Landforms from Wind Erosionand Deposition

Dana Desonie, Ph.D.

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Printed: October 7, 2013

AUTHORDana Desonie, Ph.D.

www.ck12.org Concept 1. Landforms from Wind Erosion and Deposition

CONCEPT 1 Landforms from WindErosion and Deposition

• Describe how wind erodes and deposits sediments.

What are the effects of sandblasting?

If you’ve ever been in a sand storm, you’ve felt the power of the wind blasting at your skin. Over time, this naturalsand blasting can be a tremendous erosional force on rocks or buildings. Hopefully, you won’t stay out long enoughto experience permanent damage.

Transport of Particles by Wind

Wind transports small particles, such as silt and clay, over great distances, even halfway across a continent or anentire ocean basin. Particles may be suspended for days. Wind more easily picks up particles on ground that hasbeen disturbed, such as a construction site or a sand dune. Just like flowing water, wind transports particles as bothbed load and suspended load. For wind, bed load is made of sand-sized particles, many of which move by saltation(Figure 1.1). The suspended load is very small particles of silt and clay.

Wind Erosion

Wind is a stronger erosional force in arid regions than it is in humid regions because winds are stronger. In humidareas, water and vegetation bind the soil so it is harder to pick up. In arid regions, small particles are selectivelypicked up and transported.

Deflation

As small particles are removed, the ground surface gets lower and rockier, causing deflation. What is left is desertpavement (Figure 1.2), a surface covered by gravel-sized particles that are not easily moved by wind.

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FIGURE 1.1(a) Wind transport is by suspension, saltation, and creep (bed load). (b) In a sandstorm, sand is usually within ameter of the ground. A dust storm’s smaller particles can travel higher. A dust storm as it approaches Al Asad,Iraq.

FIGURE 1.2This desert pavement formed in the Mo-jave Desert as a result of deflation.

Abrasion

Particles moved by wind do the work of abrasion. As a grain strikes another grain or surface it erodes that surface.Abrasion by wind may polish natural or human-made surfaces, such as buildings. Stones that have become polishedand faceted due to abrasion by sand particles are called ventifacts (Figure 1.3).

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www.ck12.org Concept 1. Landforms from Wind Erosion and Deposition

FIGURE 1.3As wind blows from different direction, polished flat surfaces create a ventifact.

Desert Varnish

Exposed rocks in desert areas often develop a dark brown or black coating called desert varnish. Wind transportsclay-sized particles that chemically react with other substances at high temperatures. The coating is formed of ironand manganese oxides (Figure 1.4).

FIGURE 1.4Ancient people carved these petroglyphsinto desert varnish near Canyonlands Na-tional Park in Utah.

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Wind Deposition

The main features deposited by wind are sand dunes. Loess are wind deposits of finer sediments.

Sand Dunes

Deserts and seashores sometimes have sand dunes (Figure 1.5). Beach dunes are usually made of quartz becausequartz is what’s left in humid areas as other minerals weather into clays. Sand dunes may be composed of calciumcarbonate in tropical areas. But in deserts, sand dunes are composed of a variety of minerals because there is littleweathering.

Dune sands are usually very uniform in size and shape. Larger particles are too heavy for the wind to transport bysuspension and smaller particles can’t be picked up. Particles are rounded, since rounded grains roll more easilythan angular grains.

FIGURE 1.5This sand dune in Death Valley, Californiashows secondary sand ripples along itsslip face.

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 andover the obstacle, it increases in speed. It carries the sand grains up the gently sloping, upwind side of the dune bysaltation. As the wind passes over the dune, its speed decreases. Sand cascades down the crest, forming the slip faceof the dune. The slip face is steep because it is at the angle of repose for dry sand, about 34o (Figure 1.6).

Wind deposits dune sands layer by layer. If the wind changes directions, cross beds form. Cross beds are named forthe way each layer is formed at an angle to the ground (Figure 1.7).

The type 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. Dunes may be crescent-shaped, star-shaped, parabolic, linear, orbarchan.

• An animation of the formation of the dunes at Great Sand Dunes National Park is seen on this website: http://www.nps.gov/grsa/naturescience/sanddunes.htm.

Loess

Windblown silt and clay deposited layer on layer over a large area form loess (Figure 1.8). Loess deposits formdownwind of glacial outwash or desert, where fine particles are available. Loess deposits make very fertile soils in

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www.ck12.org Concept 1. Landforms from Wind Erosion and Deposition

FIGURE 1.6Sand dunes slope gently in the upwind direction. Downwind, a steeper slip face forms.

FIGURE 1.7This sandstone in Zion National Park,Utah, shows crossbedding.

many regions of the world.

Seafloor Mud

Fine-grained mud in the deep ocean is formed from silts and clays brought from the land by wind. The particles aredeposited on the sea surface, then slowly settle to the deep ocean floor, forming brown, greenish, or reddish clays.Volcanic ash may also settle on the seafloor.

Summary

• In deserts, wind picks up small particles and leaves behind larger rocks to form desert pavement.• Moving sand may sand blast rocks and other features to create ventifacts.• The sand is transported until it is deposited in a sand dune.

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FIGURE 1.8Loess deposits form nearly vertical cliffs,without grains sliding down the face. Insome places in China, they are so thickand stable that entire structures can becarved out of them.

Practice

Use this resource to answer the questions that follow.

MEDIAClick image to the left for more content.

1. What causes erosion?

2. Why is soil erosion a problem?

3. How does wind erosion occur?

4. What are the 3 types of wind erosion?

5. What type of wind erosion moves 50% of the soil?

6. What is creep?

7. What is saltation?

8. What is suspension?

9. When is suspension easily seen?

10. What has accelerated erosion?

Review

1. How does desert varnish form?

2. Describe how sand dunes form and move.

3. Why is loess a non-renewable resource?

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References

1. (a) Courtesy of NASA; (b) Courtesy of Corporal Alicia M. Garcia/U.S. Marine Corps. . (a) Public Domain;(b) Public Domain

2. Mark A. Wilson (Department of Geology, The College of Wooster). . Public Domain3. W.H. Bradley. . Public Domain4. Peter Wey. . Used under license from Shutterstock.com5. Image copyright Dean Pennala, 2012. . Used under license from Shutterstock.com6. Courtesy of National Park Service/US Geological Survey. . Public Domain7. George Alexander Grant. . Public Domain8. Till Niermann. . CC-BY-SA 3.0

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