Of the four forces of erosion, water is the most powerful. This unit will not cover water erosion and deposition in great detail, however. Instead, all of water's erosional and depositional power (by streams, groundwater, and waves) will be given special treatment in the Water unit. This unit will cover erosion and deposition by wind, glaciers, and gravity.
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 natural sand blasting can be a tremendous erosional force on rocks or buildings. Hopefully, you won't stay out long enough to 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 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.
(a) Wind transport is by suspension, saltation, and creep (bed load). (b) In a sandstorm, sand is usually within a meter of the ground. A dust storm's smaller particles can travel higher. A dust storm as it approaches Al Asad, Iraq.
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 small particles 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.
This desert pavement formed in the Mojave Desert as a result of deflation.
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).
As wind blows from different direction, polished flat surfaces create a ventifact.
Exposed rocks in desert areas often develop a dark brown or 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).
Ancient people carved these petroglyphs into desert varnish near Canyonlands National Park in Utah.
The main features deposited by wind are sand dunes. Loess are wind deposits of finer sediments.
Deserts and seashores sometimes have sand dunes (Figure below). Beach dunes are usually made of quartz because quartz is what's left in humid areas as other minerals weather into clays. Sand dunes may be composed of calcium carbonate in tropical areas. But in deserts, sand dunes are composed of a variety of minerals because there is little weathering.
Dune sands are usually very uniform in size and shape. Larger particles are too heavy for the wind to transport by suspension and smaller particles can't be picked up. Particles are rounded, since rounded grains roll more easily than angular grains.
This sand dune in Death Valley, California shows secondary sand ripples along its slip 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 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 34o (Figure below).
Sand dunes slope gently in the upwind direction. Downwind, a steeper slip face forms.
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).
This sandstone in Zion National Park, Utah, shows crossbedding.
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, or barchan.
- 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.
Windblown silt and clay deposited layer on layer over a large area form loess (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.
Loess deposits form nearly vertical cliffs, without grains sliding down the face. In some places in China, they are so thick and stable that entire structures can be carved out of them.
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.
- saltation: The movement (or "leaping") of hard particles such as sand over an uneven surface in a turbulent flow of air or water.
- deflation: As small particles are removed, the ground surface gets lower and rockier.
- desert pavement: Ground surface covered only by large, gravel-sized particles.
- abrasion: The process of scraping or wearing away.
- ventifacts: Stones that have become polished and faceted due to abrasion by sand particles.
- dune: a mound or ridge of sand or other loose sediment formed by the wind, esp. on the sea coast or in a desert.
- loess: Windblown silt and clay deposited layer on layer over a large area.
- 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.
Use this resource to answer the questions that follow.
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?
1. How does desert varnish form?
2. Describe how sand dunes form and move.
3. Why is loess a non-renewable resource?