Issaouane Erg, Algeria. Both star and crescentic dunes can be seen in this orbital photo.

An erg (also sand sea or dune sea) is a large, relatively flat area of desert covered with wind-swept sand with little to no vegetation cover. A desert is a Landscape or region that receives very little precipitation. Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's) Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles ^[1] The term takes its name from the Arabic word erg (عرق), meaning "dune field". Arabic (ar الْعَرَبيّة (informally ar عَرَبيْ) in terms of the number of speakers is the largest living member of the Semitic language ^[2] Strictly speaking, an erg is defined to be a desert area that contains more than 125 square kilometers of eolian or wind-blown sand^[3] and where sand covers more than 20% of the surface. Square Kilometre ( US spelling square kilometer) symbol km2, is a decimal multiple of the SI unit of Aeolian (or Eolian or Æolian) processes pertain to the activity of the Winds and more specifically to the winds' ability to shape the surface of the ^[2] Smaller areas are known as dune fields. In physical Geography, a dune is a Hill of Sand built by Aeolian processes. The largest hot desert in the world, the Sahara, is 9,000,000 km² and contains several ergs, such as Erg Chech in Algeria. The Sahara (الصحراء الكبرى aṣ-ṣaḥrā´ al-kubra, "The Great Desert" is the world's largest hot Desert and the world's second largest Algeria ( ar [[Arabic]] الجزائر, Al Jaza'ir ælʤæˈzæːʔir Amazigh: ⴷⵥⴰⵢⴻⵔ Dzayer) officially the People's ^[4] Approximately 85% of all the Earth's mobile sand is found in ergs that are larger than 32,000 km². ^[3] Ergs are also found on other celestial bodies, such as Venus and Saturn's moon Titan. s are significant physical entities, associations or structures which current Science has confirmed to exist in Space.

Description

Erg Chebbi, Morocco

Ergs are concentrated in two broad belts between 20° to 40° N and 20° to 40° S latitudes, which include regions crossed by the dry, subsiding air of the trade winds. Erg Chebbi (عرج شبّي is the sole Saharan erg in Morocco. The trade winds (also called trades) are the prevailing pattern of easterly winds found in the Tropics near the Earth's Equator. Active ergs are limited to regions that receive, on the average, no more than 150 mm of annual precipitation. ^[2] The largest are in northern and southern Africa, central and western Asia, and Central Australia. North Africa or Northern Africa is the Northernmost Region of the African Continent, separated by the Sahara from Sub-Saharan Southern Africa is the Southernmost Region of the African Continent, variably defined by Geography or Geopolitics. Central Asia is a region of Asia from the Caspian Sea in the west to central China in the east and from southern Russia in the north to northern Pakistan in the south Southwest Asia or Southwestern Asia (largely overlapping with the Middle East) is the southwestern portion of Asia. Central Australia/Alice Springs Region is one of the five regions in the Northern Territory. In South America, they are areally limited by the Andes Mountains, but they contain extremely large dunes in coastal Peru and northwestern Argentina. South America is a Continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a The Andes form the world's longest exposed Mountain range. They lie as a continuous chain of highland along the western coast of South America. Peru (Perú Piruw Piruw officially the Republic of Peru ( reˈpuβlika del peˈɾu is a country in western South America. For a topic outline on this subject see List of basic Argentina topics. The only active erg in North America is in the Gran Desierto de Altar portion of the Sonoran Desert in northwestern portion of the Mexican state of Sonora, which extends northward into the Yuma Desert of Arizona and the Algodones Dunes of southeastern California. The Gran Desierto de Altar is one of the major portions of the Sonoran Desert of Mexico including the only active erg dune region in North America. The Sonoran Desert (sometimes called the Gila Desert after the Gila River or the Low Desert in opposition to the higher Mojave Desert) is Sonora is one of the 31 federal states of Mexico and is located in the northwest of the country The Yuma Desert is a lower-elevation section of the Sonoran Desert in the southwestern United States and the northwest of Mexico. The State of Arizona ( is a state located in the southwestern region of the United States. The Algodones Dunes is a large erg (sand Dune field located in southeastern portion of the U California ( is a US state on the West Coast of the United States, along the Pacific Ocean. An erg that has been fixed by vegetation forms the Nebraska Sand Hills. The Sand Hills is a region of mixed-grass Prairie in north-central Nebraska, covering just over one quarter of the state

