Erg (landform) - Description

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, ergs are limited by the Andes Mountains, but they do contain extremely large dunes in coastal Peru and northwestern Argentina. They are also found in several parts of the northeast coast of Brazil. The only active erg in North America is in the Gran Desierto de Altar (31°57′N 114°08′W / 31.95°N 114.14°W / 31.95; -114.14) that extends from the Sonoran Desert in the northwestern Mexican state of Sonora to the Yuma Desert of Arizona and the Algodones Dunes of southeastern California. An erg that has been fixed by vegetation forms the Nebraska Sandhills (42°08′N 102°11′W / 42.13°N 102.19°W / 42.13; -102.19).

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 one 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 (25°54′N 13°54′E / 25.90°N 13.90°E / 25.90; 13.90) of Libya, may attain great thicknesses (more than 1000 m) but others, such as the ergs of linear dunes in the Simpson Desert (24°57′S 137°25′E / 24.95°S 137.42°E / -24.95; 137.42) and Great Sandy Desert (19°42′S 122°37′E / 19.70°S 122.62°E / -19.70; 122.62) 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 500 m (1,600 ft). 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 1 m (3.3 ft) in the Simpson Desert, and 21–43 m (69–141 ft) in the Sahara. This is far shallower than ergs in prehistoric times were. Evidence in the geological record indicates that some Mesozoic and Paleozoic ergs reached a mean depth of several hundred meters.