Lesson Objectives

• Describe the ways particles are carried by wind.
• Discuss several ways that wind erosion changes land surfaces.
• Describe how sand dunes form.
• Describe the type of deposits formed by windborne silts and clays.

Vocabulary

• deflation
• desert pavement
• desert varnish
• loess
• sand dune
• slip face
• ventifacts

Introduction

The power of wind to erode depends on particle size, wind strength, and whether the particles are able to be picked up. Wind is a more important erosional force in arid than humid regions.

Transport of Particles by Wind

Wind transports small particles, such as silt and clay, 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 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 move by saltation (Figure below). 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 humid areas, water and vegetation bind the soil so it is harder to pick up. In arid regions, small particles are selectively picked up and transported. As they are removed, the ground surface gets lower and rockier, causing deflation. What is left is desert pavement (Figure below), a surface covered by gravel sized particles that are not easily moved by wind.

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 polished and faceted due to abrasion by sand particles are called ventifacts (Figure below).

Exposed rocks in desert areas often develop a dark brown to black coating called desert varnish. Wind transports clay-sized particles that chemically react with other substances at high temperatures. The coating is formed of iron and manganese oxides (Figure below).

Wind Deposition

Deserts and seashores sometimes have sand dunes (Figure below). Beach dunes have different compositions depending on their location. Beach dunes are usually quartz because in humid areas other minerals weather into clays. In the tropics, sand dunes may be composed of calcium carbonate, which is common. In deserts, sand dunes may be composed of a variety of minerals. There is little weathering and so less stable minerals are left behind.

Dune sands are usually very uniform in size and shape. Particles are sand-sized, because larger particles are too heavy for the wind to transport by suspension. Particles 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° (Figure below).

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 (Figure below).

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. Some dune types are shown in the Figure below.

• 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 are loess, which comes from the German word loose (Figure below). Loess deposits form downwind of glacial outwash or desert, where fine particles are available. Loess deposits make very fertile soils in many regions of the world.

Fine-grained mud in the deep ocean is formed from silts and clays brought from the land by wind. The particles are deposited 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.

Lesson Summary

• Wind can carry small particles such as sand, silt, and clay.
• Wind erosion abrades surfaces and makes desert pavement, ventifacts, and desert varnish.
• Sand dunes are common wind deposits that come in different shapes, depending on winds and sand availability.
• Loess is a very fine grained, wind-borne deposit that can be important to soil formation.

Review Questions

1. Discuss suspended load and bed load transport by wind.
2. Describe how desert pavement forms.
3. Discuss the factors necessary for sand dunes to form.
4. Name four types of sand dunes that form in desert areas.
5. Name one type of wind deposition.
6. Why is wind erosion more important in arid regions than humid areas?

Points to Consider

• Would hurricane-force winds along a coastline produce wind related erosion?
• What would be needed to convert a desert area back to a productive region for farming?
• Do you think wind could sculpt exposed rocks? Explain how this might happen.