![Linear Dunes, Namib Sand Sea](https://upload.wikimedia.org/wikipedia/commons/0/03/Linear_Dunes%2C_Namib_Sand_Sea.jpg)
An erg (also sand sea or dune sea, or sand sheet if it lacks
dune
A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...
s) is a broad, flat area of
desert
A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
covered with
wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...
-swept
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
with little or no vegetative cover. The word is derived from the
Arabic
Arabic (, ' ; , ' or ) is a Semitic languages, Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C ...
word ''ʿarq'' (), meaning "dune field".
Strictly speaking, an erg is defined as a desert area that contains more than of
aeolian or wind-blown sand and where sand covers more than 20% of the surface.
Smaller areas are known as "dune fields". The largest hot desert in the world, the
Sahara
, photo = Sahara real color.jpg
, photo_caption = The Sahara taken by Apollo 17 astronauts, 1972
, map =
, map_image =
, location =
, country =
, country1 =
, ...
, covers and contains several ergs, such as the
Chech Erg
The Erg Chech ( ar, عرق شاش), is a large erg in southwestern Algeria and northern Mali.
Geography
It is an almost uninhabited part of the greater Sahara Desert, an inhospitable desert region with long, extremely hot summers and short, very w ...
and the
Issaouane Erg
The Issaouane Erg (also called Issaouane-N-Irrararene) is an approximately 38,000 km2 erg (sand sea) in Algeria's portion of the Sahara desert, located at .
Topography
The Issaouane Erg is located near the Ahaggar Mountains. It is part of a m ...
in
Algeria
)
, image_map = Algeria (centered orthographic projection).svg
, map_caption =
, image_map2 =
, capital = Algiers
, coordinates =
, largest_city = capital
, relig ...
. Approximately 85% of all the Earth's mobile sand is found in ergs that are greater than . Ergs are also found on other
celestial bodies
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
, such as
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
,
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
, and
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
's moon
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
.
Description
![Morocco Africa Flickr Rosino December 2005 84514010](https://upload.wikimedia.org/wikipedia/commons/b/bd/Morocco_Africa_Flickr_Rosino_December_2005_84514010.jpg)
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 wind
The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...
s. Active ergs are limited to regions that receive, on average, no more than 150 mm of annual precipitation.
The largest are in
northern
Northern may refer to the following:
Geography
* North, a point in direction
* Northern Europe, the northern part or region of Europe
* Northern Highland, a region of Wisconsin, United States
* Northern Province, Sri Lanka
* Northern Range, a ra ...
and
southern
Southern may refer to:
Businesses
* China Southern Airlines, airline based in Guangzhou, China
* Southern Airways, defunct US airline
* Southern Air, air cargo transportation company based in Norwalk, Connecticut, US
* Southern Airways Express, M ...
Africa
Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
,
central
Central is an adjective usually referring to being in the center of some place or (mathematical) object.
Central may also refer to:
Directions and generalised locations
* Central Africa, a region in the centre of Africa continent, also known as ...
and
western
Western may refer to:
Places
*Western, Nebraska, a village in the US
*Western, New York, a town in the US
*Western Creek, Tasmania, a locality in Australia
*Western Junction, Tasmania, a locality in Australia
*Western world, countries that id ...
Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
, and
Central Australia
Central Australia, also sometimes referred to as the Red Centre, is an inexactly defined region associated with the geographic centre of Australia. In its narrowest sense it describes a region that is limited to the town of Alice Springs and i ...
. In
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
, ergs are limited by the
Andes Mountains
The Andes, Andes Mountains or Andean Mountains (; ) are the List of mountain ranges#Mountain ranges by length, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range i ...
, but they do contain extremely large dunes in coastal
Peru
, image_flag = Flag of Peru.svg
, image_coat = Escudo nacional del Perú.svg
, other_symbol = Great Seal of the State
, other_symbol_type = Seal (emblem), National seal
, national_motto = "Fi ...
and northwestern
Argentina
Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...
. They are also found in several parts of the northeast coast of
Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
. The only active erg in
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
is in the
Gran Desierto de Altar
The Gran Desierto de Altar is one of the major sub-ecoregions of the Sonoran Desert, located in the Sonora, State of Sonora, in northwest Mexico. It includes the only active Erg (landform), erg dune region in North America. The desert extends acro ...
that extends from the
Sonoran Desert
The Sonoran Desert ( es, Desierto de Sonora) is a desert in North America and ecoregion that covers the northwestern Mexican states of Sonora, Baja California, and Baja California Sur, as well as part of the southwestern United States (in Arizona ...
in the northwestern Mexican state of
Sonora
Sonora (), officially Estado Libre y Soberano de Sonora ( en, Free and Sovereign State of Sonora), is one of the 31 states which, along with Mexico City, comprise the Administrative divisions of Mexico, Federal Entities of Mexico. The state is d ...
to the
Yuma Desert
The Yuma Desert is a lower-elevation section of the Sonoran Desert in the southwestern United States and the northwest of Mexico. It lies in the Salton basin. The desert contains areas of sparse vegetation and has notable areas of sand dunes. With ...
of
Arizona
Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...
and the
Algodones Dunes
The Algodones Dunes is a large sand dune field, or erg, located in the southeastern portion of the U.S. state of California, near the border with Arizona and the Mexican state of Baja California. The field is approximately long by wide and exte ...
of southeastern
California
California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...
