Conservation

Dune shapes

Crescentic

The most common dune form on Earth (and on Mars) is the crescentic. Crescent-shaped mounds generally are wider than long. The slipface is on the dune's concave side. These dunes form under winds that blow from one direction, and they also are known as barchans, or transverse dunes. Some types of crescentic dunes move faster over desert surfaces than any other type of dune. A group of dunes moved more than 100 meters per year between 1954 and 1959 in China's Ningxia Province; similar rates have been recorded in the Western Desert of Egypt. The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than 3 kilometers, are in China's Taklamakan Desert.

Linear

Straight or slightly sinuous sand ridges typically much longer than they are wide are known as linear dunes. They may be more than 160 kilometers long. Linear dunes may occur as isolated ridges, but they generally form sets of parallel ridges separated by miles of sand, gravel, or rocky interdune corridors. Some linear dunes merge to form Y-shaped compound dunes. Many form in bidirectional wind regimes. The long axes of these dunes extend in the resultant direction of sand movement.

Star

Radially symmetrical, star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around the margins of the sand seas, particularly near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 meters tall and may be the tallest dunes on Earth.

Dome

Oval or circular mounds that generally lack a slipface, dome dunes are rare and occur at the far upwind margins of sand seas.

Parabolic

U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. Sometimes these dunes are called U-shaped, blowout, or hairpin dunes, and they are well known in coastal deserts. Unlike crescentic dunes, their crests point upwind. The elongated arms of parabolic dunes follow rather than lead because they have been fixed by vegetation, while the bulk of the sand in the dune migrates forward.

Longitudinal and transverse dunes

Reversing dunes

Dune types

Sub-aqueous dunes

Sub-aqueous (underwater) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous in natural channels such as rivers and estuaries, and also form in engineered canals and pipelines. Dunes move downstream as the upstream slope is eroded and the sediment deposited on the downstream or lee slope.

Lithified dunes

A lithified (consolidated) sand dune is a type of sandstone that is formed when a marine or eolian sand dune becomes compacted and hardened. Once in this form, water passing through the rock can carry and deposit minerals, which can alter the hue of the rock. Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in Zion National Park.

Coastal dunes

Ecological succession on coastal dunes

As a dune forms, plant succession occurs. The conditions on an embryo dune are harsh, with salt spray from the sea carried on strong winds. The dune is well drained and often dry. Rotting seaweed, brought in by storm waves adds nutrients to allow pioneer species to colonize the dune. These pioneer species are marram grass, sea wort grass and other sea grasses in the UK. These plants are well adapted to the harsh conditions of the fore-dune, typically having deep roots which reach the water table, root nodules that produce nitrogen compounds, and protected stoma, reducing transpiration. Also, the deep roots bind the sand together, and the dune grows into a fore dune as more sand is blown over the grasses. The grasses add nitrogen to the soil, meaning other, less hardy plants can then colonize the dunes. Typically these are heathers and gorses. These too are adapted to the low soil water content and have small, prickly leaves which reduce transpiration. Heathers add humus to the soil, but have a pH of lower than 7, so make the soil slightly acidic. Heathers are usually replaced by coniferous trees which can tolerate the low pH. Coniferous forests and heathland are common climax communities for sand dune systems.

Desertification

Examples

• The Kelso Dunes, in the Mojave desert of California.

(large expanses of dunes)

• Forvie Sands within the Ythan Estuary complex, Aberdeenshire, Scotland
• Great Sand Dunes National Park
• Western Sahara
• White Sands National Monument
• Rig-e Jenn in the Central Desert of Iran
• Indiana Dunes / Sleeping Bear Dunes National Lakeshore - Lake Michigan
• Algodones Dunes near Brawley, California
• Guadalupe-Nipomo Dunes, Central Coast California, see Dunes Center, Guadalupe, CA

World's highest dunes

Note: This table is based on estimates and incomplete information. Dune Height from Base feet/meters Height from Sea Level feet/meters Location Notes Average Highest Area Dunes 1,526/465? ~6,500/~1,980? Isaouane-n-Tifernine Sand Sea, Algerian Sahara Highest in Africa Big Daddy/Dune 7 1,256/383  ? Sossuvlei Dunes, Namib Desert, Namibia Star Dune >750/230 ~8,950/2,730 Great Sand Dunes National Park, Colorado, USA Highest in North America,

World's Highest Altitude Dunes?

Sand dune systems

(coastal dunes featuring succession)

• Kenfig Burrows. South Wales
• Margam burrows, South Wales
• Studland, Dorset, England
• Murlough Sand Dunes, Newcastle, Co Down, Northern Ireland
• Morfa Harlech sand dunes, Gwynedd, North Wales
• Newborough warren, North Wales

Extraterrestrial dunes

Dunes can likely be found in any environment where there is a substantial atmosphere, winds, and dust to be blown. Dunes are common on Mars, and have also been observed in the equatorial regions of Titan by the Cassini probe's radar.

1. ^ http://www.planetary.org/blog/article/00000733/

• The Physics of Blown Sand (1941) by Ralph Bagnold
• Nouakchott, Mauritania. NASA Earth Observatory. Retrieved on 2006-04-28.

External links

• Coastal Sand Dunes
• Magilligan Dunes, Northern Ireland
• Dune pattern identification, U.S. Army
• Treading Lightly: Minimum Impact Dune Hiking
• Dune Racers of the Empty Quarter - video of sand dunes in the United Arab Emirates
• The Bibliography of Aeolian Research

Find out more about Dune on Wikipedia