A dune is a relief made up of Sable.
All the dunes being made up of sand, one does not speak about “sand dune”, except specifying the quality of the sand which composes it: “white sand dune”.
The word dune comes from a Germanic word dun which results in: hill.
Mechanisms of formation
The dunes are formed in or starting from zones where sand abundant and is not fixed by the vegetation (desert, beach, read river with the low water level). Sand is eroded and taken charges some by the Vent (deflation). It is transported to the short-nap cloths of the ground by Saltation, then accumulates when the competence of the wind falls (Versant under the wind). A dune can move by erosion of the slope to the wind and accumulation on the opposite slope.
The same process can occur under water with the favor of a marine Running ( hydraulic dune ).
Type of dune
A dune is known as sharp when it continues to move
The dunes can be isolated or aligned.
The most common form on the Ground (and Mars) of the dunes is that in half-moon, so called transverse . The hills in half-moon are, generally, broader than short. The former part of the dune is the concave side . These dunes are formed by the winds which blow all the time in the same direction.
Some types of dunes in half-moon move in the desert with an high speed with those of the other types of dunes; for example, in the province of Ningxia in China, a group of dunes has advanced per annum at the speed of 100 meters between 1954 and 1959. Similar speeds are recorded in the the Sahara.
The most imposing dunes in half-moon are in the desert of the Taklamakan in China, where the distance between two peaks can exceed three kilometers.
These are dunes longer than broad whose right or slightly sinuous peaks can reach up to 160 kilometers. They are seldom insulated, usually laid out in parallel, separated for kilometers from sand, gravel or corridors rock. Certain linear dunes are linked, by forming Y. These formations are typical areas undergoing of the blowing winds in two directions.
Out of star
The star dunes are pyramidal, with at least three sides which start from their top. They are formed in the areas knowing of the multidirectional winds. The star dunes grow more to the top than laterally and are typical of the Eastern Sahara. Another share, they have tendency to is formed in margins of the desert, in particular close to the natural barriers. Highest are probably those of the desert of Badain Jaran in China, where they can reach 500 meters height.
They have an oval or circular form, of which it misses a part on with dimensions one to be complete. The dunes in cupola are rare and are formed only in margins of the deserts.
The dunes in Parabole are U-shaped. They are typical coastal deserts. The longest dune in known parabola measures 12 kilometers
This type of dune is formed when at the ends of a sandy formation starts to appear vegetation which stops the progression of it, while the central part continues to advance. They are formed when the wind blows only in only one dominant direction.
All these types of dunes can exist in three forms: simple , made up and complex . The simple dunes are hills with a minimal number on escarpés sides which define geometrical typology of it. The made up dunes are larger dunes, surmounted smaller similar dunes. The complex dunes, they, are made of several different types of dunes. A dune in half-moon supporting a star dune is one of the complex dunes most common. The dunes are simple when the wind remains constant during their formation.
These dunes are formed on the coasts where the winds and the contribution of sediments thanks to the currents allow the accumulation of sand on the beaches. The upper part of the beach, once it is dry, can lose of sand, particularly if it is fine, because of the wind. The formation of the dunes is done according to the direction of the wind. In the formation process of the dunes, the plants pionnières play a fundamental role, ensuring the fixing and the stability of the sand which, otherwise, would be dispersed. The dunes provide to the beach shelters, vis-a-vis the wind and with the sun.
These dunes, pushed by the winds, invade the grounds gradually.
It is for the Dunes of Flandres, which extend between Nieuport and Dunkirk, that the Spanish were beaten on sea by Maarten Tromp in 1639, and on ground by Turenne in 1658 ( to see Bataille of the Dunes .
Dunes in the world
One finds the dunes wind on the littoral or in the Désert, although the dunes are not the relief dominating of the hot deserts. The great sand extents of the the Sahara name Erg . Some have the form of a croissant : they are called the Barkhane S .
In France, the Dune of Pyla (or Pilat), on the Littoral of the the Gironde, is highest of highest Europe culminating around 100 Mr. in the world (500 meters) are those of Badain Jaran in Mongolia-Interior (China) with a strong moisture starting from 20 centimetres under the ground coming from the molten snow of the Qilian mounts, located at 500 kilometers in south-west.
Song of the dunes
See also: song of the dunes
The song of the dunes is the name given to the noise emitted by certain dunes in the deserts when the grains of sand which compose them enter in resonance. These dunes are named dunes mugissantes or dunes musicantes .
- sif erg
- Great Sand Dunes National Park and Preserves: highest dunes of North America in the Colorado.
- Dune (novel)
- the fixing of the dunes in Aquitaine
- Photographs of Dunes (Libya, Namib)
- Physical of the dunes
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