Geology 100
Lecture 14 - Deserts
Deserts: typically have <25 cm annual precipitation; may support moderate plant life (20-25 cm/year), little plant life
(15-20 cm/year) or essentially no plant life (<15 cm/year).
Nearly 25% of land area outside poles is desert.
I. What causes deserts?
Important background: cold air cannot hold much moisture; hot air can hold considerable moisture. If air with moisture
cools, the moisture will condense and precipitation will occur. If cold air with little or moderate moisture is heated, it
can hold a lot more moisture in the form of water vapor and little precipitation will result.
A) Overall atmospheric circulation patterns:
At equator, sun's rays are nearly directly overhead and so solar energy
(heat provided) is high. Therefore the air at the equator is heated and begins to rise. As the air rises, it cools and moisture
condenses, forming rain.
Rising air forms a low pressure system and usually means rain. At the equator, there is a global low pressure system.
Air that has risen at the equator spreads out to the north and south and travels to about 30 N and 30 S where it begins to
sink back down to the surface. As it descends, this cool air becomes warm. Since warm air can hold more moisture
(resulting in low precipitation), there are large deserts around 30N and 30 S.
B) Relative differences in air mass characteristics at the coasts: Southern California (e.g. LA) is a desert. Along
the coast, a south-directed water current (California current) brings cool water in from further north. This cool water cools
the air over it. When the cool air encounters warmer air onshore, the cooler air heats up and can therefore hold
considerably more moisture. Thus, a desert results (low precipitation)
C) "Rain shadow" effect by mountain ranges. Eastern Washington, Nevada and Arizona are all deserts. Here, relatively
moist eastward-flowing air from the Pacific encounters the Sierra and Cascade Mountains and is forced to rise to get over
the mountains. In rising, the air cools and moisture precipitates on the west face of the ranges. By the time the air gets over
the mountain, it is much drier. In addition, as the air moves down the east side of the mountain, it warms, enabling it to
hold more moisture. Thus, a desert results.
II. Sedimentation processes in deserts:
A) Wind is a very important agent of erosion in deserts.
The wind will entrain pieces of silt and sand and act as a `sandblaster' which quickly erodes solid rock and can shape
cobbles and large pebbles into strangely-shaped ventifacts.
Because wind can only carry fine sand and silt-sized particles, larger clasts (pebbles, cobbles, and boulders) are left
behind, forming a thin veneer of larger clasts at the surface.
This is called a "desert pavement" or deflation surface.
B) Saltation = the means by which sand particles are bounced along the ground by the wind. (They are too heavy to
be carried in suspension, but are light enough to be picked up by the wind for short distances.)
C) Most streams in deserts do not flow constantly and are called ephemeral streams (intermittent streams).
1. Dry stream beds (washes) will flow only for short periods of time in flash floods.
2. Because there is not much vegetation in deserts, much sediment is carried in flash floods. Sometimes, there is so
much sediment, that a debris flow results (these are common in Montana). Debris flows are viscous slurries that can
carry large boulders.
3. Because most streams are intermittent but yet carry large sediment loads, alluvial fans are common in deserts.
Alluvial fans may coalesce to form bajadas.
4. Because they do not flow year round, their sediment loads are often not carried far, and basins between mountain
ranges tend to fill up with sediment.
D) In addition, many deserts are internally-drained (their drainage does not reach the sea -- e.g. Great Salt Lake in Utah).
E) An intermittent (ephemeral) lake is called a playa lake. Sometimes salts are deposited in playa lake beds which may
be economic to mine (e.g. Death Valley mines).
F) Dunes (not as common as you might think) are large accumulations of wind-blown sand. Dunes migrate downwind,
giving them their shape. Barchan, transverse, parabolic, and longitudinal dunes are four main types which are a result
of the interaction of sand supply, consistency of prevailing wind, and degree of vegetation.
G) Loess is wind-blown glacial flour. It is commonly quite fertile and arable.
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