13.3: Desert Weathering and Erosion

Weathering takes place in desert climates by the same means as other climates, only at a slower rate. While higher temperatures typically spur faster chemical weathering, water is the main agent of weathering, and lack of water slows both mechanical and chemical weathering. Low precipitation levels also mean less runoff as well as ice wedging. When precipitation does occur in the desert, it is often heavy and may result in flash floods in which a lot of material may be dislodged and moved quickly.

Figure \(\PageIndex{1}\): Weathering and erosion of Canyonlands National Park have created a unique landscape, including arches, cliffs, and spires.

One unique weathering product of deserts is desert varnish. Also known as desert patina or rock rust, it is thin dark brown layers of clay minerals and iron and manganese oxides that form on very stable surfaces within arid environments. The exact way this material forms is still unknown, though cosmogenic and biologic mechanisms have been proposed.

Figure \(\PageIndex{2}\): Newspaper rock, near Canyonlands National Park, has many petroglyphs carved into the desert varnish.

While water is still the dominant agent of erosion in most desert environments, wind is a notable agent of weathering and erosion in many deserts. This includes suspended sediment traveling in haboobs, or dust storms, that frequent deserts. Deposits of windblown dust are called loess. Loess deposits cover wide areas of the midwestern United States, much of it from rock flour carried by the ice sheets that melted after the last ice age [7]. Loess was also blown from desert regions in the West. Like water, wind is a fluid and can transport sediment. Possessing lower energy than water, wind transport nevertheless moves sand, silt, and dust [8]. As noted in the chapter on Water, the load carried by a fluid is distributed among bedload and suspended load. Similar to water, the loads carried by wind depends on wind velocity.

Figure \(\PageIndex{3}\): A dust storm (haboob) appears on the front of a storm in Texas. (By Jakeorin; CC BY-SA 4.0 via Wikimedia Commons.)

Sand-sized material moves by a process called saltation in which sand grains are lifted into the moving air and carried a short distance where they drop and splash into the surface dislodging other sand grains which are then carried a short distance and splash dislodging still others [8].

Figure \(\PageIndex{4}\): Diagram showing the mechanics of saltation.

Since saltating sand grains are constantly impacting other sand grains, windblown sand grains are commonly pretty well rounded with frosted surfaces. Saltation is a cascading effect of sand movement creating a zone of windblown sand up to a meter or so above the ground. This zone of saltating sand is a powerful erosive agent in which bedrock features are effectively sandblasted. The fine-grained suspended load is effectively sorted from the sand near the surface, carrying silt and dust in haboobs. Wind is thus an effective sorting agent separating sand and dust-sized (≤70 µm) particles [9]. When wind velocity is high enough to slide or roll materials along the surface, the process is called creep.

Figure \(\PageIndex{5}\): Enlarged image of frosted and rounded windblown sand grains.

One extreme version of sediment movement was shrouded in mystery for years: sliding stones. Also called sailing stones and sliding rocks, these are large moving boulders along flat surfaces in deserts, leaving trails. This includes the famous example of the Racetrack Playa in Death Valley National Park, California. For years, scientists and enthusiasts attempted to explain their movement, with little definitive results [10; 11]. In recent years, several experimental and observational studies have confirmed that thin layers of ice allow the stones to move with high winds providing propulsive energy [12; 13]. These studies include measurements of actual movement, as well as re-creations of the conditions, with resulting movement in the lab.

Figure \(\PageIndex{6}\): A sliding stone at Racetrack Playa in Death Valley National Park, California.

The zone of saltating sand is an effective agent of erosion through sand abrasion. A bedrock outcrop which has such a sandblasted shape is called a yardang [14]. Rocks and boulders lying on the surface may be blasted and polished by saltating sand. When predominant wind directions shift, multiple sandblasted and polished faces may appear. Such polished desert rocks are called ventifacts [15].

Figure \(\PageIndex{7}\): Left: A yardang in Bolivia. Right: Ventifact from Mojave Desert near Barstow, California.

In places with sand and silt accumulations, clumps of vegetation often anchor sediment that has accumulated on the desert surface. Yet, winds may be sufficient to remove materials not anchored by vegetation. This causes a bowl-shaped depression in the sand called a blowout [16].

Figure \(\PageIndex{8}\): Blowout in Texas.

References

7. Muhs, D. R. & Bettis, E. A. Quaternary loess-paleosol sequences as examples of climate-driven sedimentary extremes. Special Papers-Geological Society of America 53–74 (2003).

8. Bagnold, R. A. The physics of blown sand and desert dunes. Methum, London, UK 265 (1941).

9. Shao, Y. Physics and Modelling of Wind Erosion. (Springer Science & Business Media, 2008).

10. Stanley, G. M. Origin of playa stone tracks, Racetrack Playa, Inyo County, California. Geol. Soc. Am. Bull. 66, 1329–1350 (1955).

11. Clements, T. Wind-blown rocks and trails on Little Bonnie Claire Playa, Nye County, Nevada. J. Sediment. Res. 22, (1952).

12. Norris, R. D., Norris, J. M., Lorenz, R. D., Ray, J. & Jackson, B. Sliding rocks on Racetrack Playa, Death Valley National Park: first observation of rocks in motion. PLoS One 9, e105948 (2014).

13. Kletetschka, G. et al. Sliding stones of Racetrack Playa, Death Valley, USA: The roles of rock thermal conductivity and fluctuating water levels. Geomorphology 195, 110–117 (2013).

14. Hedin, S. A. Central Asia and Tibet. 1, (Hurst and Blackett, limited, 1903).

15. Laity, J. E. Landforms, landscapes, and processes of aeolian erosion. in Geomorphology of Desert Environments (eds. Parsons, A. J. & Abrahams, A. D.) 597–627 (Springer Netherlands, 2009).

16. Livingstone, I. & Warren, A. Aeolian geomorphology: an introduction. (Longman, 1996).