Dunes are distinctive landforms defined as mounds or ridges of loose, granular sediment, most commonly sand, that have been shaped by the movement of a fluid. Examples include the vast sand seas of the Sahara and coastal ridges lining shorelines. This process is driven by fluid dynamics, primarily the action of wind (aeolian processes) or, in underwater environments, the flow of water. Dune building is a dynamic cycle of erosion, transport, and deposition, resulting in a landform that is in constant, though often slow, motion.
Essential Conditions for Sediment Accumulation
The formation of a dune field requires a specific set of environmental prerequisites. The first requirement is an ample and continuous supply of loose, fine-grained material, typically quartz sand, but sometimes silt, gypsum, or volcanic ash. This sediment must be readily available and not stabilized by moisture or extensive vegetation.
The second necessary element is a consistent and directional flow of energy, such as strong wind or water currents, to act as the carrying medium. Wind must reach a threshold velocity, dependent on the size of the sand grains, to initiate movement. The medium must then be capable of sustaining the transport of the particles across the landscape.
Finally, there must be an environment of deposition where the carrying medium slows down and loses its capacity to hold the sediment load. This transition from an erosional state to an accumulation state allows the scattered grains to congregate and form a mound. Without a location where the flow energy drops, the sand would simply continue to be transported away.
The Physics of Sand Movement
Once the wind velocity exceeds the threshold needed to mobilize the grains, sand is transported across the surface through three distinct mechanical processes.
Suspension
The smallest and lightest particles, generally silt or dust less than 60 micrometers in diameter, are carried high into the air within turbulent eddies, a process called suspension. These grains can travel hundreds or thousands of kilometers before settling out of the atmosphere.
Surface Creep
The largest grains, typically greater than 500 micrometers, move by surface creep because they are too heavy to be lifted directly by the wind. These particles are rolled or pushed along the ground as they are struck by other moving grains. This transfer of kinetic energy causes the heavy particles to slowly advance.
Saltation
The majority of sand, estimated to be between 75% and 95% of the total transport volume, moves by saltation, a bouncing or jumping motion. A sand grain lifted by the wind follows a ballistic trajectory, landing back on the surface at a shallow angle. The impact of the landing grain dislodges several other grains, launching them into the air and perpetuating the process.
Dune Initiation and Growth
The transition from a moving sheet of sand to an organized dune landform begins with a small irregularity on the surface. A slight obstacle, such as a rock, a clump of grass, or even a small ripple, disrupts the smooth flow of the wind. This disruption causes the airflow to separate and slow down on the sheltered side, creating a localized zone of reduced pressure and deposition.
Sand grains settle in this quiet zone, and as material accumulates, the initial mound increases in height. The continuous flow of sand is pushed up the gentle, wind-facing slope, known as the stoss side. As the grains reach the crest, they fall out of the main airflow and tumble down the steep, sheltered side, called the slip face.
The angle of the slip face is maintained at a precise physical limit known as the angle of repose, which for dry, uniform sand is typically around 32 to 34 degrees. When accumulation at the crest causes the slip face angle to momentarily exceed this limit, the sand becomes unstable and gravity triggers a small avalanche, or grain flow. This downward movement resets the slope back to the angle of repose. This perpetual cycle—sand removed from the windward side and deposited via avalanching on the leeward side—is the mechanism by which the entire dune structure slowly migrates across the landscape in the direction of the prevailing wind.
Major Dune Shapes and Their Controlling Factors
The final shape, or morphology, of a dune is determined by two primary environmental variables: the variability of the wind direction and the overall availability of sand. Dune types are a physical expression of the balance between these two factors.
Barchan dunes are crescent-shaped, isolated mounds that form under a nearly unidirectional wind regime where the sediment supply is limited. In contrast, transverse dunes also form under a unidirectional wind pattern, but they develop into long, sinuous ridges perpendicular to the wind direction due to an abundant supply of sand.
Linear dunes, sometimes called seif dunes, are long, straight ridges that can extend for many kilometers. These forms develop in environments where the wind regime is bimodal, meaning strong winds blow from two different directions. The resulting dune ridge aligns parallel to the average wind direction, often forming in areas with limited sand supply. This diversity demonstrates how the same fundamental physics, when subjected to different environmental conditions, produces a varied array of landscape features.