Dune
7890
224367436
2008-07-08T15:22:09Z
130.126.224.243
/* World's highest dunes */
{{Otheruses1|sand formations}}
In physical [[geography]], a '''dune''' is a [[hill]] of [[sand]] built by [[aeolian processes]]. Dunes are subject to different forms and sizes based on their interaction with the wind. Most kinds of dune are longer on the windward side where the sand is pushed up the dune, and a shorter "slip face" in the lee of the wind. The "valley" or trough between dunes is called a ''slack.'' A "dune field" is an area covered by extensive sand dunes. Large dune fields are known as [[erg (landform)|erg]]s.
Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the [[beach]]. In most cases the dunes are important in protecting the land against potential ravages by storm waves from the [[sea]]. Although the most widely distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient [[lake]] or [[sea]] beds.
Dunes also form under the action of water flow ([[alluvial]] processes), on sand or [[gravel]] beds of [[river]]s, [[estuary|estuaries]] and the sea-bed.
The modern word "dune" came into English from French circa 1790 but it's an old [[Indo-European languages|Indo-European]] word that's found in most of the Slavic languages as well as the Germanic and Latin languages. In ancient times words cognate to "dune" probably had the meaning of a citadel or built-up hill fortification.<ref>http://dictionary.reference.com/search?q=dune</ref>
==Conservation==
Dune habitats provide niches for highly specialized plants and animals, including numerous [[rare species|rare]] and [[endangered species]]. Due to [[overpopulation|human population expansion]] dunes face destruction through recreation and land development, as well as alteration to prevent encroachment on inhabited areas. Some countries, notably the [[United States|U.S.]], [[New Zealand]], [[Great Britain]], [[Australia]], [[Canada]] and the [[Netherlands]] have developed extensive programs of dune protection. In the UK, a [[Biodiversity Action Plan]] has been developed to assess dunes loss and prevent future dunes destruction.
==Dune shapes==
===Crescentic===
Crescent-shaped mounds are generally wider than they are 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 [[barchan]]s, 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 the [[People's Republic of 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 sea]]s, 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 (geology)|blowout]], or hairpin dunes, and they are well known in coastal deserts. Unlike crescent shaped 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 (Seif) and transverse dunes===
[[Image:Seif.gif|thumb|left|Seif dunes.]]
Longitudinal dunes (also called Seif dunes, after the Arabic word for "sword"), elongate parallel to the prevailing wind, possibly caused by a larger dune having its smaller sides blown away. Seif dunes are sharp-crested and are common in the Sahara. They range up to 300 m (900 ft) in height and 300 km (200 mi) in length. In the southern third of the Arabian Peninsula, a region called the Empty Quarter because of its total lack of populatation, a vast erg called Rub al Khali conatins seif dunes that stretch for almost 200 km and reach heights of over 300m.
Seif dunes are thought to develop from barchans if a change of wind direction occurs. The new wind direction will lead to the development of a new wing and the over development of one of the original wings. If the prevailing wind then becomes dominant for a lengthy period of time the dune will revert to its barchan form, with one exaggerated wing. Should the strong wind then return the exaggerated wing will further extend so that eventually it will be supplied with sand when the prevailing wind returns. The wing will continue to grow under both wind conditions, thus producing a seif dune. On a seif dune the slip face develops on the side facing away from the strong wind, while the slip face of a barchan faces the direction of movement. In the sheltered troughs between highly developed seif dunes barchans may be formed because the wind is unidirectional.
A transverse dune is perpendicular to the prevailing wind, probably caused by a steady buildup of sand on an already existing minuscule mound.
===Reversing dunes===
[[Image:Dune 7 in the Namib Desert.jpeg|thumb|right|Complex dune: Dune 7 in the [[Namib desert]], one of the tallest in the world.]]
Occurring wherever winds periodically reverse direction, reversing dunes are varieties of any of the above shapes. These dunes typically have major and minor slipfaces oriented in opposite directions.
All these dune shapes may occur in three forms: simple, compound, and complex. Simple dunes are basic forms with a minimum number of slipfaces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed, and complex dunes are combinations of two or more dune types. A crescentic dune with a star dune superimposed on its crest is the most common complex dune. Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed.
