Le Dune di San Vincenzo EarthCache

Le dune di San Vincenzo
 

Tipi di Duna

Una prima classificazione delle dune può essere fatta suddividendole in base alla loro forma osservata dall'alto.

A mezzaluna

La forma più comune sulla Terra (e su Marte) di una duna è la forma a mezzaluna, detta anche barcana se arcuata o trasversale se presenta una morfologia tendenzialmente rettilinea perpendicolarmente alla direzione del vento. Le dune a forma di mezzaluna, generalmente, sono più larghe che lunghe. La parte anteriore della duna è il lato concavo. Queste dune sono formate da venti che soffiano da una sola direzione.

Alcuni tipi di dune a mezzaluna si muovono, o migrano, sulla superficie del deserto con una velocità superiore a quello degli altri tipi di duna; ad esempio, nella provincia di Ningxia in Cina un gruppo di dune si sono mosse con una velocità di 100 metri all'anno tra il 1954 ed il 1959. Velocità simili sono registrate nel deserto ad ovest dell'Egitto.

Le dune a mezzaluna più imponenti sulla Terra sono nel deserto Taklamakan in Cina, dove la distanza tra le creste di due dune può superare i 3 chilometri.

Lineari

Sono dune più lunghe che larghe con creste diritte o un po' sinuose e possono raggiungere lunghezze maggiori di 160 km. Raramente sono creste isolate, mentre di solito sono disposte in file parallele separate tra loro da chilometri di sabbia, ghiaia o da corridoi rocciosi. Alcune dune lineari si uniscono tra loro formando delle conformazioni a "Y". Queste formazioni sono tipiche nelle regioni in cui sono presenti regimi di venti bidirezionali.

A stella

Le dune a stella sono formazioni collinose a piramide con simmetria radiale e con tre o più lati che partono dalla parte alta della duna. Si formano nelle aree in cui vi è la presenza di regimi di venti multidirezionali. Le dune a stella crescono più verso l'alto che lateralmente e sono tipiche del deserto del Sahara orientale. In altri luoghi hanno la tendenza a formarsi ai margini del deserto, specialmente vicino a delle barriere naturali.

Probabilmente le più alte dune a stella della Terra sono presenti nel deserto di Badain Jaran in Cina, dove possono raggiungere i 500 metri di altezza.

A cupola

Hanno forma ovale o circolare a cui manca una piccola parte su un fianco per essere complete. Le dune a cupola sono rare e si formano solitamente ai margini sopravvento del deserto.

A parabola

Le dune a parabola sono colline di sabbia a forma di "U" e sono tipiche dei deserti costieri. La duna a parabola più estesa conosciuta ha una lunghezza di 12 chilometri.

Questo tipo di duna si forma quando alle estremità della formazione sabbiosa si comincia a formare della vegetazione che ne ferma il movimento mentre la parte centrale continua ad essere libera di muoversi. Si riesce a formare solamente quando il vento spira da una sola direzione predominante.

Tipi complessi

Duna 7 nel deserto della Namibia, una delle dune più alte al mondo

Questi tipi di duna possono essere presenti in tre forme differenti: semplice, composta e complessa. Le dune semplici sono colline con un minimo numero di lati scoscesi che ne definiscono la tipologia geometrica. Le dune composte sono grandi dune sormontate da dune simili più piccole. Le dune complesse sono invece la combinazione di due o più tipi differenti di dune.

Una duna a mezzaluna con una duna a stella sovrapposta sulla relativa cresta è la duna complessa più comune. Le dune sono semplici quando il regime dei venti è rimasto costante durante la formazione della duna, mentre per le dune composte e complesse si sono avuti cambiamenti di intensità e senso del vento durante la loro creazione.

Dune costiere

Le dune possono formare sulle coste dove l'apporto di sedimenti sabbiosi da parte delle onde e delle correnti costiere permette la formazione di una spiaggia. Nella parte superiore della spiaggia (spiaggia emersa o backshore), può verificarsi l'asportazione di parte del materiale sabbioso (specialmente le frazioni granulometriche più fini) a causa dell'azione del vento. Questo materiale va a formare dune eoliche, che si sviluppano nella direzione dei venti dominanti. Nel processo di formazione delle dune le piante pioniere svolgono un ruolo fondamentale, consentendo la fissazione e la stabilizzazione della sabbia che altrimenti verrebbe dispersa altrove. Le dune forniscono alla spiaggia luoghi al riparo dal vento e dal sole.

Dune subacquee

Le dune subacquee si formano sui letti sabbiosi o ghiaiosi, sotto l'azione dei flussi dell'acqua. Sono solitamente ubicate in canali naturali come fiumi ed estuari ed anche in canali e condutture costruiti dall'uomo. Questo tipo di duna si muove verso valle; il loro materiale di costruzione è prelevato dalla parte alta mediante l'erosione effettuata dallo scorrere dell'acqua ed i depositi si hanno nei punti di minor pendenza e nella parte bassa del corso. Normalmente queste dune mostrano una somiglianza alla lunghezza ed altezza delle onde sovrastanti.

The sand dune of San Vincenzo

Lunettes

Fixed crescentic dunes that form on the leeward margins of playas and river valleys in arid and semiarid regions in response to the direction(s) of prevailing winds, are known as lunettes, source-bordering dunes, bourrelets and clay dunes. They may be composed of clay, silt, sand, or gypsum, eroded from the basin floor or shore, transported up the concave side of the dune, and deposited on the convex side. Examples in Australia are up to 6.5 km long, 1 km wide, and up to 50 metres high. They also occur in southern and West Africa, and in parts of the western United States, especially Texas.

