deserts and wind n. lindsley-griffin, 1999 chapter 14 geology today barbara w. murck brian j....

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:


“It never rains” - Wrong, rains are infrequent, but over a long enough period of time, the effects are large.

San Rafael Swell, UT


Tarbuck-Lutgens, 1998; N. Lindsley-Griffin, 1999

“Water has little effect on deserts” - Wrong, flash floods have tremendous power to erode and transport material



“All erosion is by wind” - Wrong, most desert features formed by running water during wetter Ice Age climates

Source: U.S.G.S.



“Natural arches form by wind erosion” - Wrong, mechanical weathering and mass wasting (rockfalls) are more important.

Arches National Park, UT


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.




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


Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999

Coastal deserts are caused by global oceanic and atmospheric circulation



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


Mechanical weathering is more important than chemical weathering (frost wedging, root wedging)

Capital Reef National Park, UT


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



Sediment moves by suspension, surface creep, saltation

Fig. 14.4, p. 402


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.



Fig. 14.6, p. 404



Wind-blown sand hammers at exposed rock faces to produce smooth flat surfaces

Fig. 14.5, p. 403


Ventifacts have at least one smooth abraded surface facing upwind.


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


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


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


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


Sand dune slip face: sand oversteepens at top, mass-wastes down the steep slip face of the dune


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



Transverse dunes, continuous asymmetrical ridges perpendicular to the strongest wind, require abundant sediment. (Fig. 14.9B, p. 407)

Takla Makan Desert, China



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)



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.



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


Alluvial Fans

Alluvial Fans


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


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

deserts and wind n. lindsley-griffin, 1999 chapter 14 geology today barbara w. murck brian j....

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