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CHAPTER 17 - DESERTS AND WIND ACTION
Overview
Deserts receive less than 25 cm of rain per year. Deserts may be hot or cold
and typically form in areas of descending dry air masses concentrated in a belt
10 degree - 15 degree wide centered on 30 degrees north and south latitude.
Other controls on desert location include rain shadow effects, distance from
the ocean, proximity to cold ocean currents, and in the case of Antarctica,
Greenland, and northernmost Alaska, Canada and Siberia, descending cold air
masses.
Because of low rainfall, most deserts are characterised by possession of
internal drainage (lack of through-flowing streams), flash floods (that may
include mudflows) from high runoff (the tragic 1997 flash flood in the Grand
Canyon is mentioned), normally dry stream channels (arroyos), and badland
topography. The text uses features of the southwestern United States
(Colorado Plateau and Basin and Range provinces) to introduce basic desert
geomorphology: plateau, mesa, butte, hogback, cuesta, alluvial fan, playa,
playa lake, bajada, and pediment.
Although rain is the dominant agent of landscape development in a desert,
wind action is also important, and strong winds (>100 km/hour) are common.
Winds only erode when the surface sediment is dry. Dust storms are caused
by silt and clay picked up from dry soil and carried for great distances by wind
(particles from the Sahara have been found over the West Indies). Volcanic
ash can reach the jet stream and circle the earth. In contrast, sand moves
close to the ground by saltation. Abrasion from wind-blown sand produces
ventifacts. Deflation is the removal of finer sediment by wind. That process
can form blowouts and pillars. The origin of desert pavement and varnish is
discussed in Box 17.3.
Wind deposition of silt and clay produces loess, usually associated with
deflation of outwash plains during the Quaternary. Sand dunes are
asymmetrical with steep downwind (slip face reflects angle of repose), gentle
upwind faces, and they move in the downwind direction. Dunes are classified
by their geometry: barchans, parabolic, transverse, and longitudinal;
extensive dune fields form sand seas.
Learning Objectives
1. Deserts receive less than 25 cm of rainfall (or snowfall) a year, but can
have either hot or cold climate. Most deserts are associated with falling, dry air
masses concentrated in a belt 10 degree - 15 degree wide centered on 30
degrees north and south latitude and at the poles. Other controls include rain
shadow effects, great distance from the ocean, and proximity to cold ocean
currents. Deserts typically lack through-flowing streams and have internal
drainage that may develop local base levels.
2. Running water is the dominant agent of landscape development in deserts
because of limited vegetation. Thunderstorms commonly cause flash floods
and mudflows in deserts, although stream valleys, called arroyos, are usually
dry.
3. Resistance of rocks is controlled by climate. Rocks that might be weathered
quickly in other environments (e.g. limestone) are resistant in dry desert
settings. Soils are coarse and rocky, and creep is common.
4. Resistant rocks in desert settings form flat-topped plateaus, mesas, and
buttes (depending on their size), although those features can be found in
humid climates as well. Erosion of dipping beds forms hogbacks and cuestas.
In contrast to humid regions, the landforms of desert regions are typically
angular, although shales form gentle slopes in areas of badland topography.
5. Internal drainages in desert settings allow development of playa lakes that
dry into playas, covered with either mud or salt. Coalescing alluvial fans form
bajadas near the base of mountains. Erosion of mountains eventually produces
sloping surfaces covered by a veneer of gravel (pediments) that may pass into
bajadas.
6. Wind is responsible for erosion of fine sediment in deserts. Like water, the
faster the wind blows, the more sediment it can move, but its low density
limits transportation to only the finer sediment sizes. Once eroded by
deflation, silt and clay can remain suspended for long periods and can be
carried great distances (Saharan sediment reaches the West Indies). Sand, in
contrast, moves by saltation near to the ground. Sandblasting forms ventifacts
and sculpts bedrock exposures. Blowouts are depressions caused by wind
erosion (deflation).
7. Wind deposited silt and clay is called loess and it is usually associated with
deflation of outwash plains formed during the Quaternary in the northern
hemisphere.
