www'geo'utep'edu pub hurtado2412

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www.geo.utep.edu/pub/hurtado/2412
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Wind and Deserts

• Understanding desert environments and desert
  processes is important because we live in a
  desert.
• Wind is a major surface process in desert
  environments (water is still the most important,
  though).
• Wind is also important anywhere where loose
  sediment can be eroded, transported and deposited.

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Wind and Deserts

• Wind sediment transport
• Wind erosion
• Wind deposition
• Atmospheric circulation, wind, deserts, and
  climate
• Desert characteristics and landforms

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Sediment Transport by Wind

• Like water flowing in streams, wind is the
  turbulent flow of air.
• Air is much less dense than water. So even
  though wind generally moves faster than running
  water, the size of sediment carried in suspension
  by wind is smaller than that carried by water.
• Wind also carries material in a bed load (pushed
  along the ground).
• Wind has low competence but high capacity.

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Wind
Ground
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Sediment Transport by Wind

• Bed load sediment too large (too heavy/dense)
  to be carried in suspension.
• Material in the bed load gets pushed along or
  moves by saltation (bouncing).
• Saltating sand grains moved by wind can form a
  chain-reaction

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Wind
Saltating sand moved by wind moves in the same
way as saltating grains in a water
flow. Saltating grains move close to the
surface, no more than a meter or so. Saltating
sand moved by wind can be a powerful erosive
agent (abrasion e.g. sandblasting
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Sediment Transport by Wind

• Suspended load wind can only carry very
  fine-grained material (silt- and clay-sized e.g.
  dust) in suspension. Once in suspension, fine
  sediment can be transported high into the
  atmosphere and long distances.
• BUT, it is very hard for dust to go into
  suspension, even if the wind is blowing very
  hard. Why? There is a boundary layer near the
  surface. Remember the velocity gradient due to
  surface friction

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Wind and Deserts

• Wind sediment transport
• Wind erosion
• Wind deposition
• Atmospheric circulation, wind, deserts, and
  climate
• Desert characteristics and landforms

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Erosion by Wind

• Wind is a very effective sorting agent and
  produces distinct erosional features.
• Wind erodes by
• Abrasion
• Deflation

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Erosion by Wind

• Abrasion erosion due to the impact of saltating
  sand grains on an object. Just like sandblasting
  something.
• More effective on softer rocks.
• Produces etching, pitting, smoothing, and
  polishing of objects up to about 1 m above the
  surface.

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Erosion by Wind

• Ventifacts stones that have wind-polished
  surfaces, often with pits, grooves, and facets.
• Yardangs large features, usually bedrock ridges,
  that are elongated and streamlined due to
  differential abrasion by wind. Found on Mars,
  too.
• Ventifact and yardang features are aligned with
  the predominant wind direction.

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Erosion by Wind

• Deflation removal of loose sediment by wind.
  Smaller-sized material is preferentially blown
  away. Leaves behind all the coarse material.
  Results in sorting of sediment and desert
  pavements.
• Deflation hollows (blowouts) shallow
  depressions due to differential erosion by
  deflation.

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Wind and Deserts

• Wind sediment transport
• Wind erosion
• Wind deposition
• Atmospheric circulation, wind, deserts, and
  climate
• Desert characteristics and landforms

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Wind Deposits

• Sediment transport and subsequent deposition by
  wind is more important than wind erosion.
• 2 main types of wind deposits
• Dunes sand sized particles
• Loess silt- and clay-sized particles

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Wind Deposits

• Dunes mounds or ridges of sand deposited by
  wind generally close to the sediment source.
  Form in deserts, beaches, anywhere theres lots
  of sand.
• Dunes form when the wind flows over/around
  obstructions. Wind velocity changes causing
  deposition of saltating sand in wind shadows.
• Growing dunes create larger and larger wind
  shadows.

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Wind Deposits

• Dunes have asymmetrical profiles
• Gentle slope facing the wind (windward)
• Steep slope facing away from the wind (downwind
  or lee)
• Sand saltates up the windward slope and
  accumulate on the downwind slope.
• Downwind slope is maintained at the angle of
  repose (33)
• Dunes migrate downwind by the combined process of
  saltation and slope failure.

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Wind Deposits

• Size, shape, and arrangement of dunes depends on
  wind direction(s), sediment supply, vegetation,
  bedrock, etc.
• Types of dunes
• Barchan
• Longitudinal
• Transverse
• Parabolic

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Wind Deposits

• Barchan dunes crescent-shaped dunes. Tips point
  downwind.
• Form in flat areas with little vegetation,
  limited sand supply, and constant wind direction.
• Most are small, but they can be up to 30 m high.
  Move rapidly (10 m/yr).

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Wind Deposits

• Longitudinal dunes parallel ridges of sand
  aligned parallel to the wind direction.
• Form in areas of limited sand supply where the
  wind is converging from different directions.
• Range from 3 m to 100 m high and may stretch for
  100 km.

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Wind Deposits

• Transverse dunes long parallel ridges oriented
  perpendicular to the wind direction.
• Form in areas where there is abundant sand,
  little vegetation, and constant wind direction.
• From vast dune seas with some dunes 200 m high
  and 3 km wide.

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Wind Deposits

• Parabolic dunes crescent-shaped dunes with tips
  pointing upwind (opposite of barchans).
• Common along coastlines with abundant sand,
  strong winds, and some vegetation.
• Shape is due to deflation (blowout) of the upwind
  side. Downwind side stabilized by vegetation.

