deserts

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Deserts and Wind
Chapter 14
Geology Today
Barbara W. Murck
Brian J. Skinner
Ancient sand dunes, Utah
N. Lindsley-Griffin, 1999
Deserts
Source: U.S.G.S.
What is a desert?
An area where annual precipitation is
less than 250 mm (10 in)
N. Lindsley-Griffin, 1999
Desert Myths:
N. Lindsley-Griffin, 1999
“Deserts are barren” - Wrong,
most have at least some vegetation
Organ Pipe National Monument, AZ
Desert Myths:
N. Lindsley-Griffin, 1999
“It never rains” - Wrong, rains are
infrequent, but over a long enough
period of time, the effects are large.
San Rafael Swell, UT
Desert Myths:
“Water has little effect on deserts” Wrong, flash floods have tremendous
power to erode and transport material
Tarbuck-Lutgens, 1998; N. Lindsley-Griffin, 1999
Desert Myths:
N. Lindsley-Griffin, 1999
“All erosion is by wind” - Wrong, most
desert features formed by running
water during wetter Ice Age climates
Source: U.S.G.S.
Desert Myths:
N. Lindsley-Griffin, 1999
“Natural arches form by wind erosion” Wrong, mechanical weathering and mass
wasting (rockfalls) are more important.
Arches National Park, UT
Desert Types
Subtropical deserts are controlled by global climate zones
where dry air descends to surface.
(Fig. 14.2, p. 399)
N. Lindsley-Griffin, 1999
Desert Types
N. Lindsley-Griffin, 1999
Most of the western United States is
subtropical arid and semiarid climate.
Source: U.S.G.S.
Desert Types
Rainshadow Deserts form where
mountains block moist air.
Rising winds cool, release moisture
(Seattle, Oregon, Northern California)
Dry winds
Descending winds are dry,
warm up as they compress
(Mojave Desert, Death Valley)
Desert
Wet winds
Lee Slope
Windward slope
N. Lindsley-Griffin, 1999
Desert Types
Coastal deserts are caused by global
oceanic and atmospheric circulation
Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999
Desert Types
Coastal deserts form where cold coastal
currents flow parallel to shore
Cold coastal
current
chills
onshore
winds so
they cannot
carry
moisture
Source: U.S.G.S.
N. Lindsley-Griffin, 1999
Polar Deserts - moisture frozen into ice
sheets, not available for plant growth
Air is too cold to carry moisture.
Little snow falls,
but what does fall does not melt.
Desert
Weathering
Mechanical weathering is more
important than chemical weathering
(frost wedging, root wedging)
Capital Reef National Park, UT
N. Lindsley-Griffin, 1999
Desert
Weathering
N. Lindsley-Griffin, 1999
Salt crystallization is an important
weathering process because salt
expands as it crystallizes.
Mudcracks and evaporites
Source: U.S.G.S.
Wind Erosion
Eolian processes are particularly
effective in arid and semi-arid regions
Dust Storm
Tarbuck & Lutgens, N. Lindsley-Griffin, 1999
Wind Erosion
Sediment moves by suspension,
surface creep, saltation
Fig. 14.4, p. 402
N. Lindsley-Griffin, 1999
Wind Erosion
Desert pavement forms by deflation smaller particles blow away, leaving
surface covered with tightly packed
large particles.
Fig. 14.6, p. 404
Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999
Wind Erosion
N. Lindsley-Griffin, 1999
Desert Pavement: pebble- to cobble-sized
rock fragments covering desert surface
after lighter fragments have been
Fig. 14.6, p. 404
removed by wind.
Wind Erosion
Wind-blown sand hammers at
exposed rock faces to produce
smooth flat surfaces
Fig. 14.5, p. 403
Source: U.S.G.S.
N. Lindsley-Griffin, 1999
Wind Erosion
Ventifacts have at least one smooth
abraded surface facing upwind.
Ventifact Rock faceted and
polished by windblown sand
Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999
Blowout - a small saucer-shaped depression formed by
wind erosion in dunes. Common in Nebraska sand hills
where protective vegetation has been disturbed.
