Sand dunes are mounds, hills, or ridges of sand built by wind.
How are dunes formed?
The formation of a dune begins with the transportation of sand grains by the wind. When these grains of sand are deposited in a certain area, they can form a small mound or hill.
As more and more sand is added to this mound, a dune begins to form. This process is called “saltation.”
The shape and size of dunes are determined by several factors, including the amount and size of sand grains available, the speed, direction, and frequency of the wind, and the presence of vegetation. Different combinations of these factors can result in a variety of dune types.
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Five basic dune types
There are five basic dune types: crescentic, linear, star, dome, and parabolic.
Crescentic dunes
Crescentic dunes, also known as barchans, or transverse dunes are perhaps the most common type of dunes found on Earth.
Shaped like a crescent moon, crescentic dunes form in environments with strong, unidirectional winds and limited sand supply. Crescentic dunes tend to form in areas with little or no vegetation.
The windward side of a barchan dune has a gentle slope, while the leeward side, where the sand is pushed, forms a steeper slope. The tips or “horns” of the crescent point downwind, creating a distinctive form.
A compound crescentic dune forms when smaller barchan dunes coalesce or overlap, creating larger dune structures. These formations still maintain the general crescent shape, but their size and complexity are significantly greater.
Linear dunes
Linear, also known as longitudinal or seif dunes, are long, straight dunes aligned with the dominant wind direction. Linear dune formation typically occurs in desert regions where winds blow from two directions.
Linear dunes in parts of Rub’ al Khali erg (erg means “dune sea” in Arabic), a sand sea located in the Arabian Desert, are formed by prevailing winds that result in the linear rows of sand that trend northwest-southeast. These linear dunes can be up to 75 kilometers (47 miles) in length.
Star dunes
As the name suggests, star dunes radiate at least three arms outward in a star-like pattern. They form in environments with multidirectional winds, causing the dune to grow in several directions simultaneously.
Star dunes are the tallest dunes, as sand accumulates from being blown by the wind from multiple directions. This aeolian process builds the dune upwards. The Sahara Desert’s iconic dunes are primarily star dunes.
Star dunes make up about 8.5% of the global sand dune formations. Star dunes can be found in areas such as: China’s Badain Jaran Desert, the Gran Desierto de Altar in Mexico, and the eastern sector of the Rub’ al Khali on the Arabian Peninsula.
Parabolic Dunes
In contrast to barchan dunes, parabolic dunes have their tips pointing into the wind. These dunes form in areas present with vegetation. It’s this vegetation which anchors parts of the sand, causing the U or V shape.
The unanchored sand continues to be blown by the wind, creating a hollow or ‘parabola’ within the dune. Also known as blowout, or hairpin dunes, parabolic dunes are often found along coastlines or in semi-arid regions.
Dome Dunes
Dome dunes, also known as mound dunes, are hemispherical, circular mounds of sand with no slip face. They are the rarest type of dune and are usually small.
Dome dunes typically form in areas with limited sand supply and multidirectional winds.
Where are dunes found?
Sand dunes are commonly found in desert environments and along coastlines but can be found in other types of landscapes.
Desert Dunes
The world’s largest hot desert, the Sahara, is a prime example of extensive sand dune fields. Covering a large part of North Africa, the Sahara is home to various dune types, including barchan, seif, and star dunes.
Coastal Dunes
These are found along shorelines where wind and wave action deposit sand in abundance. Coastal dunes can be observed along many coastlines worldwide, from the United States’ east and west coasts to the beaches of Australia and Europe.
Inland Dunes
Sometimes, dunes form in inland areas far from the coastline. One such example is the Nebraska Sandhills in the U.S., which is the largest sand dune formation in the Western Hemisphere.
Extraterrestrial Dunes
Sand dunes are not exclusive to Earth; they can be found on other planetary bodies as well. For example, Titan, Saturn’s moon, has longitudinal dunes, and Mars has large fields of barchan dunes.
References
Miller, E. (2021, September 28). How wind shapes White Sands. New Mexico Magazine. https://www.newmexicomagazine.org/blog/post/how-sand-dunes-move-at-white-sands-national-park/
Tsoar, H. (2001). Types of aeolian sand dunes and their formation. In Geomorphological fluid mechanics (pp. 403-429). Berlin, Heidelberg: Springer Berlin Heidelberg.
