A dune is a hill made of sand moved by wind or water. It often looks like a mound, ridge, or hill. A place with many dunes is called a dune system or a dune complex. A large group of dunes is called a dune field. Large, flat areas covered with sand and dunes, with little or no plants, are called ergs or sand seas. Dunes come in many shapes and sizes. Most dunes are longer on the stoss (upflow) side, where wind pushes sand up the dune, and have a shorter slip face on the lee side. The area between dunes is called a dune slack.

Dunes are most common in deserts because the lack of water makes it hard for plants to grow, which would otherwise stop dunes from forming. However, dunes are not only found in deserts. They can also be found along coastlines, near streams in dry areas, in places where glaciers left sand behind, and in areas where loose sandstone breaks down into sand. Dunes under water can form when water moves sand or gravel in rivers, estuaries, and the sea bed.

Some coastal areas have one or more rows of dunes that run parallel to the shoreline, just inland from the beach. These dunes often help protect land from damage caused by storm waves from the sea. People sometimes build artificial dunes to protect coasts. Wind and water can move dunes, which can cause problems. For example, the town of Eucla in Western Australia had to move in the 1890s because dunes moved and caused issues.

The word "dune" came into English from French around 1790. The French word came from the Middle Dutch word "dūne."

Formation

There is no exact difference between ripples, dunes, and draas, as they are all made of the same types of materials. Dunes are usually taller than 7 cm and can have ripples, while ripples are smaller than 3 cm. A draa is a very large landform shaped by wind, with a length of several kilometers and a height of tens to hundreds of meters. Draas may have dunes on their surface.

Dunes are made of sand-sized particles, such as quartz, calcium carbonate, snow, gypsum, or other materials. The side of a dune facing the direction the wind is coming from is called the stoss side. The opposite side, where sand slides down, is called the lee side. Sand moves up the stoss side by slowly sliding (creep) or bouncing (saltation) and then slides down the lee side. The area where sand slides down is called a slip face.

The Bagnold formula describes how fast particles can be moved by wind.

Aeolian dunes

There are five main types of dunes: crescentic, linear, star, dome, and parabolic. Dune areas can be simple, compound, or complex. Simple dunes are single, basic dunes. Compound dunes are larger dunes with smaller dunes of the same type on them. Complex dunes are combinations of different dune types.

Barchan dunes are crescent-shaped mounds that are wider than they are long. The slipfaces (slopes where sand slides) are on the concave sides of the dunes. These dunes form when the wind blows mostly in one direction. If there is not much sand, they stay as separate crescents. If there is more sand, they may join to form barchanoid ridges or transverse dunes.

Some crescentic dunes move quickly across deserts. In China’s Ningxia Province, a group of dunes moved more than 100 meters per year between 1954 and 1959. Similar speeds have been recorded in Egypt’s Western Desert. The largest crescentic dunes on Earth, with widths over three kilometers, are found in China’s Taklamakan Desert.

Barchan dunes can merge to form barchanoid ridges, which then turn into linear (or slightly curved) transverse dunes. These dunes lie across the wind direction, with the wind blowing perpendicular to their ridges.

Seif dunes are long, straight (or slightly curved) dunes with two slipfaces. They are sharp-crested and often run parallel to each other. The name "seif" comes from the Arabic word for "sword." These dunes can be over 160 kilometers long and are visible in satellite images.

Seif dunes form in areas with winds that blow in two directions. Their long axes align with the direction of sand movement. Some linear dunes merge to form Y-shaped compound dunes.

Scientists debate how seif dunes form. One theory suggests they develop when barchan dunes move into areas with bidirectional winds, causing one side of the crescent to grow longer. Another theory says they form from wind vortices in unidirectional winds. Between well-developed seif dunes, barchan dunes may form because the wind becomes unidirectional.

Seif dunes are common in the Sahara. They can be up to 300 meters tall and 300 kilometers long. In the Rub’ al Khali (Empty Quarter) of the Arabian Peninsula, seif dunes stretch nearly 200 kilometers and reach heights of over 300 meters.

Linear loess hills called pahas look similar to seif dunes. These hills formed during the last ice age under permafrost conditions with sparse tundra vegetation.

Star dunes are pyramid-shaped mounds with slipfaces on three or more arms that spread out from the top. They form in areas with winds that blow in many directions. Star dunes grow upward, not sideways. Wind constantly reshapes them, but GPS measurements show they can move. They are common in the Grand Erg Oriental of the Sahara and near topographic barriers in other deserts. In China’s Badain Jaran Desert, star dunes can be as tall as 500 meters and may be the tallest outside South America.

Dome dunes are round or oval mounds with flat tops and no slipface. Small dome dunes are low and smooth. Mega-domes are much larger, similar in size to star dunes. The largest mega-domes in Saudi Arabia reach 150 meters in height and over 1.5 kilometers in width. They are covered with secondary dunes and sometimes have sand ridges extending outward. Mega-domes are rare and found at the upwind edges of sand seas, often near star dunes.

