Dune

Aeolian dunes

Five main dune shapes exist: crescentic, linear, star, dome, and parabolic. Dune fields can be simple, with isolated basic dunes; compound, where larger dunes host smaller dunes of the same kind; or complex, mixing different types. Barchan dunes are crescent-shaped, wider than long, with their slipfaces on the inside curve. They arise from winds blowing steadily from one direction, called unimodal winds, and appear as separate crescents when sand is scarce.

More sand can cause them to merge into barchanoid ridges, then into transverse dunes. Some crescentic dunes shift faster across deserts than any other type; for instance, a group in China's Ningxia Province moved over 100 meters yearly from 1954 to 1959, with similar speeds seen in Egypt's Western Desert. The biggest crescentic dunes, averaging over three kilometers from crest to crest, are found in China's Taklamakan Desert. Seif dunes are linear or slightly wavy, with two slipfaces that give them sharp crests, named after the Arabic word for sword.

They can stretch over 160 kilometers, making them easy to spot in satellite images. These dunes are linked to bidirectional winds, and their long axes align with the net sand movement direction. Some linear dunes join to form Y-shaped compound dunes. How seif dunes form is debated.

Ralph Bagnold thought some develop when a barchan dune enters a bidirectional wind zone and one arm lengthens. Others believe vortices in a unidirectional wind create them. In the sheltered troughs between tall seif dunes, barchans can appear because the dunes force the wind to blow from only one direction.

Subaqueous dunes

Underwater dunes made of sand or gravel are shaped by flowing water. They are common in rivers, estuaries, and man-made channels like canals and pipelines. These dunes creep downstream as water erodes their upstream side and deposits material on the downstream slope. In barchan-shaped dunes, sediment is lost from the tips, called horns. Dunes usually appear in long, regular trains with very similar heights and spacing. Their shape reveals how they formed: river dunes are asymmetrical, with a steep face pointing downstream. Such ripple patterns preserved in rock layers can show which way the current once flowed. Dune fields on a channel bed greatly increase water resistance, and their growth is a key factor in river flooding.

Desertification

Sand dunes become a problem when they creep into places where people live. Wind drives their movement in several ways. In saltation, sand grains bounce along the ground like skipping balls, and when they land, they can knock other grains into motion, a process called creep. Stronger winds cause grains to collide in midair, creating sheet flows; during a major dust storm, these flows can shift dunes tens of meters.

Sand avalanches tumbling down the slipface—the side opposite the wind—also push the dune forward. Sand endangers buildings and crops in Africa, the Middle East, and China. Coating dunes with oil halts their advance but wastes a valuable resource and harms wildlife. Sand fences can slow the movement, though geologists are still studying the best designs.

Planting vegetation on dunes helps hold them in place. The United Nations Environment Programme now prioritizes protecting towns, villages, and farmland from being buried by sand.

Examples

Notable dune fields and sand seas include the Alexandria Coastal Dunefields in the Eastern Cape, South Africa; Witsand Nature Reserve in the Kalahari Desert, South Africa; the white dunes of De Hoop Nature Reserve, South Africa; the dunes of the Suguta Valley in northwestern Kenya; the dunes of the Danakil Depression in northeastern Ethiopia; the dunes of Sossusvlei in the Namib-Naukluft National Park, Namibia; Chad Basin National Park in northern Nigeria; the coastal dunes of Iona National Park in southwestern Angola; Khawa dunes in the Kgalagadi Transfrontier Park, Botswana; La Dune Rose in Gao, northern Mali; Erg Aoukar in southeastern Mauritania extending into Mali; Erg Chech in southwestern Algeria and northern Mali; Erg Chebbi and Erg Chigaga in southern Morocco; Grand Erg Oriental in northeastern Algeria and southern Tunisia; Grand Erg Occidental in western Algeria; the Idehan Ubari and Idehan Murzuq in southwestern Libya; the Corralejo dunes in the Canary Islands, Spain; the Rebiana Sand Sea in southeastern Libya; the Great Sand Sea in southeastern Libya and southwestern Egypt; the Selima Sand Sheet in northwestern Sudan; the dunes of the Bayuda Desert in northern Sudan; the dunes of the Lompoul Desert in northwestern Senegal; the coastal dunes of Bazaruto Island, Mozambique; Erg du Djourab in northern Chad; the dunes of the Mourdi Depression in northeastern Chad; the dunes of Tin Toumma Desert in southeastern Niger; Grand Erg de Bilma in the Ténéré, northern Niger; the dunes of Oursi in the Sahel Region, northern Burkina Faso; Tanzania's shifting sands near Olduvai Gorge; Sunset view dunes in Alankuda village on Kalpitiya peninsula, Sri Lanka; the dunes in the Thar Desert in India and Pakistan; Tottori Sand Dunes in Tottori Prefecture, Japan; Chara Sands in Zabaykalsky Krai, Russia; Rig-e Jenn in the Central Desert of Iran; Rig-e Lut in southeastern Iran; the Ilocos Norte Sand Dunes in the Philippines, particularly Paoay Sand Dunes; Moreeb Dune in Liwa Oasis, United Arab Emirates; Gumuk Pasir Parangkusumo near Parangtritis beach in Yogyakarta, Indonesia; Lautan Pasir, a volcanic dune in Bromo Tengger Semeru National Park, Indonesia; Mui Ne, Vietnam; Sharqiya Sands, Oman; Teri, a red dune complex in southern India; the dunes of the Taklamakan Desert in southwest Xinjiang, Northwest China; and tukulans of the Central Yakutian Lowland, Russia.