Chapter 16 Deserts

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• Chapter 16 - Deserts

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Definition of Desert

• A desert is an area with less than
• 25 cm (10 inches) of annual precipitation
• aridity index potential evaporation/precipitatio
  n
• greater than 4.0
• Deserts may be cold, temperate or hot. All major
  continents have one type of desert or the other.

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Wind-Swept Sand Dunes
Deserts cover about 1/5th of the land About 10
of that is dune covered
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Desert Life Forms (Roadrunner)
Desert animals are adapted to the scarcity of
water
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Desert Life Forms (Lizard)
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Types of Deserts

• Desert Types
• - Subtropical Desert 30o Latitude
• - Deserts on Leeward side of major Mountain
  ranges
• - Interior Deserts- center of continents far from
  ocean
• - Coastal desert- prevailing onshore wind cooled
  by cold ocean current
• - Polar deserts- extremely dry

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World's Deserts
about 1/5th of land
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Atmospheric Convection and Subtropical Deserts
Tropopause Barrier
RAIN
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A Rain-Shadow Desert
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Ocean-Current Desert
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Desert Landscape (Features)

• Weathering and desert streams create Desert
  features
• Weathering in Desert is mostly mechanical
• Little chemical weathering produces manganese and
  iron-oxide stains, called desert vanish
• Stream Erosion
• Arroyo- channel with water during periods of high
  discharge but dry most part of the year
• Pediments- large-area gently inclined sediments
• Inselberg- steep-sided knob of durable rock
• Playa- dry lake bed

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Chemical Weathering in Deserts
Desert Varnish
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Desert Landforms Produced By Water
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Uluru (Ayers Rock) Inselberg
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Playas
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Playas (cont'd)
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Playas (cont'd) A Playa in Death Valley,
California
Spadefoot Toad Anecdote
Evaporites 20 mule team Borax
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Swimmers in hypersaline Dead Sea
Evaporite deposits indicate a dry climate in the
geologic record
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Water-Carved Canyon
Dangers of Arroyos-Flash Floods
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Work of Winds

• Erosion by Wind
• Deflation- wind removes finer particles from the
  surface
• Desert pavement- layer of pebbles left behind
  after deflation
• Abrasion- sand blasting
• Ventifacts- wind-shaped stones with sharp-edge
  faces
• Yardangs- streamlined desert ridges

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Sandstorms

• In dry areas large wind-driven sandstorms, called
  black rollers in some areas, can suspend
  sand-size particles
• These sand-blast anything in their paths

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Blowout Caused by Deflation
Anecdote How fast does this occur?
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Desert Pavements
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Desert Pavements (cont'd)
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Desert Pavements (cont'd)
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Desert Pavements (cont'd)
Makes a great runway for supply aircraft Just
clear away a few big rocks We tow makeshift rakes
behind our Land Rovers
Source Martin Miller
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Desert Pavements (cont'd)
Note desert varnish, lack of sand, Iron Oxide
over everything Infiltration is slow Flash
Floods, and most water leaves the area
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Remnants of Wind AbrasionIn addition to flash
flood erosion(surfaces are sand blasted)
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Origin of Ventifacts
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Origin of Ventifacts (contd)
Henry Bunn, Left Handed Smoke Shifter
Wind
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Yardangs , White Desert, Egypt
Sphinx?
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Transport By Wind

• Suspension- most suspended grains consist of dust
  (silt, clay, pollen, bacteria, salt crystals from
  evaporation, etc. Can be high and travel
  thousands of miles.
• Bed Load- sediments moved along or near the
  ground
• Moves by Traction (rolling) or Sliding (rare)
• or saltation (bouncing) - bed loads lifted off
  the ground momentarily due to force of collision
  with other grains. If windy enough, one starts a
  chain reaction

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Transport of Wind-Borne Sediment
Sahara to Caribbean Effect on Hurricanes
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Deposition of Dunes

• Reduced wind velocity results in sediments
  deposition
• Dunes are hills of loose wind-born sand
• Size, shape, and orientation of dune are
  determined by available sand, vegetation, and
  wind

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Beach Sand Dunes
Sandy Hook, Island Beach
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Deposition of Winds Bed Load
Rain Shadow Desert in Lee Of Mountains
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Large Scale Dunes (Gobi Desert)
Movement vs. small dunes
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Dune Migration
Just like ripples in a stream
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Dune Migration (cont'd)
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Deposition and Dune Types

• Dune Types
• Transverse- ridges that are perpendicular to
  prevailing wind direction
• Longitudinal- ridges that are parallel to
  prevailing wind direction
• Barchans- crescent-shaped with horns pointing
  downwind
• Horseshoe (Parabolic)- crescent-shaped with horns
  pointing upwind
• Star- winds from three or more directions

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Transverse Dunes
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Transverse Dunes
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Longitudinal Dunes
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Longitudinal Dunes
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Barchan Dunes
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Barchan Dunes in Baja California
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Parabolic Dunes
Horns pinned by plants
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Star Dunes
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Star Dunes
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Lithified Sand Dunes (Jurassic Navajo Sandstone)
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Loess from the Columbia River Basin

• Loess deposits are (usually) of glacial outwash
  silt. These thin downwind.

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Desertification

• Desertification- invasion of desert conditions
  into formerly non-desert areas
• Drought and overpopulation are main causes
• Signs
• Lowering of water table
• Marked reduction of water supply
• Increased salinity in water and soil
• Progressive destruction of native vegetation
• Accelerated soil erosion
• Anecdote the role of goats, Acacia Commifera
  removal and evaporation. Past and current
  drought.
• Death of cattle water dependant foods.

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Desertification- Map of the Sahel
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End of Chapter 16