Geology 111G/Lecture 19
Wind and Deserts
Desert Types
Wind Transport
Wind Erosion
Wind Deposition
Loess
Dunes
Desertification
I. Distribution
of Deserts. Regions of low
moisture, generally where evapotranspiration exceeds precipitation. In the
II. Types of Deserts. Aridity has several causes.
A. Topographic deserts or rain
shadows: These are cutoff from moisture by mountain ranges. Basin and Range,
B. Subtropical deserts: Lie between
15¡ and 30¡ north and south of the equator, in belts of warm air convergence
that create high-pressure zones.
These are the world great deserts:
C. Continental deserts. Lie far from ocean moisture sources.
D. Coastal deserts: Created by cold surface water lying offshore that force moisture from air masses moving onshore. Dries places on earth. Rain falls at sea. Atacama, Kalahari.
III. Wind. Lack of veetation in desert areas allows wind to have a direct influence on the erosion, transport and deposition of sediment.
A. Erosion. Two processes at work here.
1. Abrasion: moving sand grains sandblast stationary material, fenceposts and ventifacts.
2. Deflation: removal of unconsolidated fine material, leaving a desert pavement (deflation lag) of larger particles. Interlocking mosaic of stones that protects the desert surface from further erosion.
B. Transport: Wind transports sand and dust.
1. Dust: Clay and silt-sized particles, less than 1/16 mm. A dust storm consists of dust carried to heights of hundreds to thousands of feet; small particle sizes permit suspension in air.
2. Sand. Smaller grain sizes are transported, up to about 1/3 mm. A sand storm is a carpet of wind-driven sand usually not more than 1 m above the ground. Sand is not suspended, but travels in other ways:
a. Saltation: Transport by bouncing and high-velocity impacts among grains. Grains follow parabolic paths.
b. Rolling.
C. Deposition. Wind deposits consist of dust and sand.
1. Loess: Deposit of unstratified angular dust fragments,
commonly blown out of river channels during low water. May be quite thick. Along the Yellow River in
2. Sand Dunes: major accumulations of wind-blown sand. Generally asymmetrical in cross section with a stoss side and a steep lee side formed by avalanching of grains at angle of repose (about 34¡ in dry fine sand). There are several dune types.
a. Transverse dunes. Straight crest at 90¡ to prevailing wind direction. Abundant sand supply, consistent wind.
b. Barchan dunes: croissant shaped, with horns pointing downwind. Limited sand supply, consitent wind direction.
c. Linear dunes: long ridges of sand oriented parallel to
prevailing wind direction. Moderate
sand supply, consistent wind direction.
Usually a few meters high and tens of meters long, but big ones in
d. Star dunes: Multipronged, multicrested dunes. Limited sand supply, many wind directions.
IV. Anecdote: Wind storm of December 20, 1977, Southern San Joaquin Valley.
A. Velocity: toppled high-voltage line towers designed to withstand 250 kph (150 mph) wind.
B. Transport: Egg-sized pebbles were lifted into the
air, presumably saltating, and a huge dust cloud was blown toward the
B. Deflation: Wind removed up to 60 cm (24 in) of soil locally. 50 million metric tons of soil were removed from 200 km2 of agricultural and range land. This equals 25,000 tons/km2 or about 50 lbs/m2.
V. Desertification: invasion of desert into formerly non-desert areas. Often applied to a conversion that results from land-use practices. Symptoms include:
A. Declining water tables.
B. Increased salinity of water and soil.
C. Reduction of surface water supply.
D. High rates of soil erosion.
E. Loss of much native vegetation.
F. Causes:
1. Climatic change or drought: drought of early 1970's caused
2. Human activities: groundwater withdrawal; irrigation;
grazing. These practices probably
increased severity of