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WHAT IS A DESERT?
A desert is a barren area of land where little precipitation occurs and consequently
living conditions are hostile for plant and animal life.
WHAT ABOUT THIS?
WHAT CAUSES DESERTS?
1)
Earth’s air circulation patterns:
Warm air rises at the equator, and cools.
Since cool air holds less water, precipitation is common at the equator.
The dry air mass moves north and south to N30 degrees and S30 degrees.
At ± 30 degrees, the dry air sinks and warms. The warmer air can hold more
water resulting in evaporation of water and the formation of deserts.
The equatorial region is tropical and ± 30 degrees is desert.
WHAT CAUSES DESERTS?
2)
Rain shadow effect:
Hot rising air cools as it approaches a mountain. Less water can be held and
that side of the mountain gets rain.
The cooler air that crosses over mountains is dry, so deserts form on the
other side of the mountain range.
In the US, major rain shadow effects occur in northern Arizona, Nevada,
and California due to the Sierra Nevada range.
WHAT CAUSES DESERTS?
3)
Distance from the ocean.
Some land is so far from oceans (where air absorbs most of its water) that
deserts form.
4)
Near cold ocean currents (along the western edge of continents).
The cold ocean air warms as it passes over continents.
The warmer air holds more moisture and causes evaporation and the
formation of deserts.
WIND EROSION
Wind erodes the desert in two ways: Deflation and Abrasion
Deflation is the lowering of the land surface due to removal of fine-grained particles
by the wind. Deflation concentrates the coarser grained particles at the surface,
eventually resulting in a surface composed only of the coarser grained fragments
that cannot be transported by the wind. Such a surface is called desert pavement.
Abrasion happens when windblown sand cuts and polishes exposed rock surfaces
STREAMS IN THE DESERT
Desert streams are present but are intermittent (often only flow after a storm).
STREAMS IN THE DESERT
These dry streams are called arroyos or dry washes. They often form dendritic
patterns due to the large amount of gravels in deserts.
Desert rains often come all at once, resulting in flooding.
Many rivers never reach the sea. They have internal drainage and a local base level
(example: Death Valley).
DESERTS IN THE UNITED STATES
The Colorado Plateau (Colorado, Utah, Arizona, New Mexico)
Occurs at 30 degrees north latitude.
There is also a rain shadow effect of the Sierra Nevada.
DESERTS IN THE UNITED STATES
The plateau is made of flat-lying sedimentary rocks (sandstone and limestone).
Due to elevation of 1500 meters above sea level, the plateau is undergoing erosion.
Plateau: Large, flat elevated region usually bounded by cliffs.
Mesa: Small region of plateau that resisted erosion.
Lava-capped mesa by the Rio Grande southwest of Taos, NM
DESERTS IN THE UNITED STATES
Butte: An even smaller, steep sided region that escaped erosion.
A butte near Sedona, Arizona.
Mesas and buttes are often capped with erosion-resistant rock that protects underlying
sediments from erosion.
DESERTS IN THE UNITED STATES
The Basin and Range (Nevada, Utah, California, Arizona)
The Basin (valley) and Range (mountain) region is characterized by parallel sequences
of steep mountain ranges separated by flat valleys
Copper Canyon Turtleback fault in Death
Valley
DESERTS IN THE UNITED STATES
The boundaries between mountains and valleys represent normal faults in which the
valleys dropped down relative to the adjacent mountain ranges.
The origin of basin (graben) and range (horst)
by faulting
DESERTS IN THE UNITED STATES
The mountains result in more localized rain shadow effects.
Storms in the mountains often result in alluvial fans which transition into dry lakes called
playas at the centers of valleys.
Alluvial fan in Death Valley
DESERTS IN THE UNITED STATES
Over time, separate alluvial fans may join up to produce a bajada.
Bajada in Death Valley
WIND EROSION IN THE DESERT
Water is the main mechanism of erosion in the desert, often due to episodic storms.
However, wind can make a very significant contribution to erosion because 1) wind is
continuous and 2) wind is not restricted to channels.
Only the smallest particles can be transported by winds. Silt and clay can become
airborne whereas sand can move by a series of small jumps called saltation.
Ventifacts - Desert rocks often become sandblasted by wind and shaped into faceted
blocks called ventifacts.
Antarctica ventifacts resulting from wind
erosion
WIND EROSION IN THE DESERT
Desert varnish - Many rocks become coated by a dark surface layer called desert
varnish (Mn may play a role).
Newspaper Rock near Utah's Canyonlands
National Park
WIND EROSION IN THE DESERT
Blowout - Wind deflation of soil can result in the removal of silt and clay, producing a
deflated bowl called a blowout.
Desert pavement - A thin layer of densely packed pebbles that can cover a desert
surface. Two theories of pavement formation have been proposed: 1) origin as a lag
deposit and 2) formation by swelling of the ground and the upward movement of
pebbles.
Desert Pavement near Reno, Nevada
WIND DEPOSITION IN THE DESERT: SAND DUNES
Wind-generated sand dune deposits can consist of quartz, quartz mixed with feldspar, or
carbonates.
The sand grains tend to be well-rounded and well-sorted (wind is very efficient at sorting).
Dunes are not stationary: they migrate over time. Wind-generated dunes have shallow face
and a steep face (slip face). Migration is in the direction of the slip face.
The angle of repose for loose sand is 34 degrees.
The shape of a dune depends on several
factors:
1) Wind speed/direction (constant vs. nonconstant).
2) The amount of sand available.
3) The amount of vegetation (which can
stabilize dunes).
WIND DEPOSITION IN THE DESERT: SAND DUNES
Barchan dunes
-Crescent dunes with slip face on the concave side.
-Horns point downwind.
-Form when the sand supply is limited.
WIND DEPOSITION IN THE DESERT: SAND DUNES
Transverse Dunes –
Straight dunes running perpendicular to the
wind.
Form when more sand is available.
WIND DEPOSITION IN THE DESERT: SAND DUNES
Parabolic Dunes –
These are the reverse of barchan dunes: the
horns point upwind.
Form around blowouts.
WIND DEPOSITION IN THE DESERT: SAND DUNES
Longitudinal Dunes - Long ridges of sand that form parallel to the prevailing wind.
Their formation is not entirely understood, but may involve circular wind patterns.