A dune is a mound, ridge, or hill of loose sediment shaped and deposited by the flow of wind or water. Dunes are dynamic, shifting structures that migrate across the landscape over time. They are most often composed of sand-sized particles, such as quartz or calcium carbonate, and are found in environments ranging from vast deserts (ergs) to coastal shorelines and riverbeds. The existence of a dune system requires three elements: a large supply of loose sediment, a fluid medium like wind or water to move it, and a location where the sediment can accumulate.
The Mechanics of Dune Formation
The physical process of dune building begins with the transportation of sediment by wind, a force known as an aeolian process. Sand is moved through three primary mechanisms based on the size of the grain and the strength of the wind. The smallest, lightest particles, like silt and clay, are carried high and far by the wind in a process called suspension.
Medium-sized sand grains, typically between 0.1 and 0.5 millimeters in diameter, move by saltation, the most significant method of sand transport. During saltation, grains bounce across the surface in short leaps, and their impact dislodges other grains, causing them to hop up and continue the process.
The largest particles, over 0.5 millimeters, are too heavy to jump. They are pushed or rolled along the ground by the force of the wind and the impact of saltating grains in a mechanism known as surface creep.
As these grains accumulate, they form a characteristic structure with two main slopes. The gentle slope facing the wind is called the windward or stoss side, where sand is pushed or bounced up toward the crest. Once the sand reaches the crest, it slides down the steeper side, known as the slipface, which maintains an angle approximating the sediment’s angle of repose. This continuous cycle of sand movement up the windward slope and down the slipface causes the entire dune to slowly migrate downwind.
Classifying Dune Shapes and Locations
The final shape of a dune is a direct reflection of the amount of available sand, the consistency of the wind direction, and the presence of vegetation. Crescentic dunes, or Barchans, form where sand is limited and the wind blows consistently from one direction, creating a crescent shape with two horns pointing downwind. Linear dunes, also called seif dunes, are straight, long ridges that form parallel to the sand movement in areas with a bidirectional wind regime and moderate sand supply.
Where sand is abundant and the wind direction is multi-directional, star dunes develop, forming a pyramid shape with multiple ridges radiating from a central peak. These are often the largest and tallest dunes, sometimes reaching heights of hundreds of meters. Parabolic dunes are u-shaped forms common in coastal areas, where vegetation anchors the arms and the middle is blown out, causing the tips to point upwind.
Dunes are geographically classified by their setting, most commonly found in desert sand seas and along coastlines. Coastal dunes form when onshore winds blow sand inland from the beach, often aided by an obstacle like driftwood or plants. Beyond the surface, subaqueous dunes form underwater on the beds of rivers and estuaries, where flowing water, instead of wind, acts as the fluid medium to shape the sediment.
The Ecology of Dune Systems
The shifting, nutrient-poor environment of a dune system supports a highly specialized ecology. Plants that colonize these areas, such as Marram Grass, are uniquely adapted to withstand constant sand burial and high salinity. Their extensive root and rhizome systems trap sand, holding the dune in place and allowing it to grow vertically and stabilize.
The fauna of dune systems adapts to the harsh conditions, including extreme temperature swings and a lack of moisture. Many insects, reptiles, and small mammals, like the sand lizard, are burrowing species, using the sand for shelter from the heat and wind. The biodiversity of dunes is often highest in the more stable, inland sections where vegetation is denser and organic material has accumulated to retain moisture.