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Wind Deposits

• Loess windblown deposits of silt- and
  clay-sized quartz, feldspar, mica, and calcite
  particles. Covers about 10 of Earths surface
  and 30 of USA.
• Source material from deserts, glacial outwash,
  arid river floodplains. Accumulates in places
  where it is stabilized by moisture and vegetation
  ? not found in deserts, but in areas around
  (possibly very far from) deserts.
• Very soft and easily eroded. Very fertile and
  makes good soil

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Wind and Deserts

• Wind sediment transport
• Wind erosion
• Wind deposition
• Atmospheric circulation, wind, deserts, and
  climate
• Desert characteristics and landforms

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Atmospheric Circulation Winds

• Atmospheric circulation patterns are due to air
  pressure, solar radiation, and Earths rotation.
• Heated air is less dense than cold air and will
  rise. Cold air is dense and will sink.
• Equatorial regions are heated by the sun more
  than polar regions.
• The air at the equator is warmed and rises ? low
  air pressure.
• Opposite at the poles ? high air pressure

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Atmospheric Circulation Winds

• Air will flow from high pressure to low pressure
• BUT Earths rotation bends the path of the wind
  (the Coriolis effect) ? winds are deflected and
  travel in east-west directions.

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Atmospheric Circulation Winds

• In 3D, air circulates in convection cells
  called Hadley cells. Air rises (warms up) at
  equator and travels to poles. There air sinks
  (cools) and travels back to equator.
• Rising air at equator will cause rainfall. By
  the time air reaches poles, it is dry.
• Descending air traveling from pole back to
  equator warms up again and is dry ? mid-latitude
  deserts.

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Deserts and Climate

• Dry climates (due to atmospheric circulation
  patterns) mainly occur at low and mid-latitudes.
  Also occur at high latitudes (poles) and
  elsewhere.
• Deserts occur where yearly evaporation gt yearly
  precipitation. (lt 25 cm/yr of rain)
• Dry (arid and semi-arid) climates cover 30 of
  Earths land surface.

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Deserts and Climate

• Rainshadow desert moist marine air moves inland
  and meets a mountain range. Air is forced
  upward. It cools, clouds form, and precipitation
  falls on the windward side. Air decending onto
  the leeward side is dry.
• Continental interiors are typically dry because
  they are far from the ocean.
• Cold desert the extreme polar areas (like
  Antarctica) are very dry.

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Wind and Deserts

• Wind sediment transport
• Wind erosion
• Wind deposition
• Atmospheric circulation, wind, deserts, and
  climate
• Desert characteristics and landforms

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Desert Characteristics

• Low latitude deserts are very hot during daytime
  summers. Average of gt 32 C (90 F). Record is
  58 C (136 F) in Libya.
• But, it can get very cold at night, especially
  the winter ? extreme temperature fluctuations.

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Desert Characteristics

• Dryness due to atmospheric circulation,
  rainshadow, distance from ocean, etc.
• Amount of rainfall is low, but unpredictable.
  All of yearly rainfall could fall in one storm.
  And then no rain for several years!
• Despite extreme heat and dryness, often a wide
  variety of plants and animals.

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Desert Characteristics

• Mechanical weathering dominates in deserts
• Temperature fluctuations
• Frost wedging
• Chemical weathering and physical weathering that
  relies on water (plants, salt crystals growth) is
  minimal.

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Desert Characteristics

• Rock varnish thin red, brown, or black coating
  on desert rocks of all types. Varnish composed
  of Mn and Fe oxides.
• Not produced from the rock. Probably from
  windblown dust or from microorganisms.

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Desert Characteristics

• Desert soils are poorly developed due to lack of
  chemical weathering.
• Sparse vegetation and periodic, intense rainfall
  serve to strip away any soil that does form.

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Desert Characteristics

• Running water is the most important erosive force
  in deserts, even though deserts are so dry!
• During storms, there is intense rainfall, so much
  that it cannot infiltrate the surface ? large
  amount of surface runoff ? flash floods.
• Rapid moving surface runoff and channel flow
  causes rapid erosion (flows have high competence
  and capacity).

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Desert Characteristics

• Arid regions are characterized by internal
  drainage. This means streams that start in
  deserts dont make it to the ocean before they
  evaporate.
• Some streams start outside of deserts and flow
  through them (like the Rio Grande). Humans put
  intense pressure on these rivers.

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Desert Characteristics

• Groundwater in deserts is the prime water supply.
• The water table in deserts is very deep except
  near rivers.
• Groundwater not often replenished
• Water supply is limited
• Groundwater is old

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Desert Characteristics

• Wind is a secondary surface process in deserts.
• Primary effect of wind is the transport of sand,
  silt, and dust.
• Even so, only 25 of worlds deserts are sandy.
  Most are bare rock and desert pavement.

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Desert Landforms

• Playa Lake temporary lake in a low area (like
  the middle of a graben). Shallow, high
  evaporation, salty (evaporites form).
• Alluvial fans fan shaped deposits at the mouths
  of desert canyons.
• Bajada adjacent fans that have grown together.

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Desert Landforms

• Pediment gently sloping bedrock surfaces that
  slope away from desert mountains. Often covered
  by alluvial fans and bajadas.
• Inselbergs isolated, highly resistant,
  steep-sided blocks of rock (often
  mountain-sized).

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Desert Landforms

• Mesa a broad, flat-topped erosional remnant
  surrounded on all sides by steep cliffs.
• Butte like a mesa, but smaller. Pillar-like.

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www.geo.utep.edu/pub/hurtado/2412