What are sand dunes, and how do they get established? A dune is a hill of sand that forms where the combination of winds and/or the contribution of sediment by water currents allow the accumulation of sand: the wind lifts the accumulated sand from the sea and carries it inland, until it meets an obstacle that will force the sand to settle. Dunes are therefore subject to continuous movement and resizing depending on the direction and strength of the wind.
Types of Sand Dunes
The type of dune is determined by its shape. Geologists have classified dune types into five main categories: barchan or crescent dunes, parabolic dunes, transverse dunes, linear dunes and star dunes.
Barchan Dunes / Crescent Dunes
This is the traditional dune form: when viewed from above, Barchan dunes resemble crescent moons, with convex (outward) backs and steep, concave (inward) faces. The crescent’s curving tips or wings point downwind, partially enclosing a single slip-face. Barchans arise when there is a limited quantity of sand, reasonably flat ground, and a consistent wind flow from one direction. A barchan dune can reach the height of a multi-story skyscraper. The most frequent are barchan dunes, which can be found in deserts all around the world.
A parabolic dune resembles a barchan in shape, however it is the exact opposite. The dune’s tips face the wind, while the dune’s main body moves with the wind, forming a depression between the tips. Blowing out dunes are sometimes called as parabolic dunes because of this structure. When vegetation stabilizes sediments and a U-shaped blowout forms between clusters of plants, these dunes arise.
Transverse dunes are long, asymmetrical dunes that grow at right angles to the direction of the wind. They form when there is a lot of sand and the winds aren’t too strong. These dunes have a single, steeply sloping slip face. On a broad scale, a group of transverse dunes resembles sand ripples.
A linear dune (or longitudinal dune) arises when there is plenty of sand and strong cross winds blow from at least two directions, forcing the sand into long lines or ridges. Linear dunes’ crests or summits are frequently straight or slightly curved, with slipfaces on both sides. Linear dunes can be as tall as 655 feet (200 meters) and as long as 62 miles (103 kilometers).
Three or more sinuous ridges radiate out from a central sand peak in what makes a pyramidal or star-shaped dune. Where there is enough of sand and strong breezes from all directions, a star dune forms. There are three or more slipfaces on this dune. It develops upwards rather than moving along the ground. The largest and highest dunes are star dunes.
Sand dunes F.A.Q.
Are sand dunes constructive or destructive?
Sand dunes are constructive: they are built via deposition by the constructive force of wind which shifts rock particles on top of each other until a sandhill is formed.
Are sand dunes formed by erosion or deposition?
Sand dunes are an example of deposition. They are formed by the shifting of sand particles by the wind, sea waves, or both.
What are “singing” and “booming” sands?
In a few places in the world a phenomenon can occur that causes the shifting sands of a dune to produce an audible sound. These dunes are called singing dunes or booming dunes.
Sources: Sand Dunes of the Southwest