Erg (landform)
An erg (also sand sea or dune sea, or sand sheet if it lacks
.Geography
Sand seas and dune fields generally occur in regions
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 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.
Description
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
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
By nature, ergs are very active. Smaller dunes form and migrate along the flanks of the larger dunes and sand ridges. Occasional
Individual dunes in ergs typically have widths, lengths, or both dimensions greater than 500 m (1,600 ft).[2] 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.[12]
Extraterrestrial ergs
Ergs are a geological feature that can be found on
Venus
At least two ergs have been recognized by the
Mars
Mars shows very large ergs, especially next to the polar caps, where dunes can reach a considerable size.[15] 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
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.[18]
See also
- Aeolian processes – Processes due to wind activity
- Blowout (geomorphology) – Depressions in a sand dune ecosystem caused by the removal of sediments by wind
- Desert pavement – Type of desert earth surface
- Hamada – Desert landscape with mostly rock instead of sand
- List of ergs
- Médanos (geology) – type of sand dune
- Yardang – Streamlined aeolian landform
References
- ^ "Issaouane Erg, Algeria". NASA Earth Observatory. Archived from the original on 2006-10-01. Retrieved 2006-05-18.
- ^ a b c d "Summary: Sand Seas/Ergs/Dune Fields". Desert Guide. United States Army Corps of Engineers. Retrieved 2006-05-18.[permanent dead link]
- ISBN 978-0-231-10207-0.
- ^ Landforms in the World: Aeolian Landform (08. Erg)
- NASA Goddard Space Flight Center. Archived from the originalon 2006-08-28. Retrieved 2006-05-18.
- ISBN 978-0-520-02280-5.
- ISBN 978-0-19-160983-1.
- ^ Wilson, I. 1971. Desert sandflow basins and a model for the development of ergs. Geographical Journal, v. 137, Pt. 2, pp. 180–199.
- ^ Glennie, K. W. 1970. Desert sedimentary environments: Developments in sedimentology 14, Enclosure 4. New York: American Elsevier Publishing Co.
- ^ Breed, C. S., and T. Grow. 1979. Morphology and distribution of dunes in sand seas observed by remote sensing. In A study of global sand seas, edited by E. D. McKee. U.S. Geological Survey Professional Paper 1052, pp. 253–302.
- ^ Breed, C. S., S. G. Fryberger, S. Andrews, C. K. McCauley, F. Lennartz, D. Gebel, and K. Horstman. 1979. Regional studies of sand seas using Landsat (ERTS) imagery. In A study of global sand seas, edited by E.D. McKee. U.S. Geological Survey Professional Paper 1052, pp. 305–397.
- ISBN 978-3-540-85909-3.
- ^ Fenton, L. K. (2005). "Seasonal Movement of Material on Dunes in Proctor Crater, Mars: Possible Present-Day Sand Saltation" (PDF). Lunar and Planetary Science XXXVI (2005).
- ^ Greeley, R., et al. (1992), Aeolian features on Venus: Preliminary Magellan results Archived 2008-06-15 at the Wayback Machine, Journal of Geophysical Research, 97(E8), 13,319–13,345.
- ^ Britt, Robert Roy (2003-11-10). "Sand Dunes on Mars Reach Dizzying Heights". Space.com. Archived from the original on 2006-03-07.
- ^ Stiles, Lori (2006-05-04). "Titan's Seas Are Sand". UA News. University of Arizona. Archived from the original on October 23, 2007.
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- ^ Goudarzi, Sara (2006-05-04). "Saharan Sand Dunes Found on Saturn's Moon Titan". Space.com.