A sand dune is a mound or ridge of sand built by the wind. Dunes appear where three ingredients come together: a supply of loose sand, winds strong enough to move it, and little vegetation to hold the grains in place. Deserts, coastlines, and dry lake beds can all meet those conditions. How Desert Mirages Actually Form.

Wind moves sand in three main ways. Very fine dust can be lifted high and carried in suspension for long distances. Sand-sized grains mostly travel by saltation, a series of short bouncing hops close to the ground. The largest, heaviest grains are not lifted at all; they roll or slide slowly along the surface in a motion called creep, often nudged along by the impacts of saltating grains.

A dune begins when airflow slows down, for example where wind meets a small obstacle or a patch of rougher ground. Slower air can carry less sand, so grains settle out and a small pile forms. That pile then slows the wind further and traps more sand, and the mound grows. As the dune gets larger, sand is pushed up the gentler windward slope until it reaches the crest and avalanches down the steeper sheltered side, called the slip face. Through this cycle of erosion on one side and deposition on the other, the whole dune slowly migrates in the direction the wind blows.

The shape of a dune records the local wind pattern and sand supply. Crescent-shaped barchan dunes form where wind blows steadily from one direction and sand is scarce. Transverse dunes form long ridges across the wind where sand is plentiful. Star dunes grow upward where winds blow from several directions, and parabolic dunes form where vegetation anchors parts of the sand.

The short answer

Sand dunes form when wind strong enough to lift and bounce sand grains loses speed and drops its load. The growing pile traps more sand, avalanches down its sheltered slip face, and slowly migrates, taking shapes that match the local winds and sand supply.

Sources

  1. U.S. National Park Service, "Monitoring Aeolian Features and Processes." β€” https://www.nps.gov/articles/aeolian.htm
  2. U.S. National Park Service, Great Sand Dunes National Park & Preserve, "Dune Types." β€” https://www.nps.gov/grsa/learn/nature/dune-types.htm
  3. Encyclopaedia Britannica, Dune. https://www.britannica.com/science/dune-geology

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