How Wind Causes Erosion
Wind by itself is the weakest agent of erosion. Water, waves, moving ice, and even mass movement have more effect on the land. Yet wind can be a powerful force in shaping the land in areas where there are few plants to hold the soil in place. For example, few plants grow in deserts, so wind can easily move the grains of dry sand. Wind causes erosion through deflation and abrasion.
The main way that wind causes erosion is by deflation. Geologists define deflation as the process by which win d removes surface materials. When wind blows over the land, it picks up the smallest particles of sediment. This sediment is made of bits of clay and silt. The stronger the wind, the larger the particles that it is able to pick up. Slightly heavier particles, such as sand, might skip or bounce for a short distance. Shortly after, the sand falls back to the ground.
Deflation does not usually have a great effect on land. However, in parts of the Great Plains in the 1930s, deflation caused the loss of about 1 meter of topsoil in just a few years. In deserts, sometimes deflation can create an area of rock fragments called desert pavement. In this area, the wind has blown away smaller sediment, leaving only rocky materials that are to heavy to be moved by wind. Where there is already a slight depression in the ground, deflation can produce a bowl-shaped hollow called a blowout.
Abrasion by wind-carried sand can polish rock, but only causes little erosion. At one time, geologists thought that the sediment carried by wind cut the stone shapes seen in deserts. But now evidence shows that most desert landforms are the result of weathering and water erosion.
All the sediment picked up by wind eventually falls back to the ground. This happens when the wind slows down or some obstacle, such as a boulder or a clump of grass, traps the windblown sand sediment. Wind erosion and deposition may form sand dunes and loess deposits. When the wind strikes an obstacle, the result is usually a sand dune. Sand dunes can be seen on beaches and in deserts where windblown sediment has built up.
Sand dunes come in various shapes and sizes. Some are long, with parallel ridges, while others are U-shaped. They can be very large, or very small. Actually, some sand dunes in China are up to 500 meters tall. Sand dunes move over time. Little by little, the sand shifts with the wind from one side of the dune to the other. Sometimes, plants start to grow on the dunes. Plants can help to anchor the dune in one place.
Sediment that is finer than sand, such as particles of clay and silt, is sometimes deposited in layers far from its source. This fine, wind-deposited sediment is loess (LES). Large loess deposits are found in central China and in states such as Nebraska, South Dakota, Iowa, Missouri, and Illinois. Loess helps to form fertile soil. Many areas with thick loess deposits are valuable farmlands.