High School Earth Science/Wind Erosion and Deposition
Moving water does much of the work of erosion that shapes the land surface of our Earth. Wind also flows over the Earth's surface, sometimes carrying particles long distances before they are deposited. Wind blows from areas of high pressure to areas of lower pressure. The erosive power of wind varies with the strength of the winds that blow, but usually wind transports smaller particles like silt and clay. Somewhat larger particles may be bumped or rolled along by the wind. Wind can carry particles across ocean basins and to great heights within our atmosphere. Wind is a stronger erosional force in arid regions than humid areas for two reasons. In arid regions, temperatures change greatly from night to day, which produces wind. Even strong winds in humid areas are less effective erosional agents because the ground is wet, so soil particles are heavier and less likely to be removed or transported by wind.
- 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.
Transport of Particles by Wind
Wind is able to transport the smallest particles of sediment, like silt and clay, over great distances and areas. Once these particles become mixed into the air, wind can keep them suspended for hours or maybe even days at a time. If nothing disturbs these tiny particles, wind would have trouble picking them off the ground surface. This is because very close to the ground, there is very little motion due to wind. Look behind a car or truck as it drives over an unpaved road. You will see a big cloud of dust that wasn't there before the truck disturbed the ground surface. Once these fine particles are disturbed, wind easily picks them up and distributes them.
Just as water carries different size particles in various ways, wind also transports particles as both bed load and suspended load. For wind, sand sized particles make up the bed load. These sand grains are moved along by the wind in a bump, roll and jump kind of motion. First, a grain of sand gets knocked into the air. It is too heavy to have wind carry it for long in suspension, so it falls back to the ground, possibly knocking another sand grain into the air as it hits the ground. This starts the process all over again. This process is called saltation, which comes from a Latin word meaning 'to leap' (Figure 10.24). The suspended load for wind will always be very small particles of silt and clay, which are still able to be carried suspended in the air by wind.
Erosion by Wind
As wind moves sand sized particles, they will remain close to the ground, usually less than a meter from the ground even in the strongest winds. In a sandstorm, about a quarter of the particles are sand which moves as bed load. In arid regions, a sandstorm actually moves much smaller particles than sand in the winds. Wind can carry these small particles high into the air and these particles can infiltrate cracks around windows and doors in a dust storm (Figure 10.25).
Sometimes these small particles are deposited in areas relatively close to their original source, but often silts and clays have been carried halfway across a continent or from desert areas on one continent across an entire ocean basin. Wind is more effective at erosion in arid regions because in humid regions smaller particles are held together by the moisture in the soil and by plant roots from the vegetation. Where it is dry, plants don't grow as well, so both these factors increase the ability of wind to transport particles, eroding the landscape.
The process of smaller particles being selectively transported by the wind is called deflation (Figure 10.26). This means the ground surface gets lower and rockier, as more and more small particles are blown away. Eventually, most of the smaller particles will have been removed and the rockier surface left behind is called desert pavement. This surface is covered by pebbles and gravel sized particles that are not easily moved by wind. If no disturbance from vehicles or animals disrupts the surface, deflation will stop once this rocky surface has formed.
All the particles moved by wind, whether as suspended load or by saltation as bed load, do the work of abrasion. As one particle hits another, each grain erodes another. You may have seen workers sandblasting the front of a building in order to remove paint or dirt. In the natural situation, erosion by wind polishes surfaces of rock. In the desert, rocks or boulders develop different polished flat surfaces as wind blows from different directions. These polished stones are called ventifacts (Figure 10.27).
Exposed rocks in desert areas often develop a dark brown to black coating called desert varnish. This coating forms on stable rock surfaces that don’t get much precipitation. The first part of the process is the transport of clay sized particles by wind which chemically react with other substances at high temperatures. The coating is formed of iron and manganese oxides. Ancient people carved into these darkened surfaces to make petroglyphs (Figure 10.28).
Deposition by Wind
When you think of a desert or perhaps even a beach, the image that comes to mind might include sand dunes (Figure 10.29). In coastal regions, you will find sand dunes in the landward direction of the beach. Sand dunes form here as sand is blown from the shore inland. The sand dunes along a beach are likely to be composed of individual grains of the mineral quartz, unless the beach is in a tropical area. In humid regions, other minerals break down readily to form clays, leaving behind only the more resistant quartz. In the tropics, sand dunes may be composed of calcium carbonate. In a desert, the sand dunes may be composed of a variety of minerals. This is because a desert region, by definition, has very little water. This means that mostly mechanical weathering and very little chemical weathering occurs here. So desert sand dunes will include even unstable minerals.
Just as water waves are very selective about the size particles they carry and deposit, so will the size of the sand grains in a dune be very uniform. The sand dunes are formed of a particular size particle which is too heavy for the wind to transport. This process is sometimes so selective that wind will transport and carry rounded grains of sand, which roll easily, more readily than angular grains.
The faster and stronger the wind, the more particles it can carry. As wind slows down, it will drop off the heaviest particles first. This often happens as wind moves over some type of obstacle, such as a rock or an area of vegetation. As the wind moves up and over the obstacle, it increases in speed, but as soon as it passes the article, wind speed decreases. That is why you will often see deposits of sand on the downwind side of an obstacle. These deposits are the starting material for formation of sand dunes. This is the first condition needed for dunes to form.