Linear

Large linear dunes in the Great Sand Sea in southwest Egypt, seen from the International Space Station. The distance between each dune is 1.5-2.5 km.

Straight or slightly sinuous sand ridges typically much longer than they are wide are known as linear dunes. They may be more than 160 kilometres (100 miles) long. 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.

Linear loess hills known as pahas are superficially similar. These hills appear to have been formed during the last ice age under permafrost conditions dominated by sparse tundra vegetation.

Star

An isolated star dune.

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 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

Schematic of coastal parabolic dunes.

U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms are held in place by vegetation; the largest arm known on Earth reaches 12 km. Sometimes these dunes are called U-shaped, blowout, or hairpin dunes, and they are well known in coastal deserts. Unlike crescent shaped dunes, their crests point upwind. The bulk of the sand in the dune migrates forward.

In plan view, these are U-shaped or V-shaped mounds of well-sorted, very fine to medium sand with elongated arms that extend upwind behind the central part of the dune. There are slip faces that often occur on the outer side of the nose and on the outer slopes of the arms.

These dunes often occur in semiarid areas where the precipitation is retained in the lower parts of the dune and underlying soils. The stability of the dunes was once attributed to the vegetative cover but recent research has pointed to water as the main source of parabolic dune stability. The vegetation that covers them—grasses, shrubs, and trees—help anchor the trailing arms. In inland deserts, parabolic dunes commonly originate and extend downwind from blowouts in sand sheets only partly anchored by vegetation. They can also originate from beach sands and extend inland into vegetated areas in coastal zones and on shores of large lakes.

Most parabolic dunes do not reach heights higher than a few tens of metres except at their nose, where vegetation stops or slows the advance of accumulating sand.

Simple parabolic dunes have only one set of arms that trail upwind, behind the leading nose. Compound parabolic dunes are coalesced features with several sets of trailing arms. Complex parabolic dunes include subsidiary superposed or coalesced forms, usually of barchanoid or linear shapes.

Parabolic dunes, like crescent dunes, occur in areas where very strong winds are mostly unidirectional. Although these dunes are found in areas now characterized by variable wind speeds, the effective winds associated with the growth and migration of both the parabolic and crescent dunes probably are the most consistent in wind direction.

The grain size for these well-sorted, very fine to medium sands is about 0.06 to 0.5 mm. Parabolic dunes have loose sand and steep slopes only on their outer flanks. The inner slopes are mostly well packed and anchored by vegetation, as are the corridors between individual dunes. Because all dune arms are oriented in the same direction, and, the inter-dune corridors are generally swept clear of loose sand, the corridors can usually be traversed in between the trailing arms of the dune. However to cross straight over the dune by going over the trailing arms, can be very difficult. Also, traversing the nose is very difficult as well because the nose is usually made up of loose sand without much if any vegetation.

A type of extensive parabolic dune that lacks discernible slip faces and has mostly coarse grained sand is known as a Zibar. The term zibar comes from the Arabic word to describe "rolling transverse ridges ... with a hard surface". The dunes are small, have low relief, and can be found in many places across the planet from Wyoming (United States) to Saudi Arabia to Australia. Spacing between zibars ranges from 50 to 400 metres and they don't become more than 10 metres high. The dunes form at about ninety degrees to the prevailing wind which blows away the small, fine-grained sand leaving behind the coarser grained sand to form the crest.

Longitudinal (Seif) dunes

Rub' al Khali (Arabian Empty Quarter) sand dunes imaged by Terra (EOS AM-1). Most of these dunes are seif dunes. Their origin from barchans is suggested by the stubby remnant "hooks" seen on many of the dunes. Wind would be from left to right.

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 (980 ft) in height and 300 km (190 mi) in length. In the southern third of the Arabian Peninsula, a vast erg called the Rub' al Khali or the Empty Quarter, contains seif dunes that stretch for almost 200 km and reach heights of over 300 m.

Seif dunes are thought to develop from barchans if a change of the usual 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 build-up of sand on an already existing minuscule mound.

Reversing dunes

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.

Sub-aqueous dunes

Dunes (foreground) formed sub-aqueously at Tyne Sands in England, UK. Here the dunes are temporarily exposed to the air by a neap tide.

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 in typical bedform construction.
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 color of the rock. Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in the Zion National Park in the western United States.

A slang term that is used in the Southwestern States (of the U.S.A.) for those consolidated and hardened sand dunes is "slickrock", a name that was introduced by pioneers of the Old West because their steel-rimmed wagon wheels could not gain traction on the rock.

Nabkha dunes

A nabkha, or coppice dune, is a small dune anchored by vegetation. They usually indicate desertification or soil erosion, and serve as nesting and burrow sites for animals.

Coastal dunes

Coastal dunes covered in grasses around the mouth of the Liver Å river in Denmark. Dunes form where constructive waves encourage the accumulation of sand, and where prevailing onshore winds blow this sand inland. There need to be obstacles—for example, vegetation, pebbles and so on—to trap the moving sand grains. As the sand grains get trapped they start to accumulate, starting 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

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, 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.

These plants are well adapted to the harsh conditions of the foredune 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 foredune 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, heaths and gorses. These too are adapted to the low soil water content and have small, prickly leaves which reduce transpiration.

Heather adds humus to the soil and is usually replaced by coniferous trees, which can tolerate low soil pH, caused by the accumulation and decomposition of organic matter with nitrate leaching, Coniferous forests and heathland are common climax communities for sand dune systems.