Wind erosion, also known as aeolian erosion, describes the geological process where moving air detaches, transports, and deposits loose surface materials like soil, sand, and dust. This mechanism shapes vast landscapes, particularly where ground cover is sparse. It involves the physical movement of sediment, which can occur over short distances or span continents. The process is a significant agent of change in dry environments, altering surface topography and influencing soil fertility.
Environmental Conditions Necessary for Erosion
The initiation of wind erosion requires a specific convergence of physical and environmental factors. A lack of moisture is a prerequisite, as dry soils lack the cohesion that binds particles together. Without this natural cementation, individual soil grains become easily detachable by air currents. This vulnerability increases in regions experiencing extended periods of drought or low annual precipitation, often falling below 500 millimeters a year.
Loose, finely textured sediment provides the necessary source material. This material must also be exposed, meaning the land surface must be largely devoid of vegetation cover. Plant roots anchor the soil, and a closed canopy can almost completely prevent the wind from reaching the surface.
The wind itself must reach a certain minimum speed, known as the threshold velocity, to initiate particle movement. This required speed varies depending on the size of the surface grains. Winds between 6 and 8 meters per second, measured at a standard height of ten meters, are often sufficient to trigger erosive action. Once this threshold is surpassed, the energy available for detachment and transport increases exponentially, mobilizing significant amounts of material.
The Three Ways Wind Moves Sediment
Once the necessary environmental conditions are met, the wind transports sediment through three distinct physical processes: saltation, suspension, and surface creep. The size of the particle determines which mode of transport dominates.
Saltation is the most common mode of transport, typically accounting for 50 to 75 percent of the total sediment flux. This process involves the bouncing or hopping motion of medium-sized grains, generally sand particles ranging from 0.1 to 0.5 millimeters in diameter. The wind applies a shear force across the surface, causing these grains to lift off briefly before following a parabolic trajectory back to the ground.
The impact of a saltating grain striking the surface drives the larger process of erosion, as the collision often dislodges several other stationary particles. These newly dislodged grains are either launched into the air or nudged forward along the ground. This mechanical chain reaction facilitates the movement of the other two particle types.
Suspension involves the smallest and lightest particles, such as silt and clay, which are less than 0.1 millimeters in diameter. These fine grains are easily picked up by turbulent air currents and can be carried high into the atmosphere, remaining airborne for hours or even days. This mechanism is responsible for the vast, long-distance transport of dust, which can travel across entire continents or ocean basins.
Suspension often creates massive dust storms that visibly reduce air quality and visibility. While it accounts for a smaller percentage of the total deflation (approximately 30 to 40 percent), it is the most significant process for the removal of fertile topsoil.
Surface creep involves the largest, heaviest particles, generally greater than 0.5 millimeters in size. These grains are too heavy to be lofted into the air by the wind’s direct force. Instead, they move by rolling or sliding along the surface, relying almost entirely on the impact energy transferred by saltating particles. Surface creep typically represents only 5 to 25 percent of the total sediment movement.
Observable Effects and Landforms
The sustained action of wind erosion and subsequent deposition sculpt the landscape into recognizable geological features. One direct result of wind lifting fine material is deflation, the gradual lowering of the land surface. When the wind selectively removes silt and dust, it leaves behind a protective layer of larger, non-transportable gravel and pebbles, creating a hard, rocky surface known as desert pavement.
Deflation can also create shallow, closed depressions known as deflation hollows or blowouts, where localized turbulence has vacuumed loose material. Another effect is abrasion, which acts like a natural sandblaster, where saltating sand particles strike stationary rock surfaces. This constant impact polishes and wears down the rock, often creating smooth, faceted stones known as ventifacts.
Abrasion can also sculpt larger formations, such as mushroom rocks, where the lower portions of a rock column are eroded more quickly due to the concentration of saltating particles near the ground. The end result of wind transport is deposition, which occurs when the wind velocity drops and the suspended or saltating load is released.
The most common depositional landform is the sand dune, which forms when large volumes of transported sand accumulate after encountering an obstacle or a sudden decrease in wind speed. The finest materials carried high in suspension, silt and clay, are eventually deposited far from their source to form extensive, blanket-like layers called loess. These loess deposits can be tens or even hundreds of meters thick and form some of the most fertile agricultural soils in the world.