Sand seas and dune fields generally occur in regions downwind of copious sources of dry, loose sand, such as dry riverbeds and deltas, floodplains, glacial outwash plains, dry lakes, and beaches. A delta is a Landform where the mouth of a River flows into an Ocean, Sea, Estuary, Lake or another river ||-||-||-||-||-||-||-||}A floodplain, or flood plain, is flat or nearly flat land adjacent to a Stream or River that experiences occasional or periodic A sandur (plural sandar is a glacial outwash Plain formed of Sediments deposited by meltwater at the terminus of a Glacier. A lake (from Latin lacus) is a Terrain feature (or Physical feature) a body of Liquid on the surface of a world that is localized to the Almost all major ergs are located downwind from river beds in areas that are too dry to support extensive vegetative cover and are thus subject to long-continued wind erosion. Sand from these abundant sources migrates downwind and builds up into very large dunes where its movement is halted or slowed by topographic barriers to windflow or by convergence of windflow. Topography ( topo-, "place" and graphia, "writing" is the study of Earth 's Surface features or those of Planets Entire ergs and dune fields tend to migrate downwind as far as hundreds of kilometers from their sources of sand. Such accumulation requires long periods of time. At least 1 million years are required to build ergs with very large dunes, such as those on the Arabian Peninsula, in North Africa, and in central Asia. The Arabian Peninsula (in Arabic: شبه الجزيرة العربية šibh al-jazīra al-ʻarabīya or جزيرة العرب jazīrat al-ʻarab) ^[5] Sand seas that have accumulated in subsiding structural and topographic basins, such as the Murzuk Sand Sea of Libya, may attain great thicknesses (more than 1000 m^[6]) but others, such as the ergs of linear dunes in the Simpson and Great Sandy Deserts of Australia, may be no thicker than the individual dunes superposed on the alluvial plain. Murzuk or Murzuq (مرزق is a town in south west Libya. It is one of the main settlements of the Murzuq municipality Libya ( ليبيا ar-Latn Lībiyā; Libyan vernacular: Lībya; Amazigh:) officially the Great Socialist People's Libyan Arab The Simpson Desert occupies approximately 176500 square kilometres of central Australia. There is another desert called " Great Sandy Desert " in Oregon, US. An alluvial plain is a relatively flat Landform created by the deposition of sediment over a long period of time by one or more Rivers coming from highland regions Within sand seas in a given area, the dunes tend to be of a single type. For example, there are ergs or fields of linear dunes, of crescentic dunes, of star dunes, and of parabolic dunes, and these dune arrays tend to have consistent orientations and sizes. ^[7][8]

By nature, ergs are very active. Smaller dunes form and migrate along the flanks of the larger dunes and sand ridges. Occasional precipitation fills basins formed by the dunes; as the water evaporates, salt deposits are left behind. In Meteorology, precipitation (also known as one class of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric Salt is a Dietary mineral composed primarily of Sodium chloride that is essential for Animal life but toxic to most land plants

Individual dunes in ergs typically have widths, lengths, or both dimensions greater than 500 m. ^[2] Both the regional extent of their sand cover and the complexity and great size of their dunes distinguish ergs from dune fields.

Extraterrestrial ergs

Ergs located in Proctor Crater, Mars, as taken by the Mars Global Surveyor. The Mars Global Surveyor ( MGS) was a US Spacecraft developed by NASA and the Jet Propulsion Laboratory and launched

See also: List of extraterrestrial dune fields

Ergs are a geological feature that can be found on planets where an atmosphere capable of significant wind erosion acts on the surface for a significant period of time, creating sand and allowing it to accumulate. This is a list of Dune fields not on Earth which have been given official names by the International Astronomical Union. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " Aeolian (or Eolian or Æolian) processes pertain to the activity of the Winds and more specifically to the winds' ability to shape the surface of the Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles Today at least three bodies, apart from Earth, are known in the solar system to feature ergs on their surface: Venus, Mars and Titan. The Solar System consists of the Sun and those celestial objects bound to it by Gravity.