==Dune types==
[[Image:DunePyla2.jpg|thumb|[[The Great Dune of Pyla]] is the largest dune in Europe.]]
===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 [[Channel (geography)|channels]] such as rivers and estuaries, and also form in engineered [[canal]]s and pipelines. Dunes move downstream as the upstream slope is eroded and the sediment deposited on the downstream or lee slope.
These dunes most often form as a continuous 'train' of dunes, showing remarkable similarity in [[wavelength]] and height.
Dunes on the bed of a channel significantly increase flow resistance, their presence and growth playing a major part in river [[flooding]].
===Lithified dunes===
A [[lithified]] (consolidated) sand dune is a type of [[sandstone]] that is formed when a marine or aeolian 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]].
A local slang term used for these consolidated dunes is "slickrock", a name that was introduced by pioneers of the old west because their steel-rimmed wagon wheels could not gain purchase on the rock.
===Coastal dunes===
[[Image:SandDunesSutherlandShire.jpg|thumb|Coastal dunes on the [[Cronulla sand dunes, Kurnell Peninsula|Kurnell Peninsula]] ]]
Dunes form where '''constructive waves''' encourage the accumulation of sand, and where prevailing '''onshore winds''' blow this sand inland. There needs to be obstacles e.g. vegetation, pebbles etc. to trap the moving sand grains. As the sand grains get trapped they start to accumulate, this is the start of dune formation. The wind then starts to affect the mound of sand by eroding sand particles from the windward side and depositing them on the leeward side. Gradually this action causes the dune to “migrate” inland, as it does so it accumulates more and more sand.
Dunes provide [[privacy]] and shelter from the [[wind]].
====Ecological succession on coastal dunes====
[[Image:Curonian spit.jpg|thumb|left|270px|Coastal dunes in [[Curonian spit]].]]
As a dune forms, plant [[Ecological succession|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, and composed of calcium carbonate from seashells. 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 [[United Kingdom|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 [[heather]]s and [[gorse]]s. 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 [[conifer]]ous trees which can tolerate the low [[pH]]. Coniferous forests and [[heathland]] are common [[climax community|climax communities]] for sand dune systems.
Young dunes are called [[yellow dune]]s, dunes which have high humus content are called [[grey dune]]s. Leaching occurs on the dunes, washing humus into the slacks, and the slacks may be much more developed than the exposed tops of the dunes. It is usually in the slacks that more rare species are developed and there is a tendency for the dune slacks soil to be waterlogged and where only marsh plants can survive. These plants would include: creeping willow, cotton grass, yellow ins, reeds, and rushes. As for the species, there is a tendency for natterjack toads to breed here.
==Desertification==
{{main|Desertification}}
[[Image:Highway sand dune.JPG|thumb|right|One way of preventing sand from accumulating in roadways is planting trees and vegetation along the road.]]
One of the biggest problems posed by sand dunes is their encroachment on human habitats. Sand dunes move through a few different means, all of them helped along by wind. One way that dunes can move is through [[saltation (geology)|saltation]], where sand particles skip along the ground like a rock thrown across a pond might skip across the water's surface. When these skipping particles land, they may knock into other particles and cause them to skip as well. With slightly stronger winds, particles collide in mid-air, causing [[sheet flow]]s. In a major [[dust storm]], dunes may move tens of meters through such sheet flows. And like snow, sand [[avalanche]]s, falling down the steep slopes of the dunes that face away from the winds, also moving the dunes forward.
Sand threatens buildings and crops in Africa, the Middle East and China. Drenching sand dunes with oil stops their migration, but this approach is highly destructive to the dunes habitat and uses a finite resource. [[Sand fence]]s might also work, but researchers are still analyzing optimum fence designs. Preventing sand dunes from overwhelming cities and agricultural areas has become a priority for the [[United Nations Environment Programme]].
==Examples==
[[Image:KhongorynEls.jpg|thumb|right|Sand dunes of Khongoryn Els, [[Gobi Gurvansaikhan National Park|Gurvansaikhan NP]], [[Mongolia]].]]
