Erg (landform)

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. The term takes its name from the Arabic word erg (), meaning "dune field." Strictly speaking, an erg is defined to be a desert area that contains more than 125 square kilometers of eolian sand and where sand covers more than 20 percent of the surface. Smaller areas are known as dune fields. The largest desert in the world, the Sahara, is 7 million km² and contains several ergs, such as Erg Chech in Algeria. Approximately 85% of all the Earth's mobile sand is found in ergs that are larger than 32 000 km².

Description

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. Active ergs are limited to regions that receive, on the average, no more than 150 mm of annual precipitation. The largest are in northern and southern Africa, central and western Asia, and Central Australia. In South America, they are areally limited by the Andes Mountains, but they contain extremely large dunes in coastal Peru and northwestern Argentina. The only active erg in North America is in the Gran Desierto de Altar portion of the Sonoran Desert in northwestern Sonora, Mexico, which extends northward into the Yuma Desert of Arizona and the Algodones Dunes of southeastern California. An erg that has been fixed by vegetation forms the Nebraska Sand Hills.

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. 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. 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. 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) 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. 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 fills basins formed by the dunes; as the water evaporates, salt deposits are left behind.

Individual dunes in ergs typically have widths, lengths, or both dimensions greater than 500m. 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 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. Today at least two bodies, apart from Earth, are known in the solar system to feature ergs on their surface.

Ergs on Mars

Mars shows very large ergs, expecially 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.

Ergs on 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^*. 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.^*