   #copyright

Dune

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Mesquite Flat Dunes in Death Valley National Park
   Enlarge
   Mesquite Flat Dunes in Death Valley National Park

   In physical geography, a dune is a hill of sand built by eolian
   processes. Bare dunes are subject to shifting location and size based
   on their interaction with 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 ergs.

   Some coastal areas have one or more sets of dunes running parallel to
   the shoreline directly inland from the beach. In most such 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 rivers, estuaries and the sea-bed.

   The word 'dune' derives from a medieval Germanic or Norse word - "dun",
   a hill.

Conservation

   A footbridge provides beach access and protects dunes
   Enlarge
   A footbridge provides beach access and protects dunes

   Dune habitats provide niches for highly specialized plants and animals,
   including numerous rare and endangered species. Due to 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 USA and Great Britain 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

   A sand dune in Namibia.
   Enlarge
   A sand dune in Namibia.

Crescentic

   The most common dune form on Earth (and on Mars) is the crescentic
   dune. 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 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 centre 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 dune partially stabilized by marram grass
   Enlarge
   Parabolic dune partially stabilized by marram grass

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

   Seif dunes
   Enlarge
   Seif dunes

   Longitudinal dunes, also called Seif dunes, 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.

   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 overdevelopment 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

   Complex dune: Dune 7 in the Namib desert, one of the tallest in the
   world.
   Enlarge
   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

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.

   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 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.

   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

   Coastal dunes in Curonian spit.
   Enlarge
   Coastal dunes in Curonian spit.
   The fore dune and first yellow dune at Studland, England.
   Enlarge
   The fore dune and first yellow dune at Studland, England.

   Dunes form on coasts where the backshore can support and onshore winds
   encourage the accumulation of sand blown inland from off a beach. Any
   part of the upper beach, once dry, can lose sand to the wind,
   especially if the sand is fine, and dune formation proceeds in the
   direction towards which the predominant wind direction is blowing.

   Dunes provide privacy and shelter from the wind.

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.

   Young dunes are called yellow dunes, dunes which have high humus
   content are called grey dunes. 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.

   For the snow analogue to a sand dune see sastruga.

Desertification

   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, 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 flows. In a major dust storm, dunes may move
   tens of meters through such sheet flows. And like snow, sand
   avalanches, 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 fences 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

     * 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, Colorado, USA
     * 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 Centre,
       Guadalupe, CA
     * Lencoi Maranhenses in the state of Maranhao Brazil.

World's highest dunes

   CAPTION: 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?
   Dune of Pilat ~345/105 ~699/130 Bay of Arcachon, Aquitaine, France
   Highest in Europe
   Mount Tempest ~920/280 ~920/280 Moreton Bay, Brisbane, Australia
   Highest in Australia
   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?
   Badain Jaran Dunes ~1640/500 ~6640/2,020 Badain Jaran Desert, Alashan
   Plain, Inner Mongolia, Gobi Desert, China World's Tallest 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
     * Bamburgh Dunes, Northumberland, England

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.

   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.
   Retrieved from " http://en.wikipedia.org/wiki/Dune"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