*The dunes in the [[Thar]] desert in [[Rajasthan]], [[India]]
*[[Tottori Sand Dunes]],[[Tottori Prefecture]],[[Japan]]
*The [[Kelso Dunes]], in the [[Mojave desert]] of [[California]].
*[[Sands of Forvie]] within the [[Ythan Estuary]] complex, [[Aberdeenshire]], [[Scotland]].
*[[Great Sand Dunes National Park]], [[Colorado]], [[United States]].
*[[Western Sahara]]
*[[White Sands National Monument]], USA.
*[[Rig-e Jenn]] in the Central Desert of [[Iran]].
*Rig-e Lut in the Southeast of [[Iran]].
*[[Indiana Dunes]] / [[Sleeping Bear Dunes National Lakeshore]], [[Lake Michigan]], USA.
*[[Algodones Dunes]] near [[Brawley, California]], USA.
*[[Guadalupe-Nipomo Dunes]], Central Coast California, USA.
*[[Lencoi Maranhenses]] in the state of [[Maranhão]], [[Brazil]].
*[[Mer'eb Dune]] (also written as Merheb) in United Arab Emirates, used as an arena for motor sports and skiing.
*Monahans Sandhills State Park near [[Odessa, Texas]].
*Oxwich Dunes, near Swansea on the Gower Peninsula in South Wales.
*[[Winterton Dunes]]-[[Norfolk]],[[England]]
*The Killpecker sand dunes of Southwestern Wyoming, USA
*Jockey's Ridge State Park - Outer Banks, North Carolina
===World's highest dunes===
{| class="wikitable"
|+ Note: This table is based on estimates and incomplete information.
|- style=background:silver; color:black
! width="200"|Dune !! width="130"|Height from Base feet/meters !! width="160"|Height from Sea Level feet/meters !! width="460"|Location !! width="180"|Notes
|-
!Cerro Blanco
| ? || ~6791/2,080 || [[Sechura Desert]], Nazca || Highest in the world
|-
!Badain Jaran Dunes
| ~1640/500 || ~6640/2,020 || [[Badain Jaran Desert]], Alashan Plain, Inner Mongolia, [[Gobi Desert]], China || World's Tallest Stationary Dunes<ref>http://www.newscientist.com/article.ns?id=dn6712</ref>
|-
!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 || ? || Sossusvlei Dunes, [[Namib Desert]], Namibia / Near Walvis Bay [[Namib Desert]], Namibia ||
|-
!Mount Tempest
| ~920/280 || ~920/280 || [[Moreton Bay]], [[Brisbane]], Australia || Highest in Australia
|-
!Star Dune
| >750/230 || ~8,950/2,730 || [[Great Sand Dunes National Park and Preserve]], Colorado, USA || Highest in North America
|-
![[Dune of Pyla]]
| ~345/105 || ~699/130 || Bay of [[Arcachon]], Aquitaine, France || Highest in Europe
|-
!Ming-Sha Dunes
| ? || 5,660/1,725 || [[Dunhuang]] Oasis, Taklamakan Desert, Gansu, China ||
|-
!Medanoso Dune
| ? || 4,921/1,500 or 7,923/2,415? || [[Atacama Desert]], Chile || Highest in South America?
|}
===Sand dune systems===
{{Globalize/UK}}
[[Image:Studlandfencing.jpg|thumb|An example of fencing at Studland]]
<!-- Commented out because image was deleted: [[Image:Coastal Dune.JPG|thumb|Typical dune found along the Gulf of Mexico. This dune is on Perdido Key Florida, in the Gulf Islands National Seashore]] -->
:(coastal dunes featuring succession)
*[[Ashdod Sand Dune]], [[Israel]]
*[[Kenfig Burrows]], [[South Wales]]
*[[Margam burrows]], [[South Wales]]
*[[Studland]], [[Dorset]], [[England]]
*[[Murlough Sand Dunes]], [[Newcastle, County Down|Newcastle]], [[County Down|Co Down]], [[Northern Ireland]]
*[[Morfa Harlech sand dunes]], [[Gwynedd]], [[North Wales]]
* [[Newborough warren]], [[North Wales]]
* [[Bamburgh Dunes]], [[Northumberland]], England
*[[Oregon Dunes National Recreation Area]], [[North Bend]], [[Oregon]], [[United States]]
*Mediterranean Dune in the southern coast of [[Lazio]], [[Italy]] ([[Circeo National Park]])
*Winterton, [[Norfolk]], [[England]]
*[[Ynyslas Sand Dunes]], [[Wales]]
*[[Sleeping Bear Dunes National Lakeshore]], [[Michigan]], [[United States]]
===Extraterrestrial dunes===
{{seealso|List of extraterrestrial dune fields}}
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 (planet)|Mars]], and have also been observed in the equatorial regions of [[Titan (moon)|Titan]] by the [[Cassini probe]]'s radar.
