Figure 16.31, “Examples of sand seas, or ergs,” 4CE, p. 521 (fig. 15.8; 3CE, p. 460).
Dunes are dynamic features – they are constantly changing. They tend to advance, pushed by prevailing winds.
Study Pages 522-3; (Figure 16.1, 4CE) “Dune cross section,” (Figure 15.9; 3CE, p. 461). Note:
- The effective (prevailing) wind direction
- The direction of sand movement (pushed along by the wind)
- The windward (stoss) slope (up which sand particles are pushed)
- The slipface (leeward slope) – down which particles fall
- Previous slipfaces.
Winds characteristically create a gently sloping windward side (stoss side), with a more steeply sloped slipface on the leeward side. The angle of a slipface is the steepest angle at which loose material is stable—its angle of repose. Thus, the constant flow of new material makes a slipface a type of avalanche slope: Sand builds up as it moves over the crest of the dune to the brink; then it avalanches, falling and cascading as the slipface continually adjusts.
Dunes come in all shapes and sizes depending on local wind and geologic conditions. Look over Pages 522-3, (4CE), or Figure 15.10, “Major dune forms,” 3CE, pp. 462-3. You are not responsible for knowing all the different dune forms!
- Loess
Loess (also Lőss; pronounced “luss” for you German speakers!) refers to wind-blown deposits of fine grained clays and silts which came from glacial outwash deposits. After glaciers retreated, much of the finest sediment they left behind was redistributed by wind as vast blankets of very fine sediment that covered existing landscapes. Loess, fine-textured (no rocks) and rich in minerals is excellent for agriculture, making these very fertile regions.
The vast deposits of loess in China, covering more than 300 000 km2, are derived from windblown desert sediment. Accumulations in the Loess Plateau of China are more than 300 m thick, forming complex weathered badlands and good agricultural land. These windblown deposits are interwoven through much of Chinese history and social customs; in some areas, dwellings are carved into the strong vertical structure of loess cliffs.
See Figure 16.35, 4CE, p. 525 (Figure 15.12, 3CE, p. 464) “Loess bluff, western Iowa,” and Figure 16.34, 4CE, p. 525 (figure 15.13, 3CE, p. 465) “Global loess deposits,” . Note that these are some of the world’s most productive agricultural regions.
Worth Reflecting On …
Gerard Manley Hopkins writes:
| Pied Beauty |
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What are your thoughts on the themes Hopkins brings up? Feel free to discuss this quote on the course discussion site,
See also: “The Trees Clap their Hands: What Does It Mean to Say Creation Praises Its Creator ” by Ruth M. Bancewicz
To review …
Check out the resources at www.masteringgeography.com
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