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| Sunday, 23 April 2006 |
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Junior Observer |  |
| News Business Features Editorial
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Dunes in the desert We have already featured deserts and sandstorms. This week, we will talk about sand dunes, another interesting sand-related geological feature. In physical geography, a dune is a hill of sand built by eolian (wind-related) 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.
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 (damage) 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. Sand dune systems are of great importance to bio-diversity, and many are conserved. Nearest the sea shore, the dunes are usually dominated by marram grass (Ammophila arenaria) and the vegetation is 'open', ie with a lot of bare ground between the marram plants. Further back from the sea, the vegetation is 'closed', the marram cannot survive, and the dune surface is covered by various other kinds of vegetation and plant communities, many of which are grass-dominated (especially by Festuca rubra), while others are dominated by bryophytes, lichens ('grey dunes'), or bracken (Pteridium aquilinum). The younger dunes are rich in calcium carbonate from seashells, allowing lime-loving species to flourish, but older parts of most dune systems are acidic, owing to the leaching (filtering) effect of rain, and can support plants such as heathers (Erica spp. and Calluna vulgaris). The inland portions of many dune systems have been used as pine plantations. Dune slacks The hollows between dunes are called slacks. Often, but not always, they are wet and marshy and many of the plants found in them are also characteristic of base-rich fens (low lying areas) and lake-margins. Blow-outs Damage to the vegetation surface allows the wind to excavate hollows called blow-outs, which can eventually develop into new slacks. *** Types of dunes 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 are also 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 metres 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 three kilometres are in China's Taklamakan Desert. Linear Straight or slightly sinuous (curved) sand ridges, typically much longer than they are wide, are known as linear dunes. They may be more than 160 kilometres 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 during bi-directional wind blows. The long axes of these dunes extend in the resultant direction of sand movement. Star Radially symmetrical (divided on any point to cross the centre, the two sides would be similar), 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 multi-directional wind blows. Star dunes grow upward rather than laterally (sideways). 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 (position of rivers, roads, etc.). In the southeast Badain Jaran Desert of China, the star dunes are up to 500 metres 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 (lengthened) 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 upward. 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. Sub-aqueous dunes Sub-aqueous (underwater) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous (found everywhere) 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. Longitudinal and transverse 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. 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. |
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