Aeolian processes

Aeolian processes refer to the activity of wind in shaping the Earth's surface. The term comes from Aeolus, the Greek god who was the keeper of the winds. These processes involve the erosion, transport, and deposition of sediments. While water is a more powerful eroding force, wind is a major agent in arid and semiarid environments.

Wind is most effective where there is little vegetation, low soil moisture, and plenty of loose, unconsolidated sediment. These processes are not limited to deserts, however. They also occur along shorelines, near river courses, and in areas of glacial outwash.

Wind erodes the landscape through three primary mechanisms: deflation, abrasion, and attrition. Deflation is the removal of loose, fine-grained particles by wind turbulence. This happens through three specific movements. Larger grains move via traction, also known as surface creep, by sliding or rolling across the ground. Medium particles move through saltation, which is a series of short bounces. Tiny particles are carried by suspension, staying fully entrained in the air to travel long distances. Saltation is the most common, accounting for 50% to 70% of deflation.

Abrasion, or corrasion, occurs when wind-driven grains strike and wear down landforms. This acts like a natural sandblasting process. While once thought to be a primary cause of many desert features, modern science views its role as less significant than previously believed. For example, features like mushroom rocks are now often attributed to rainwash or deflation. However, abrasion still creates unique structures like yardangs. These are streamlined rock ridges that can be kilometers long and tens of meters high.

Attrition is a related process where particles collide while moving through the air. These collisions cause the particles to break down into smaller pieces. Attrition is very effective at rounding sand grains and creating a distinctive frosted texture on their surfaces. This process is a major source of fine dust, specifically particles in the 2-5 micron size range. This dust is often created when the weathered clay coating is stripped from the grains.

Transport describes how wind moves sediment from one location to another. To begin this movement, the wind must reach a specific velocity called the fluid threshold. Once movement starts, a cascade effect occurs where dislodged grains hit others and tear them loose. This allows transport to continue even if the wind slows down to the dynamic threshold.

Wind is highly efficient at separating sediments by size. It can carry clay and silt over vast distances, even across entire oceans. For instance, Saharan dust travels to the Amazon basin and helps form red clay soils in southern Europe.

When wind energy decreases, it results in deposition. The wind is excellent at separating sand from silt and clay, creating distinct types of deposits. Sandy deposits are called ergs, or sand seas. Silty deposits are known as loess. Loess can form extremely thick layers. The Loess Plateau in China contains deposits up to 900 meters thick. In North America, the Peoria Loess in Iowa can reach 6 meters in thickness. These loess soils are often highly productive for agriculture.

Scientists have spent decades classifying these wind-driven landforms. Ralph Alger Bagnold, a British army engineer, performed vital research in Egypt before World War II. He studied the physics of moving particles and identified dune types like the crescentic "barchan" and the linear "seif" dune. In 1941, John Tilton Hack added parabolic dunes to these classifications. Today, researchers use computer simulations and space exploration to study these processes.

In 1971, the Mariner 9 spacecraft observed a massive dust storm on Mars that covered the entire planet for one month. Studying these patterns helps us understand the history and intensity of past and present climates.