Desert pavement
Encyclopedia
A desert pavement is a desert
surface that is covered with closely packed, interlocking angular or rounded rock
fragments of pebble
and cobble
size.
, 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.
Finally, a new theory suggests that they form as a consequence of the entrapment of fine-grained material which infiltrates below the rocky surface. This surface continues to be pushed up above the fine grained material by microbial activity which makes small bubbles that help raise the more coarse grained materials to their surface layer; this cycle of events can continue to make a thick layer of windblown dust below the pavement over the millennia. Frequently the stones are polished by the abrasion of wind-borne dust and may even be reduced to a streamlined shape with the narrow point on the windward side, becoming ventifact
s.
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.
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.
, which refers to a sandy desert area.
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
surface that is covered with closely packed, interlocking angular or rounded rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
fragments of pebble
Pebble
A pebble is a clast of rock with a particle size of 4 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered to be larger than granules and smaller than cobbles . A rock made predominantly of pebbles is termed a conglomerate...
and cobble
Cobblestone
Cobblestones are stones that were frequently used in the pavement of early streets. "Cobblestone" is derived from the very old English word "cob", which had a wide range of meanings, one of which was "rounded lump" with overtones of large size...
size.
Formation
Several theories have been proposed for their formation. The more common theory is that they form by the gradual removal of the sandSand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, 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.
Finally, a new theory suggests that they form as a consequence of the entrapment of fine-grained material which infiltrates below the rocky surface. This surface continues to be pushed up above the fine grained material by microbial activity which makes small bubbles that help raise the more coarse grained materials to their surface layer; this cycle of events can continue to make a thick layer of windblown dust below the pavement over the millennia. Frequently the stones are polished by the abrasion of wind-borne dust and may even be reduced to a streamlined shape with the narrow point on the windward side, becoming ventifact
Ventifact
Ventifacts are rocks that have been abraded, pitted, etched, grooved, or polished by wind-driven sand or ice crystals. These geomorphic features are most typically found in arid environments where there is little vegetation to interfere with aeolian particle transport, where there are frequently...
s.
Desert pavement surfaces are often coated with desert varnish
Desert varnish
Desert varnish, or rock varnish is a orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is usually around one micron thick and present nanometre-scale layering...
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.
Local denominations
Stony deserts may be known by different names according to the region. A few examples are:Gibbers
Covering extensive areas in AustraliaAustralia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
such as parts of the Tirari-Sturt stony desert
Tirari-Sturt stony desert
The Tirari-Sturt stony desert is a large dry World Wildlife Fund ecoregion of southern Australia.-Location and description:The Tirari-Sturt stony desert ecoregion contains the gibber plains and red sands of the large Sturt Stony Desert, the Tirari Desert to its southwest and the Flinders and...
ecoregion
Ecoregion
An ecoregion , sometimes called a bioregion, is an ecologically and geographically defined area that is smaller than an ecozone and larger than an ecosystem. Ecoregions cover relatively large areas of land or water, and contain characteristic, geographically distinct assemblages of natural...
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 ErgErg (landform)
An erg is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. The term takes its name from the Arabic word ʿarq , meaning "dune field"...
, which refers to a sandy desert area.
See also
- Aeolian processes
- Desert varnishDesert varnishDesert varnish, or rock varnish is a orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is usually around one micron thick and present nanometre-scale layering...
- Eduction (geology)Eduction (geology)In geology, eduction is a process in which the Earth crust spreads sideways, exposing deep-seated rocks.It is prominent in middle layers of the Himalayas, where gravity pushes the mountains down. Together with a high grade of erosion, this activity brings deep rocks to the surface, many from more...
, a mechanism of surface rock formation - ErgErg (landform)An erg is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. The term takes its name from the Arabic word ʿarq , meaning "dune field"...
- Saltation (geology)Saltation (geology)In geology, saltation is a specific type of particle transport by fluids such as wind or water. It occurs when loose material is removed from a bed and carried by the fluid, before being transported back to the surface...
- VentifactVentifactVentifacts are rocks that have been abraded, pitted, etched, grooved, or polished by wind-driven sand or ice crystals. These geomorphic features are most typically found in arid environments where there is little vegetation to interfere with aeolian particle transport, where there are frequently...