Desert pavement

Geologists debate the mechanics of pavement formation and their age. Studies look at pavements on alluvial surfaces and show that older surfaces have more developed pavements with small, angular rocks that are well sorted and tightly packed together.

Formation

Desert pavement formation can be attributed to deflation, the erosion of fine-grained material, or an upward migration of gravel. There are specific factors involved for the development of desert pavements, which include the necessity for low slope angles and dry conditions, alongside geomorphic and vegetative settings.

Several hypotheses have been proposed for the formation of desert pavements. A common hypothesis suggests that they form through the gradual removal of sand, dust and other fine-grained material by the wind and intermittent rain, leaving the larger fragments behind. The larger fragments are shaken into place through the forces of rain, running water, wind, gravity, creep, thermal expansion and contraction, wetting and drying, frost heaving, animal traffic, and the Earth's constant microseismic vibrations. The removal of small particles by wind does not continue indefinitely, because once the pavement forms, it acts as a barrier to resist further erosion. The small particles collect underneath the pavement surface, forming a vesicular A soil horizon (designated "Av").

A second hypothesis supposes that desert pavements form from the shrink/swell properties of the clay underneath the pavement; when precipitation is absorbed by clay it causes it to expand, and when it dries it cracks along planes of weakness. Over time, this geomorphic action transports small pebbles to the surface, where they stay through lack of precipitation that would otherwise destroy the pavement by transport of the clasts or excessive vegetative growth.

A newer hypothesis suggests that desert pavements form through deposition of windblown dust atop preexisting rocks. The dust then settles beneath the rocks, forming a layer of soil, while the rocks at the top crack and begin to merge into desert pavement. In 1995, researchers conducted a study in the Mojave Desert using helium-3 dating where they determined the rocks to be of older age than the soil beneath, leading to the conclusion of "stone pavements [being] born at the surface."

Aeolian processes play a key role in the evolution of desert pavements especially in arid environments. This process focuses on deposition, erosion, and transportation of sediments as a result of wind. Fine dust and sediment gets deposited by the wind in-between surface rocks, which helps contribute soil formation under the pavement. This would ultimately tightly pack the stones together on the surface, contributing to the sustainability and long-term development of desert pavements. The continuous accumulation of dust allows for the pavement to be more mature and be resistant to disturbances, allowing for long-term stability.

Desert pavement surfaces are often coated with desert varnish, a dark brown, sometimes shiny coating that contains clay minerals. In the US a famous example can be found on Newspaper Rock in southeastern Utah. Desert varnish is a thin coating (patina) of clays, iron, and manganese on the surface of sun-baked boulders. Micro-organisms may also play a role in their formation. Desert varnish is also prevalent in the Mojave desert and Great Basin geomorphic province.