Titan's dunes include large expanses with modal lengths of about 20-30 km. The regions are not topographically confined, resembling sand seas. These dunes are interpreted to be longitudinal dunes whose crests are oriented parallel to the dominant wind direction, which generally indicates west-to-east wind flow. The sand is likely composed of hydrocarbon particles, possibly with some water ice mixed in.<ref>[http://www.planetary.org/blog/article/00000733/ Peeking Through the Haze: Titan's Surface, part II - The Planetary Society Blog | The Planetary Society<!-- Bot generated title -->]</ref>
==Images==
<gallery>
Image:Sand dune formation.png|A diagram showing the formation of a dune with a slipface
Image:Mesquite Sand Dunes.JPG|Mesquite Flat Dunes in [[Death Valley National Park]]
Image:Libya 4608 Idehan Ubari Dunes Luca Galuzzi 2007.jpg|Erg Awbari (Idehan Ubari) in the Sahara desert region of Fezzan in Libya
Image:Dune boarding.jpg|Some dunes are suitable for dune-boarding
Image:Send dunes in death valley national park.jpg|Sand dunes in [[Death Valley National Park]]. The image shows coppice dunes,which are formed around vegetation
Image:Beach access.JPG|A [[footbridge]] provides beach access and protects dunes
Image:Sossusvlei_oPEYRE.jpg|A sand dune in [[Namibia]]
Image:Dune sunrise.jpg|A dune in [[Morocco]]
Image:Sanddune.JPG|Sand dunes with scattered vegetation in [[UAE]]
Image:Parabolic dune.jpg|Parabolic dune partially stabilized by [[marram grass]]
</gallery>
==References==
* ''[[The Physics of Blown Sand]]'' (1941) by [[Ralph Bagnold]]
* {{cite web | url=http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17168 | title=Nouakchott, Mauritania | publisher=[[NASA Earth Observatory]] | accessdate=2006-04-28 }}
==Footnotes==
{{reflist}}
==External links==
*[http://www.geography-site.co.uk/pages/physical/coastal/dunes.html Coastal Sand Dunes]
*[http://www.geographyinaction.co.uk/Magilligan/Mag_intro.html Magilligan Dunes, Northern Ireland]
*[http://www.tec.army.mil/research/products/desert_guide/lpisheet/lpdunes1.htm Dune pattern identification, U.S. Army]
*[http://www.geocities.com/Yosemite/Falls/9200/lnt_dune_hiking.html Treading Lightly: Minimum Impact Dune Hiking]
*[http://www.andycarvin.com/archives/2005/06/dune_racers_of.html Dune Racers of the Empty Quarter] - video of sand dunes in the United Arab Emirates
*[http://www.lbk.ars.usda.gov/wewc/biblio/bar.htm The Bibliography of Aeolian Research]
*[http://www.saharasafaris.org/understanding/sanddunes.htm Sahara Dunes as investigated by native offroaders from Egypt]
*[http://www.goldcoastcity.com.au/dunepolicy Gold Coast Dune Management Policy]
==See also==
{{commons|Dune}}
*[[Antidune]]
*[[Barchan]]
*[[Earth science]]
*[[List of landforms]]
*[[Machair (geography)|Machair]]
*[[Masseira]]
*[[Sandhill]]
*[[Strand plain]]
*[[Singing Sand Dunes]]
[[Category:Coastal geography]]
[[Category:Dunes]]
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