A dune is a landform—a mound, ridge, or hill of loose sediment, typically sand, that has been built by wind or water flow. These dynamic features are accumulations of granular material resulting from the continuous process of sediment transport and deposition. Dunes occur in many different sizes and shapes and can migrate across landscapes over time.
The Mechanics of Dune Formation
The formation of a dune requires three conditions: a substantial supply of loose, dry sand, a consistent wind to move the grains, and an obstruction to initiate deposition. Wind-driven sediment transport is known as an aeolian process. The most significant transport mechanism for sand-sized particles is saltation, where wind causes grains to bounce along the ground in short, hopping trajectories.
When a hopping grain lands, it may dislodge other grains, causing them to move in surface creep, or it may launch itself back into the air. Saltation accounts for the vast majority of sand movement in a dune system. The wind carries this moving sand up the gentler, windward side of an existing mound or obstacle, which is called the stoss slope.
As the wind reaches the top of the mound, its speed decreases significantly, causing the sand grains to fall out of the air and accumulate at the dune’s crest. This accumulation eventually exceeds the angle of repose, the steepest angle at which the loose sand can remain stable. When this angle is surpassed, the sand avalanches down the steeper, downwind side of the dune, creating the slip face. This constant cycle of windward erosion and slip face deposition causes the entire dune structure to migrate downwind.
Major Types of Dunes and Their Shapes
Dune shape is a direct consequence of the local wind regime and the available sand supply. Crescentic dunes, also known as barchans, are the most common type. They form where wind blows consistently from one primary direction and the sand supply is limited. These dunes are wider than they are long, featuring a convex windward slope and two “horns” that point downwind, flanking the concave slip face.
Longitudinal or seif dunes are long, straight, or slightly sinuous ridges that can extend for over 100 kilometers. They form under a bidirectional wind regime where winds blow from two distinct directions. Seif dunes often have a sharp crest and two slip faces, and they form in areas where sand supply is moderate.
Star dunes are pyramidal mounds characterized by three or more arms radiating from a high central peak. These dunes develop where the wind direction is highly variable and multidirectional, causing the sand to pile upward rather than migrating laterally. They require a large volume of sand for their formation and are among the tallest dunes on Earth.
Parabolic dunes are U-shaped mounds whose crescent tips point upwind. They form in areas with strong, unidirectional winds where vegetation is present to anchor the arms of the dune. The wind erodes a blowout depression in the less-vegetated center, pushing the sand downwind while the vegetated arms remain stable. Parabolic dunes are common in coastal environments.
Geographical Settings and Dune Environments
Dunes occur in diverse environments beyond the typical desert landscape. The largest dune systems are found in inland arid regions, where vast fields of sand, called ergs or sand seas, cover broad, flat areas. These dune fields require an ample sediment source and continuous aeolian processes to maintain their scale and mobility.
Dunes are also found along coastlines, forming when dry sand is blown inland from a wide beach. Coastal dunes are often stabilized by specialized vegetation, which helps to bind the sand and protect the land from storm waves. This plant life distinguishes the ecological setting of coastal dunes from the environment of desert dunes.
Subaqueous dunes form on the beds of rivers, estuaries, and the sea floor. These underwater features are shaped by the flow of water rather than wind. Dunes can also form in periglacial regions, where sandy deposits are exposed and transported by wind action in cold, dry climates.