. An erg that has been fixed by vegetation forms the
Nebraska Sandhills
The Sandhills, often written Sand Hills, is a region of mixed-grass prairie on grass-stabilized sand dunes in north-central Nebraska, covering just over one quarter of the state. The dunes were designated a National Natural Landmark in 1984.
...
.
Sand seas and dune fields generally occur in regions downwind of copious sources of dry, loose sand, such as dry
riverbed
A stream bed or streambed is the bottom of a stream or river (bathymetry) or the physical confine of the normal water flow ( channel). The lateral confines or channel margins are known as the stream banks or river banks, during all but flood ...
s and
deltas
A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarel ...
,
floodplain
A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...
s,
glacial outwash plains, dry
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
s, and
beach
A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles, etc., or biological sources, such as mollusc shel ...
es. 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
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary scie ...
barriers to windflow or by convergence of windflow. 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 one million years is required to build ergs with very large dunes, such as those on the
Arabian Peninsula
The Arabian Peninsula, (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") or Arabia, is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plate ...
, in North Africa, and in central Asia. Sand seas that have accumulated in subsiding structural and topographic basins, such as the
Murzuk Sand Sea of
Libya
Libya (; ar, ليبيا, Lībiyā), officially the State of Libya ( ar, دولة ليبيا, Dawlat Lībiyā), is a country in the Maghreb region in North Africa. It is bordered by the Mediterranean Sea to the north, Egypt to Egypt–Libya bo ...
, may attain great thicknesses (more than 1000 m) but others, such as the ergs of linear dunes in the
Simpson Desert
The Simpson Desert is a large area of dry, red sandy plain and dunes in the Northern Territory, South Australia and Queensland in central Australia. It is the fourth-largest Deserts of Australia, Australian desert, with an area of .
The desert ...
and
Great Sandy Desert
The Great Sandy Desert is an interim Australian bioregion,[IBRA Version 6.1](_blank)
data of Australia, may be no thicker than the individual dunes superposed on the
alluvial plain
An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the sma ...
. 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.
By nature, ergs are very active. Smaller dunes form and migrate along the flanks of the larger dunes and sand ridges. Occasional
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
fills basins formed by the dunes; as the water evaporates,
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
deposits are left behind.
Individual dunes in ergs typically have widths, lengths, or both dimensions greater than .
Both the regional extent of their sand cover and the complexity and great size of their dunes distinguish ergs from dune fields. The depth of sand in ergs varies widely around the world, ranging from only a few centimeters deep in the Selima Sand Sheet of Southern Egypt, to approximately in the Simpson Desert, and in the Sahara. This is far shallower than ergs in prehistoric times were. Evidence in the geological record indicates that some
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
and
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
ergs reached a mean depth of several hundred meters.
Extraterrestrial ergs
![Marsdunes mgs big](https://upload.wikimedia.org/wikipedia/commons/6/64/Marsdunes_mgs_big.gif)
Ergs are a geological feature that can be found on
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
s where an
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
capable of significant
wind erosion
Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erode, transport, and deposit materials ...
acts on the surface for a significant period of time, creating
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
and allowing it to accumulate.
Today at least three bodies in the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
, apart from Earth, are known to feature ergs on their surface: Venus, Mars and Titan.
Venus
At least two ergs have been recognized by the
Magellan probe
The ''Magellan'' spacecraft was a robotic space probe launched by NASA of the United States, on May 4, 1989, to map the surface of Venus by using synthetic-aperture radar and to measure the planetary gravitational field.
The ''Magellan'' pro ...
on
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
: the Aglaonice dune field, which covers approximately , and the Meshkenet dune field (~). These seem to be mostly transverse dune fields (with dune crests perpendicular to prevailing winds).
Mars
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
shows very large ergs, especially next to the polar caps, where dunes can reach a considerable size. 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
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
in October 2005 show sand dunes at Titan's equator much like those in deserts of Earth. One erg was observed to be more than long. Dunes are a dominant landform on Titan. Approximately 15-20% of the surface is covered by ergs with an estimated total area of 12–18 million km
2 making it the largest dune field coverage in the Solar System identified to date.
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.
See also
*
*
*
*
*
*
*
References
{{Deserts
Dunes
Aeolian landforms
Erosion landforms