Desert: a region with so little vegetation that no significant population can be supported on that land. This implies that it is dry, but not necessarily hot.
There are several kinds of deserts, but they all have one thing in common - they each receive less than 25 cm (~10") of precipitation per year.
• Polar ice and snow deserts - the cold climate produces both low precipitation and relatively high evaporation rates, e.g., Antarctica. • Middle latitude deserts in the interiors of continents - characterized by low rainfall and high summer temperatures, e.g., Gobi Desert of Mongolia. • Trade wind deserts - roughly centered at latitudes of 30°N and 30 °S; high level winds descend from the equator and from higher latitudes, i.e., they are located at boundaries between Hadley circulation cells, e.g., Sahara and Simpson deserts. The descending air masses are dry. • Coastal deserts where cold oceans lie next to hot coastal regions - most of the precipitation falls over the oceans before it gets to the land, e.g., Atacama and Kalahari deserts. • Orographic deserts - deserts that receive little moisture due to the "rain shadow" effect of nearby mountains, e.g., Basin and Range of Nevada and eastern Oregon's deserts.
Desert Processes
Aeolian processes - caused by the wind (remember, the wind is a low density, low viscosity fluid)
• abrasion - a physical weathering process. • ventifacts - stones that have been sculpted by the wind. • desert varnish - the patina of iron and maganese oxides left on rocks after they have undergone long periods of chemical weathering in the desert. • desert pavement: forms when wind removes all of the fine-grained sand from a system, leaving only the coarse gravel behind. Below are three examples of a desert pavements, or regs. The first is a region that is actively undergoing deflation (a process in which the finer material is removed). The second photo shows the process continuing, whereas the third photo is a desert pavement, or reg.
Depositional features - sand dunes In a desert, material moves primarily by the wind through a process called saltation. This is the same type of jumping processes in which some material is carried as bedload in a stream. However, like in a stream, the most erosive agent is water. Even though rainfall is scarce, it usually comes in a few sudden, intense thunderstorm events. The water is not able to soak into the ground in such a short amount of time, so it flows across the ground as sheetflow, coalescing into gullies and rivers that can erode arroyos into the landscape very quickly. An arroyo is a term for a gully in an arid environment that has almost vertical sides and a flat channel floor.
Saltation may not move the larger particles; they may be left and form a desert pavement. Any vegetation, or other item, may also block the wind. That would allow small dunes to begin to form in the object's windshadow. In the images below, the wind is blowing from left to right and a windshadow is being created downwind of the object - allowing the finer material to accumulate.
There are many different types of sand dunes found in the desert. Below are some drawings from the US Geological Survey that show some of the major types of sand dunes. and how they relate to the various wind directions. barchan dune: crescent shaped dune with "horns" pointing downwind
Above is an isolated barchan dune that has some small transverse dunes nearby.
Above is a barchan dune that has various features labeled. longitudinal dune: linear dune that moves ~parallel to the wind transverse dune: linear dune that moves ~perpendicular to the wind star dune: dune that forms when there are a number of dominant wind directions
The airphoto above is a good example of star dunes coppice dune: a small dune that has been stabilized by vegetation
Parabolic dunes are similarly shaped as barchan dunes (crescent shaped) except that in this case, the wind is blowing so that the "horns" are pointing upwind since they have been stabilized.
When the winds are primarily from one direction, but occasionally reverse directions a feature known as a Chinese Wall can develop on the crest of dunes. The photograph below shows a good example of Chinese Walls on dunes in a dune field. If the dune field is very large, it is called a sand sea or erg.
If the wind erodes all of the fine material from an area it may start to erode into the local bedrock. This is called a blowout. If the erosion continues streamlined bedrock hills may form - they are called yardangs (see the airphoto below)
Ephemeral streams in steep sided valleys (arroyos and canyons) are very common in arid and semiarid environments.
• In general, V-shaped gullies (type A) with short, steep gradients are indicative of granular and semigranular, nonplastic, noncohesive soils, i.e., sand, gravel, and combinations of the two. As silt and clay particles are added, the V-shape broadens and becomes rounded, the gradient shallows, and the slope lengthens. Alluvial fans are common at mouths of A-type gullies. • V-shaped gullies with steep sides and flat bottoms (type B) have long slopes and gentle gradients ending in steep faces. This type of cross section is characteristic of many loess deposits. Alluvial fans are not formed at the mouths of these gullies. • Box-shaped gullies (type C) with vertical sides, and flat bottoms with long, gentle gradients are characteristic of water-deposited silt and marine sand and clay, as on the eastern U.S. coastal plain. This shape is also characteristic of subsurface drainage or ground water sapping, and it is found in granular and silty soils resting on a nonpermeable or semipermeable layer. • Saucer-shaped gullies (type D) with gentle side slopes and broad, rounded cross sections are characteristic of nongranular, cohesive, plastic soils, i.e., clay and silty clay. Gradients of these gullies are very gentle, and the gully extends for great distances, gradually feathering out to become indistinguishable from the surrounding terrain.
Alluvial fans - debris that accumulates at the foot of a mountain range in an arid environment. As a stream's gradient is abruptly decreased, the bedload and suspended load drops out. In the airphoto below, the number "1" points out the edge of a single alluvial fan that has been cut by an arroyo.
Bajadas - coalescing alluvial fans. In the airphoto below, the bajada is the region between the dashed blue lines.
Playas - ephemeral lake; in the airphoto below, the a playa can be seen as the bright patch southwest of the number "2".
In the photo above, another feature that is common in some deserts can be seen. Immediately to the east (right) of the number "1" is an inselberg - the top of a peak in which the rest of the mountain has been covered by sediment. As you can imagine, this process has taken millions of years to happen.
Terms for desert surfaces
• hamada: bare rock or rock with a thin veneer of pebble (Arabic?) • gobi: rock floored desert plain (Mongol) • gibber plain: stony (often ventifacts) desert (Australian) • reg: desert pavement of loose stones (N. Africa) • serir: desert pavement of loose stones, coarser and older than reg (N. Africa) • debba: extensive sandy plain (N. Africa) • dune field: extensive group of dunes • erg: vast region deeply covered with sand and topped by sand dunes (N. Africa) • playa, pan: extremely flat, vegetation free area of silt or clay in the lowest part of a closed basin (N. America, SW. Africa) • salina: playa with saline water or kept moist by rising groundwater (N. & S. America, Spain) • chott: closed basin containing a dry lake or playa (N. African) • sabkha, sebka: temporary lake or playa; marsh, salt flat or salt lake (N. African) • salar, salina: desert floor basin with salt deposit (Chile & Peru) • balat: desert marsh, depression or mudflat (N. African) • mbuga: temporary swamp or black clay pan (SW. Africa)