Venus

Three dune fields have been recognized by the Magellan probe on Venus. The Magellan spacecraft was a space probe sent to the planet Venus, the first post- Voyager unmanned spacecraft to be launched by NASA The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University ^[9] These seem to be mostly transverse dune fields (with dune crests perpendicular to prevailing winds).

Mars

Mars shows very large ergs, especially next to the polar caps, where dunes can reach a considerable size. ^[10] Ergs on Mars can exhibit strange shapes and patterns, due to complex interaction with the underlying surface and wind direction.

Titan

Radar images captured by the Cassini spacecraft as it flew by Titan in October 2005 show sand dunes at Titan's equator much like those in deserts of Earth. Cassini–Huygens is a joint NASA / ESA / ASI Robotic spacecraft mission currently studying the planet Saturn and its TemplateInfobox Planet.--> Titan (ˈtaɪtən, or as ^[11] The sand dunes are believed to be formed by wind generated as a result of tidal forces from Saturn on Titan's atmosphere.

The images are evidence that these dunes were built from winds that blow in one direction before switching to another and then back to the first direction and so on, causing the sand dunes to build up in long parallel lines. These tidal winds combined with Titan's west-to-east zonal winds create dunes aligned west-to-east nearly everywhere except close to mountains, which alter wind direction.

The sand on Titan might have formed when liquid methane rained and eroded the ice bedrock, possibly in the form of flash floods. Alternatively, the sand could also have come from organic solids produced by photochemical reactions in Titan's atmosphere. ^[12]

List of ergs

Dasht-e Lut, Iran, as seen from space.

Africa Algeria Grand Erg Oriental (extends into Tunisia) Grand Erg Occidental Erg Chech (extends into Mali) Erg Iguidi (extends into Mauritania) Erg er-Raoui Erg Tiferine Erg Tidohaine Erg Ferradj Erg Admer Botswana Kalahari Desert Chad Erg Djourab Erg Idrisi Egypt Ramla El Kebir Erg Somaya Erg Abunugar Erg Abu Ramada Negev Desert (extends into Israel) Libya Erg Awbari Erg Ubari Erg Titersine Erg Idehan Murzuq Erg Ouan Kasa Erg Tannezouft Erg Takarkouri Erg Mehedjibat Erg Tassegefit Mali Erg Achaif Erg Atouila Erg Outouila Mauritania Erg Aoukar Erg Taokest Morocco Erg Chebbi Erg Lihoudi Erg Chegaga Erg Mghiti Erg Jerboia Erg Zemoul Namibia Namib Desert Niger Jadal Erg Brousset Ténéré Tafassasset Great Bilma Erg or 'Kaouar' (extends into Chad) Sudan Erg Selima

Asia China Taklamakan, Xinjiang Dzungaria, Xinjiang Tengger Desert, Ningxia Ordos Desert, Inner Mongolia Iran Dasht-e Lut Kazakhstan Moin Kum Kyzyl Kum (extends into Uzbekistan and Turkmenistan) Mongolia Nomin Mingan Gobi Pakistan and India Thar Desert Saudi Arabia Rub' al Khali Dahna Great Nefud Jafurah Nefud Es Sirr Nefud Eth Thuwairat Nefud Shudaiah Turkmenistan Karakum Desert Turan Zaunguz Australia Gibson Desert Great Sandy Desert Great Victoria Desert Simpson Desert North America Mexico Sonoran Desert United States Navajo Sandstone (former Jurassic age erg has since formed sandstone) Nebraska Sand Hills, Nebraska (active 800-1000 years ago, now vegetated) Yuma Desert, Arizona Algodones Dunes, California South America Atacama Desert, Chile Chile Atacama Desert Peru Great Ica Desert

See also

• Hamada

References

External links

• The Bibliography of Aeolian Research