Desert pavement

Desert pavement near Barstow, California, in the Mojave Desert

A desert pavement is a desert surface that is covered with closely packed, interlocking angular or rounded rock fragments of pebble and cobble size.

Formation

Several theories have been proposed for their formation. The more common theory is that they form by the gradual removal of the sand, dust and other fine grained material by the wind and intermittent rain leaving only the larger fragments behind. This does not continue indefinitely, however, because once the pavement has been formed it can act as a barrier to further erosion. Secondly, it has been proposed that desert pavement forms from the shrink/swell properties of the clay underneath the pavement; when precipitation is absorbed by clay it causes it to expand and later when it dries it cracks along planes of weakness. This geomorphic action is believed to have the ability to transport small pebbles to the surface over time; it stays this way due to the lack of abundant precipitation that would otherwise destroy the pavement development through transport of the clasts or excessive vegetative growth.

The newest theory of pavement formation comes from careful studies of places like Cima Dome, in the Mojave Desert of California, by Stephen Wells and his coworkers. Cima Dome is a place where lava flows of recent age, geologically speaking, are partly covered by younger soil layers that have desert pavement on top of them, made of rubble from the same lava. Obviously the soil has been built up, not blown away, and yet it still has stones on top. In fact, there are no stones in the soil, not even gravel.

There are ways to tell how many years a stone has been exposed on the ground. Wells used a method based on cosmogenic helium-3, which forms by cosmic ray bombardment at the ground surface. Helium-3 is retained inside grains of olivine and pyroxene in the lava flows, building up with exposure time. The helium-3 dates show that the lava stones in the desert pavement at Cima Dome have all been at the surface the same amount of time as the solid lava flows right next to them. It's inescapable that in some places, as he put it in a July 1995 article in Geology, "stone pavements are born at the surface." While the stones remain on the surface due to heave, deposition of windblown dust must build up the soil beneath that pavement.

For the geologist, this discovery means that some desert pavements preserve a long history of dust deposition beneath them. The dust is a record of ancient climate, just as it is on the deep sea floor and in the world's ice caps.

Desert pavement surfaces are often coated with desert varnish which is a dark brown, sometimes shiny coating that contains clay minerals. In the USA a famous example can be found on Newspaper Rock in Canyonlands, Utah.

According to Ronald I. Dorn and Theodore M. Oberlander (Science Volume 213, 1981), desert varnish is a thin coating (patina) of manganese, iron and clays on the surface of sun-baked boulders. Desert varnish is formed by colonies of microscopic bacteria living on the rock surface for thousands of years. Desert varnish is also prevalent in the Mojave desert and Great Basin geomorphic province.

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  Desert pavement, Ahaggar Mountains of central Sahara, southern Algeria
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  Desert pavement covering an aeolian deflation basin in the Rub' al-Khali desert. Scale cube is 1cm each side.
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  Desert pavement in the Cima Volcanic Field of the Mojave Desert

Local denominations

The desert known as Reg de l'Adrar in Mauritania

Stony deserts may be known by different names according to the region. A few examples are:

Gibbers

Covering extensive areas in Australia such as parts of the Tirari-Sturt stony desert ecoregion desert pavements are called Gibber Plains after the pebbles or gibbers, and so Gibber is also used in describing ecological communities such as Gibber Chenopod Shrublands or Gibber Transition Shrublands.

Reg

In Northern Africa, a vast stony desert plain is known as Reg. This term is in contrast with Erg, which refers to a sandy desert area.

Hamada

A stone strewn surface in the Sahara.

See also

• Aeolian processes
• Desert varnish
• Eduction (geology), a mechanism of surface rock formation
• Erg
• Saltation (geology)
• Ventifact

References

• Al-Qudah, K.A. 2003. The influence of long-term landscape stability on flood hydrology and geomorphic evolution of valley floor in the northeastern Badin of Jordan. Doctoral thesis, University of Nevada, Reno, 208 pp.
• Anderson, K.C. 1999. Processes of vesicular horizon development and desert pavement formation on basalt flows of the Cima Volcanic Field and alluvial fans of the Avawatz Mountains Piedmont, Mojave Desert, California. Doctoral thesis, University of California, Riverside, 191 pp.
• Goudie, A.S. 2008. The history and nature of wind erosion in deserts. Annual Review of Earth and Planetary Sciences 36:97-119.
• Grotzinger, et al. 2007. Understanding Earth, fifth edition. Freeman and Company. NY, NY. 458-460
• Haff, P.K. and Werner, B.T. 1996. Dynamical processes on desert pavements and the healing of surficial disturbance. Quaternary Research 45(1):38-46.
• Meadows, D.G., Young, M.H. and McDonald, E.V. 2006. Estimating the fine soil fraction of desert pavements using ground penetrating radar. Vadose Zone Journal 5(2):720-730.
• Qu Jianjun, Huang Ning, Dong Guangrong and Zhang Weimin. 2001. The role and significance of the Gobi desert pavement in controlling sand movement on the cliff top near the Dunhuang Magao Grottoes. Journal of Arid Environments 48(3):357-371.
• Rieman, H.M. 1979. Deflation armor (desert pavement). The Lapidary Journal 33(7):1648-1650.
• Williams, S.H. and Zimbelman, J.R. 1994. Desert pavement evolution: An example of the role of sheetflood. The Journal of Geology 102(2):243-248.

External links

Media related to Desert pavement at Wikimedia Commons

• Desert Processes Working Group Retrieved 1 Nov 2005
• The Bibliography of Aeolian Research
• The gibber plains of the Sturt Stony Desert in South Australiua