Fixed crescentic dunes that form on the leeward sides of playas (dry lake beds) and river valleys in arid and semiarid regions are called lunettes, source-bordering dunes, bourrelets, or clay dunes. They can be made of clay, silt, sand, or gypsum. These materials are eroded from the basin floor or shore, carried up the concave side of the dune, and deposited on the convex side. In Australia, some lunettes are up to 6.5 kilometers long, 1 kilometer wide, and 50 meters high. They also occur in parts of Africa and the United States, especially Texas.

Parabolic dunes are U-shaped mounds of sand with convex noses and long arms trailing upwind. They form when wind erodes vegetated sand, creating a U-shaped depression. The arms are held in place by vegetation, with the longest arm on Earth reaching 12 kilometers. These dunes are also called U-shaped, blowout, or hairpin dunes and are common in coastal deserts. Unlike crescent dunes, their crests point upwind. Most parabolic dunes are not very tall, except at their noses, where vegetation stops or slows sand accumulation.

In plan view, parabolic dunes are U- or V-shaped mounds of well-sorted, fine to medium sand with elongated arms extending upwind. Slipfaces often appear on the outer side of the nose and on the outer slopes of the arms.

Parabolic dunes are found in semiarid areas where water is retained in the lower parts of the dune and underlying soil. Their stability was once thought to depend on vegetation, but recent research suggests water is the main factor. Vegetation like grasses, shrubs, and trees help anchor the arms. In inland deserts, parabolic dunes form from blowouts in sand sheets with limited vegetation. They can also form from beach sands and extend inland into vegetated areas near lakes or coasts.

Simple parabolic dunes have one set of arms trailing upwind. Compound parabolic dunes have multiple sets of arms. Complex parabolic dunes include smaller dunes or merged forms, such as barchanoid or linear shapes.

Like crescent dunes, parabolic dunes form in areas with strong, mostly unidirectional

Subaqueous dunes

Subaqueous (underwater) dunes form on a bed of sand or gravel due to the movement of water. These dunes are commonly found in natural water channels, such as rivers and estuaries, as well as in man-made canals and pipelines. As water flows, dunes shift downstream because the upstream side is worn away, and sediment is deposited on the downstream side. In subaqueous barchan dunes, sediment is lost from the ends, called horns.

These dunes often appear as a continuous line, with each dune having similar size and spacing. The shape of a dune provides clues about the environment where it formed. For example, rivers create asymmetrical ripples, where the steeper side faces the direction of water flow. Ripple marks found in rock layers can help scientists determine the direction of ancient water currents and identify where sediments came from.

Dunes on the bottom of a channel increase resistance to water flow. Their presence and growth can greatly influence the likelihood of river flooding.

Lithified dunes

A hardened sand dune is a type of sandstone that forms when a sand dune from the ocean or wind becomes pressed together and solid. When this happens, water moving through the rock can carry minerals that change the rock’s color. Layers of these hardened dunes stacked on top of each other can create patterns that look like crossed lines, such as those found in Zion National Park in the western United States.

A nickname for these hardened sand dunes in the southwestern United States is "slickrock." This term was first used by people from the Old West because their wagon wheels made of steel could not move easily over the smooth, hard surface of the rock.

Desertification

Sand dunes can harm humans when they move into areas where people live. Dunes move because of wind, and there are several ways this happens. One way is called saltation, where sand particles bounce along the ground like a ball. When these bouncing particles land, they can push other particles, causing them to move slowly in a process called creep. With stronger winds, sand particles may collide in the air, creating sheet flows that move large groups of sand together. During big dust storms, dunes can move tens of meters through these sheet flows. Additionally, sand can slide down the slipface of dunes—the side facing away from the wind—helping the dunes move forward.

Sand dunes threaten buildings and crops in Africa, the Middle East, and China. Covering dunes with oil can stop them from moving, but this uses a valuable resource and harms the homes of animals that live there. Sand fences can also slow dunes, but scientists are still studying the best fence designs. Stopping dunes from taking over towns, villages, and farmland is now a key goal for the United Nations Environment Programme. Planting plants on dunes helps keep them stable.

Conservation

Dune habitats offer homes for many plants and animals, some of which are rare or endangered. As more people live in areas near dunes, these habitats are being damaged by building homes, recreational activities, and efforts to stop sand from moving into populated areas. Countries like the United States, Australia, Canada, New Zealand, the United Kingdom, Netherlands, and Sri Lanka have created important programs to protect dunes by keeping them stable. In the United Kingdom, a Biodiversity Action Plan has been made to study how dunes are being lost and to stop dunes from being destroyed in the future.

Examples

Dunes can be found in places that have a thick atmosphere, strong winds, and dust that can be blown by the wind. Dunes are common on Mars and in the equatorial regions of Titan.

Titan's dunes cover large areas, with average lengths of about 20–30 kilometers. These regions are not limited by natural landforms and look like sand seas. Scientists believe these dunes are longitudinal, meaning their crests run parallel to the main wind direction, which is usually from west to east. The sand is likely made of carbon-based particles, with some water ice possibly mixed in.

Dunes are a common theme in science fiction. They appear in depictions of dry desert planets, such as in the 1956 film Forbidden Planet and Frank Herbert’s 1965 novel Dune. The desert planet Arrakis (also known as Dune) in the Dune series inspired the Star Wars franchise, which includes fictional planets like Tatooine, Geonosis, and Jakku that have dunes as a major feature.