Deserts and WindDeserts and Wind
N. Lindsley-Griffin, 1999
Chapter 14
Geology Today
Barbara W. Murck
Brian J. Skinner
Ancient sand dunes, Utah
What is a desert?An area where annual precipitation is less than 250 mm (10 in)
DesertsDeserts
Source: U.S.G.S. N. Lindsley-Griffin, 1999
Desert Myths:Desert Myths: “Deserts are barren” - Wrong, most have at least some vegetation
N. Lindsley-Griffin, 1999Organ Pipe National Monument, AZ
Desert Myths: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:Desert Myths:
Tarbuck-Lutgens, 1998; N. Lindsley-Griffin, 1999
“Water has little effect on deserts” - Wrong, flash floods have tremendous power to erode and transport material
Desert Myths: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: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
N. Lindsley-Griffin, 1999
Subtropical deserts are controlled by global climate zones where dry air descends to surface. (Fig. 14.2, p. 399)
Desert TypesDesert Types
Source: U.S.G.S.
Desert TypesDesert Types Most of the western United States is subtropical arid and semiarid climate.
N. Lindsley-Griffin, 1999
Desert TypesDesert Types
N. Lindsley-Griffin, 1999
Rainshadow Deserts form where mountains block moist air.
Dry winds
Wet winds
Rising winds cool, release moisture (Seattle, Oregon, Northern California)
Descending winds are dry, warm up as they compress(Mojave Desert, Death Valley)
Desert
Lee Slope
Windward slope
Desert TypesDesert Types
Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999
Coastal deserts are caused by global oceanic and atmospheric circulation
Desert TypesDesert Types
Source: U.S.G.S. N. Lindsley-Griffin, 1999
Coastal deserts form where cold coastal currents flow parallel to shore
Cold coastal current chills
onshore winds so
they cannot carry
moisture
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
Desert Weathering
N. Lindsley-Griffin, 1999
Mechanical weathering is more important than chemical weathering (frost wedging, root wedging)
Capital Reef National Park, UT
N. Lindsley-Griffin, 1999
Salt crystallization is an important weathering process because salt expands as it crystallizes.
Source: U.S.G.S.
Desert Weathering
Desert Weathering
Mudcracks and evaporites
Wind ErosionWind Erosion
Tarbuck & Lutgens, N. Lindsley-Griffin, 1999
Eolian processes are particularly effective in arid and semi-arid regions
Dust Storm
Wind ErosionWind Erosion
N. Lindsley-Griffin, 1999
Sediment moves by suspension, surface creep, saltation
Fig. 14.4, p. 402
Wind ErosionWind Erosion
Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999
Desert pavement forms by deflation - smaller particles blow away, leaving surface covered with tightly packed large particles.
Fig. 14.6, p. 404
Desert Pavement: pebble- to cobble-sized rock fragments covering desert surface after lighter fragments have been removed by wind.
Wind ErosionWind Erosion
N. Lindsley-Griffin, 1999
Fig. 14.6, p. 404
Wind ErosionWind Erosion
Source: U.S.G.S. N. Lindsley-Griffin, 1999
Wind-blown sand hammers at exposed rock faces to produce smooth flat surfaces
Fig. 14.5, p. 403
Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999
Ventifacts have at least one smooth abraded surface facing upwind.
Wind ErosionWind Erosion
Ventifact - Rock faceted and polished by wind-
blown sand
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
Wind Deposition
N. Lindsley-Griffin, 1999
Sand dunes - Mounds of wind-blown sand
Loess - Thick deposits of cohesive,
unstratified wind-blown dust
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
Tifernine dune field, Africa (space shuttle)
Barchan dunes
Coral Pink Sand Dunes State Park, AZ
Sand DunesSand Dunes
N. Lindsley-Griffin, 1999
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
Wind Deposition
Source: U.S.G.S.
Sand ripples in Monument ValleyWind Deposition
Wind Deposition
N. Lindsley-Griffin, 1999
Sand moves up windward slope by creep and saltation. It drops out as the wind loses velocity in the lee of the dune.
Sand DunesSand Dunes
N. Lindsley-Griffin, 1999
Barchan Dunes
Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999
Sand dune slip face: sand oversteepens at top, mass-wastes down the steep slip face of the dune
Sand DunesSand Dunes
Houghton Mifflin, 1998; N. Lindsley-Griffin, 1999
Transverse dunes Longitudinal dunes Barchans
Star dunesParabolic dunes
Sand DunesSand DunesThe type of sand dune that forms depends on amount of sediment supply and wind direction
Sand DunesSand Dunes Barchan dunes, crescent-shaped with horns pointing downwind, require moderate sediment supply and wind strength. (Fig. 14.9A, p. 407)
N. Lindsley-Griffin, 1999Danakil Depression, Ethiopia
Sand DunesSand Dunes
N. Lindsley-Griffin, 1999
Transverse dunes, continuous asymmetrical ridges perpendicular to the strongest wind, require abundant sediment. (Fig. 14.9B, p. 407)
Takla Makan Desert, China
Sand DunesSand Dunes
N. Lindsley-Griffin, 1999
300 m high star dunes in Libya; radar satellite image
Star dunes, isolated and stationary, form where winds blow from all directions. (Fig. 14.9C, p. 407)
Sand DunesSand Dunes
N. Lindsley-Griffin, 1999
Common in Africa and Australia
(Fig. 14.9E, p. 407)
Longitudinal dunes, long narrow ridges parallel to wind, form where little sediment is available and winds are strong.
Sand DunesSand Dunes
N. Lindsley-Griffin, 1999
Ancient sand dune deposits can be recognized by the steep sets of cross beds, separated by gently dipping sets.
Steep sets = slip faceGentle sets = windward face
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.
Eolian SedimentEolian Sediment
Checkerboard Mesa - pattern caused by intersection of joints and eolian cross beds
Source: U.S.G.S. N. Lindsley-Griffin, 1999
Alluvial Fans
Alluvial Fans
N. Lindsley-Griffin, 1999
Streams deposit sediment at slope change at mouth of canyon
Basins of internal drainage are common in the Basin and Range region of the U.S.
Sediment-filled Playa
Fault-block Mountains
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
Nebraska’s Sand Hills - a giant sand dune field formed near the end of the Ice AgesSand HillsSand Hills
NE Conservation & Survey, J.R. Griffin , 1999 Yellow area - Sand hills
AGI/NAGI, J.R. Griffin, 1999
Wind Deposits
Wind Deposits
Deposits of eolian sand and loess, Midwestern United States
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.
Desertification in the central Plains, 1930s
Desertification and Land DegradationDesertification and Land Degradation
N. Lindsley-Griffin, 1999
Houghton Mifflin 1998; N. Lindsley-Griffin, 1999
This desert was once 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.
The Aral SeaThe Aral Sea
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