8. Sand dunes are mounds of loose sand deposited by wind. The sand grains
can be of any composition, but are typically well sorted and well rounded. The
dunes are asymmetric, steep on their downwind side and gentle on their
upwind side. The downwind side or slip face is steep because the slope is at
the angle of repose for dry sand (34 degrees). Slip faces form the crossbedding preserved in dune sandstones. Movement of the dune is in the
direction of the slip face, due to erosion on the upwind side, transportation
over the dune crest and deposition on the slip face.
9. Dune geometry is controlled by wind velocity, direction, sand supply, and
vegetation cover. Large dune fields are sand seas. Many dunes are irregular,
but some patterns may be recognized. Low sand supply is characterized by
barchan dunes, with horns pointing downwind. Areas of high sand supply may
form transverse or longitudinal dunes that are elongate, or parabolic dunes
around blowouts, with horns pointing upwind.
Boxes
17.1 - ENVIRONMENTAL GEOLOGY - EXPANDING DESERTS
A desert must not only receive less than 25 cm of rain yearly, but also lack significant
vegetation. Overpopulation and overgrazing of dry regions, in addition to climatic
cycles, produces desertification by removing the sparse vegetation, and promoting
wind erosion. Desertification has occurred recently in the Sahel region of Africa and
also characterized the Dust Bowl era in the Midwestern United States during the
1930s.
17.2 - IN GREATER DEPTH – MOVING SANDS IN NORTHERN CANADA: THE ATHABASCA
SAND DUNES
The world’s northernmost sand dune field lies in northwestern Saskatchewan along the
southern shore of Lake Athabasca. Sands forming the dunes are derived from glacial
deposits formed by reworking of soft sandstones of the Athabasca Formation. The sand
dunes, which reach heights of up to 30m are protected within Athabasca Sand Dunes
Provincial Park.
17.3 IN GREATER DEPTH – DESERT PAVEMENT AND DESERT VARNISH
Cycles of wetting and drying of desert soils are thought to bring pebbles to the surface
to form desert pavement, rather than deflation processes. Rocks on the desert surface
exhibit desert varnish, a dark, often shiny, coating of manganese dioxide and clay
minerals. Origin of this varnish is unclear. It may form from windblown clay caught on
dew-dampened rocks that in turn draws manganese dioxide dissolved in water from the
soil by capillary action. Alternatively, desert varnish may be deposited by microbes.
Short Discussion/Essay
1. Why is desert pavement composed of gravel?
2. Why are alluvial fans and bajadas so characteristic of desert settings?
3. How would you distinguish a desert dune from a marine sand wave, if the
two were composed of the same size sediment?
4. What effect does saltation have on the surface of sand grains transported
by wind?
5. Why is water the principal means of landscape modification in a desert?
Longer Discussion/Essay
1. Explain the distribution of loess in North America.
2. How will natural climatic changes in the coming centuries influence the
distribution of deserts? What effect will human activity have on this
distribution?
3. The land speed record was broken on a playa (Bonneville salt flats). Why
are the playa surfaces so flat?
4. Explain how Mayan cities in Central America have become buried in
sediment in only the last 900 years (e.g. Tikal, Guatemala).
5. Explain the mechanics of sand formation and movement.
Selected Readings
Dorn, R.I. 1991. "Rock varnish," American Scientist 79(6): 542-544.
Nori, F., Sholtz, P. and Bretz, M. 1997. "Booming Sand," Scientific American
277: 84-89.
Ritter, D.F. 1995. Process Geomorphology. 3rd Edition. Dubuque, IA: Wm. C.
Brown Publishers.
Racetrack Playa in Death Valley is the site of the famous roving rocks. These
two articles are short and well worth reading concerning this unusual
phenomenon.
Cahill, D.B. and Tombrello, T.A. 1996. "Sailing Stones on Racetrack Playa,"
Journal of Geology 104: 121-125.
O'Hanlon, L. 1996. "The Roving Rocks of Racetrack Playa," Earth 5: 58-59