Blowout
Wind
Deposition
N. Lindsley-Griffin, 1999
Sand dunes - Mounds of wind-blown sand
Loess - Thick deposits of cohesive,
unstratified wind-blown dust
Sand Dunes
Barchan
dunes
Dunes form wherever
a supply of sand is available,
strong wind blows constantly,
and a barrier causes wind to
lose velocity so sand can be
deposited
Coral Pink Sand Dunes State Park, AZ
Tifernine dune field, Africa (space shuttle)
N. Lindsley-Griffin, 1999
Wind
Deposition
Houghton Mifflin 1998; N. Lindsley-Griffin, 1999
Sand dunes dip gently on the upwind or
windward side, are steep on downwind slip
face.
Wind
Deposition
Source: U.S.G.S.
Sand ripples in Monument Valley
N. Lindsley-Griffin, 1999
Sand Dunes
Sand moves up windward slope by creep
and saltation. It drops out as the wind
loses velocity in the lee of the dune.
Barchan Dunes
N. Lindsley-Griffin, 1999
Sand Dunes
Sand dune slip face: sand oversteepens
at top, mass-wastes down the steep slip
face of the dune
Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999
Sand Dunes
Transverse dunes
Parabolic dunes
Houghton Mifflin, 1998; N. Lindsley-Griffin, 1999
The type of sand dune that forms
depends on amount of sediment supply
and wind direction
Longitudinal dunes
Star dunes
Barchans
Sand Dunes
N. Lindsley-Griffin, 1999
Barchan dunes, crescent-shaped with
horns pointing downwind, require
moderate sediment supply and wind
strength. (Fig. 14.9A, p. 407)
Danakil Depression, Ethiopia
Sand Dunes
Transverse dunes, continuous
asymmetrical ridges perpendicular to the
strongest wind, require abundant sediment.
(Fig. 14.9B, p. 407)
N. Lindsley-Griffin, 1999
Takla Makan Desert, China
Sand Dunes
300 m high star
dunes in Libya;
radar satellite
image
N. Lindsley-Griffin, 1999
Star dunes, isolated and stationary,
form where winds blow from all
directions. (Fig. 14.9C, p. 407)
Sand Dunes
Common in Africa
and Australia
(Fig. 14.9E, p. 407)
N. Lindsley-Griffin, 1999
Longitudinal dunes, long narrow ridges
parallel to wind, form where little sediment
is available and winds are strong.
Sand Dunes
Ancient sand dune
deposits can be
recognized by the
steep sets of cross
beds, separated by
gently dipping sets.
Steep sets = slip face
Gentle sets = windward
face
N. Lindsley-Griffin, 1999
Eolian Sediment
Fine-grained
(sand, silt, clay)
Well-sorted
Distinctive cross bedding
style: low angle layers
separating steep layers
Sand grains rounded
with frosted surfaces
Source: U.S.G.S.
Checkerboard Mesa - pattern caused by
intersection of joints and eolian cross beds
Source: U.S.G.S.
N. Lindsley-Griffin, 1999
Alluvial
Fans
N. Lindsley-Griffin, 1999
Streams deposit sediment at slope
change at mouth of canyon
Fault-block Mountains
Sediment-filled Playa
Basins of internal drainage are common in
the Basin and Range region of the U.S.
Sand Hills
Nebraska’s Sand Hills - a giant sand dune
field formed near the end of the Ice Ages
Sand dunes held in place by
vegetation - if grass is destroyed,
blowouts will form
Dark green areas on map are up
to 75% eolian sand
NE Conservation & Survey, J.R. Griffin , 1999
Yellow area - Sand hills
Wind
Deposits
Deposits of eolian sand
and loess, Midwestern
United States
AGI/NAGI, J.R. Griffin, 1999
Desertification and Land Degradation
Desertification - invasion of
desert conditions into
nondesert areas:
Drought or climate change in
areas that are already semiarid.
Land degradation desertification caused by
human activities:
Too many people for resource
base, poor agricultural practices,
overgrazing, deforestation, misuse
of surface and groundwater, civil
war.
N. Lindsley-Griffin, 1999
Desertification in the central Plains, 1930s
The Aral Sea
Aral Sea is drying up as water from tributary rivers is diverted
to irrigate new cropland.
Climate: winters colder, summers
hotter, dust storms common.
Economy: fishing industry gone,
salinization is ruining soil.
This desert was once the Aral Sea
Houghton Mifflin 1998; N. Lindsley-Griffin, 1999
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