In order for sand dunes to form, two more conditions must be met. The first of these conditions is that there is an abundant supply of sand. The last condition is that there are steady winds. As steady winds blow over an ample supply of sand, sand grains will bump and roll along, moving by saltation up the gently sloping, upwind side of the dune. As a grain of sand reaches the crest of the dune, it cascades down the steeper, downwind side of the dune, forming the slip face of the dune. The slip face is steep because it forms at the angle of repose for dry sand, which is about 34° (Figure 10.30).
So as wind erodes and transports sand grains along the gently sloped upwind side of a dune, it deposits sand along the downwind slip face. As each new layer of sand falls down the slip face of the dune, cross beds are formed. Cross beds are named for the way each layer is formed at an angle to the ground. Some of the most beautiful sandstones are crossbedded sandstones (Figure 10.31). These sandstones preserve sands originally deposited as sand dunes in deserts millions of years ago.
Sand is always moving up the gently sloped side of a dune, and depositing on the downwind side, which means that dunes themselves slowly migrate in the downwind direction. This means that over a period of years, sand dunes will move many meters downwind. This is something that beach house owners need to consider if they live near coastal sand dunes. Once a sand dune becomes stabilized by vegetation, such as sea grasses, its migration will stop. Beach goers need to be careful not to disturb these grasses when they go to and from the beach.
Reducing Wind Erosion – Types of Sand Dunes
There are several different forms that sand dunes will take. The differences are due to the amount of sand available to be moved, the character and direction of the wind and the type of ground the sand is moving over. The most usual crescent shaped dune is called a barchan dune (Figure 10.32). This type of dune forms when there is an adequate amount of sand being moved by constant winds that blow from one direction over hard ground. The crescent shape will curve in the direction the wind blows. In an area of constant winds with an abundant supply of sand, barchan dunes blend together into large scale sand ripples called transverse dunes. Many coastal dunes are this type of dune. Desert areas that are completely sand covered can join many transverse dunes together into a sand sea.
In many parts of the world, the finest grains of windblown silt and clay are deposited layer on layer, covering whole regions with these tiny particles (Figure 10.33). Geologists call these deposits loess, which comes from the German word 'loose'. These deposits form downwind of areas of glacial outwash or desert areas. There are extensive loess deposits in China where deserts are the original source for these fine grained, windborne particles. One unusual characteristic of loess deposits is their ability to form nearly vertical cliffs, without grains sliding or slumping down the face. In China, people once built homes directly into these deposits because they are easy to dig into and they keep their shape. Loess deposits are also the source of wind transported materials that make very fertile soils in many regions of the world.
Much of the fine grained mud that covers the deepest parts of the ocean floor comes from silts and clays brought there by winds from the land. These tiny particles are easily carried long distances by wind. Once they are deposited on the water surface, they settle ever so slowly to the deep ocean floor, forming brown, greenish or reddish clays. Another source of windborne particles is volcanic activity. Explosive volcanoes eject volcanic ash and dust high into the air, sometimes reaching the stratosphere. Once these fine grained particles are airborne, they can travel hundreds or thousands of kilometers. Regions closest to the volcano are the areas with thickest deposits, but volcanic ash has even completely circled the Earth in extremely violent eruptions like Krakatau in 1883. Windborne volcanic ash can produce spectacularly beautiful sunsets, as well as decreasing worldwide temperatures, as ash and dust block out incoming sun's rays.
- Wind can carry small particles like sand, silt, and clay.
- Wind erosion produces sand blasting of surfaces and produces desert pavement, ventifacts, and desert varnish.
- Sand dunes are some of the most common wind born deposits, which come in many different shapes and sizes.
- Loess is a very fine grained, wind borne deposit that is important to soil formation in many regions.
- Discuss suspended load and bed load transport by wind.
- Describe how desert pavement forms.
- Discuss the factors necessary for sand dunes to form.
- Name four types of sand dunes that form in desert areas.
- Name one type of wind deposition.
- Why is wind erosion more important in arid regions than humid areas?
- barchan dune
- Crescent shaped dune that forms in regions of ample sand, constant winds, and hard ground.
- bed load
- The portion of sand carried by wind as grains roll, bump, and jump along the ground surface.
- The process of wind removing finer grains of silt and clay; causes the ground surface to subside.
- desert pavement
- Rocky, pebbled surface created as finer silts and clays are removed by wind.
- desert varnish
- Dark mineral coating that forms on stable, exposed rock surfaces as windborne clays are deposited.
- Extremely fine-grained, windborne deposit of silts and clays; forms nearly vertical cliffs.
- Rock carvings formed by cutting into desert varnish of exposed rock surfaces.
- Movement of sand sized particles by rolling, bumping, and jumping along the ground surface.
- sand dune
- Deposit of sand formed in regions of abundant sand and constant winds.
- slip face
- Steeper, downwind side of a dune; region where sand grains fall down from the crest of the dune.
- suspended load
- Particles of silt and clay carried in the air by the energy of winds.
- Polished, faceted stones formed by abrasion of sand particles.
Points to Consider
- Do you think strong hurricane winds along a coastline would 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.