An erg (also sea sand or dune sea , or sand sheet if it does not have sand dunes) is large, the flat area of ââthe desert is covered with wind-swept sand with little or no vegetative cover. This term takes its name from the Arabic word ? Arq ( ??? ), which means "dune field". Strictly speaking, erg is defined as a desert region containing more than 125 km <1> (48Ã, sqÃ, mi) of aeolian or windblown sand and where the sand covers more than 20% of the surface. The smaller area is known as the "field mound". The world's largest hot desert, the Sahara, covers 9 million square kilometers (3.5 ÃÆ'â ⬠10 6 sqÃ, mi) and contains some ergs, like Chech Erg ( 24,57 à ° N 2.59 à ° W / 24,57; -2,59 ) and Issaouane Erg ( 31.18 à ° N 7.93 à ° E / 31.18; 7.93 ) in Algeria. About 85% of all Earth's mobile sand is found in ergs greater than 32,000 km 2 (12,355 sq mi). Erg is also found in other celestial bodies, such as Venus, Mars, and Saturn's moon Titan.
Video Erg (landform)
Description
Ergs are concentrated in two large belts ranging from 20 ° to 40 ° N and 20 ° to 40 ° latitude, covering the area through which the dry air drifts and subsides from the pasat wind. Erg is actively limited to the receiving region, on average, no more than 150 mm of annual rainfall. The largest are in northern and southern Africa, central and western Asia, and Central Australia. In South America, erg is limited by the Andes Mountains, but they contain enormous sand dunes on the coast of Peru and northwest Argentina. They are also found in some parts of the northeast coast of Brazil. The only active erg in North America is at Gran Desierto de Altar ( 31,95 à ° N 114,14 à ° W / 31.95; -114.14 ) that stretches from the Sonoran Desert in the northwestern state of Sonora to the Arizona Yuma Desert and Algodones Dunes from southeastern California. An erg that has been improved by vegetation to form Nebraska Sandhills ( 42.13Ã, à ° N 102.19Ã, à ° W / 42.13; -102.19 ).
Sea sand and dunes usually occur in areas with low wind direction from dry sources, loose sand, such as dry and delta river beds, floodplains, glacial washing plains, dry lakes, and beaches. Almost all of the major ergs are located in the wind direction of the river bed in an area that is too dry to support large vegetation cover and thus can cause long wind erosion. Sand from these abundant sources migrates against the wind and builds very large sand dunes where the movement stops or is slowed by topographic obstacles to wind flow or by wind convergence. All ergs and deserts tend to migrate against the wind hundreds of kilometers from their sand source. Such accumulation takes a long time. At least a million years is needed to build erg with enormous dunes, such as in the Arabian Peninsula, in North Africa, and in Central Asia. Sea sand that has accumulated in the ruins of structural basins and topography, such as the Sand Sea Murzuk ( 25,90 à ° N 13.90 à ° E / 25,90; 13.90 ) from Libya, can reach a large thickness (more than 1000 m) others, such as linear hill ergs in the Simpson Desert ( 24,95 à ° S 137.42 à ° E / -24.95; 137.42 ) and Great Sandy Desert ( 19,70 à ° S 122,62 à ° E / -19,70; 122,62 ) from Australia, probably no thicker than each of the sand dunes placed on the alluvial plain. In a sea of ââsand in a particular area, the dunes tend to be of a single type. For example, there are erg or linear hill fields, crescent moon hills, star sand dunes, and parabolic dunes, and these dunes tend to have consistent orientation and size.
Naturally, erg is very active. Smaller dunes are formed and migrate along the sides of larger sand dunes and sand. The rain deposit fills the basin formed by the dunes; when the water evaporates, the salt deposit is left behind.
Individual hills in erg usually have width, length, or both dimensions greater than 500 m (1,600 ft). Both the area coverage of their sand cover and the complexity and large size of their sand dunes distinguish erg from the field of dunes. The depth of sand in erg varies worldwide, ranging from just a few centimeters away at Selima Sand Sheet in Southern Egypt, up to about 1 m (3.3 m) in the Simpson Desert, and 21-43 m (69- 141Ã, ft) in Sahara. This is much more banal than erg in prehistoric times. The evidence in the geological record shows that some Mesozoic and Paleozoic ergos reach an average depth of several hundred meters.
Maps Erg (landform)
Extraterrestrial ergs
Ergs is a geological feature that can be found on planets where atmospheres capable of significant wind erosion act on the surface for significant periods of time, creating sand and allowing it to accumulate. Today at least three bodies, other than Earth, are known in the solar system to display erg on its surface: Venus, Mars, and Titan.
Venus
At least two erg have been recognized by the Magellan probe on Venus: the field of Aglaonice mound, which covers about 1,290 km 2 (498Ã, sqÃ, mi), and Meshkenet mound fields (~ 17,120 km 2 or 6.610Ã, sqÃ, mi). These areas appear to be mostly sand dunes (with the tops of dunes perpendicular to the prevailing winds). Mars
Mars shows a very large erg, especially next to the polar cap, where the dunes can reach a size large enough. Ergs on Mars can show strange shapes and patterns, because of the complex interactions with the underlying surface and wind direction.
Titan
The radar images captured by the Cassini spacecraft while flying by Titan in October 2005 show sand dunes on the Titan's equator like the one in the desert Earth. One observed erg has a length of over 930 miles (1,497 km). The dunes are the dominant land form in Titan. About 15-20% of the surface is covered by ergs with an estimated total area of ââ12-18 million km 2 making it the largest hill field coverage in the solar system identified to date.
The dunes are believed to be formed by the wind generated as a result of the tidal forces of Saturn in Titan's atmosphere. The images are proof that these dunes are built from winds that blow in one direction before switching the other way and then back in the first direction and so on, causing the sand dunes to form on long parallel lines. This tidal wind combined with the west-to-east zonal wind of Titan makes the hills aligned west-to-east almost everywhere except close to the mountains, which change the direction of the wind.
Sand on Titan may have formed when liquid methane showered and eroded the ice sheet, possibly in the form of banjir bandang. Or, the sand can also be derived from the organic solids produced by photochemical reactions in the Titan atmosphere.
See also
- Aeolian process
- Explosion (geology)
- desert sidewalks
- Hamada
- List erg
- MÃÆ'Ã and danos (geology)
- Yardang
References
External links
- Aeolian Research Bibliography
Source of the article : Wikipedia