What is a Desert

1
What is a Desert?
Deserts

• Arid, receive lt25 cm of rain per year
• Poorly developed soils
• High evaporation rates
• Sparse vegetation

2
What Creates a Desert?

• Global position/Coriolis Effect
• Rainshadows
• Mid-continent position

3
Facts about Deserts

• Daily temperature swings can be extreme
• Weathering in deserts is primarily mechanical
  aridity limits chemical weathering

4
Facts about Deserts

• Weathering surfaces known as rock varnish occur
  on bare rock surfaces in deserts this is a
  biogenic deposit produced by bacteria living on
  rock surfaces and digesting organic debris

5
Water in Deserts

• Running water is the primary agent of erosion in
  deserts
• Braided streams are common these are often
  ephemeral
• Water table is often lower than stream beds, thus
  many streams are dry except in rainy seasons

6
Deserts and Winds

• Wind is a powerful and ubiquitous agent of
  erosion in deserts creates many unique
  structures
• Wind transports sediments in the desert much like
  stream waters air acts as a fluid
• Sediments include clay and silt-sized particles
  can be carried aloft in a suspended load, and
  sand sized and larger particles are moved along
  the ground as a bed load by creeping or saltating

7
(No Transcript)
8
Wind Erosion

• Wind erodes via two processes abrasion and
  deflation
• Abrasion sandblasting by wind-borne sand
  particles creates etching, pitting, and
  polishing, creates
• Ventifacts stones whose surfaces have been
  polished and faceted by the wind
• Yardangs elongated, streamlined rock bodies
  aligned parallel to prevailing winds
• Hoodoos columns or towers of rock capped by a
  erosion resistant rock layer
• Deflation is the removal of loose surface
  sediments by the wind, creates deflation hollows
  or blowouts, and desert pavement

9
Wind Deposition

• Running water is the prime erosive agent in the
  desert, but desert winds create the major
  depositional forms, including dunes and loess
  deposits
• Dunes winded deposited mounds and ridges of
  sand asymmetric, with about a 30o angle of
  repose
• Loess fine-grained windblown deposits produced
  by glacial erosion

10
(No Transcript)
11
Wind Deposition

• Dunes form as sand grains accumulate to form
  gradually steeper mounds grains saltate up a
  gentle slope until the angle of repose is
  exceeded and the lee side slope collapses,
  repetition of this process causes dune migration

12
Types of Dunes

• Barchan dunes crescent-shaped dunes that form on
  generally flat, vegetation free surfaces with a
  limited supply of sand small (30 m) and mobile
  (up to 10 m per year)

13
Types of Dunes

• Transverse dunes form as long ridges
  perpendicular to the prevailing wind direction in
  areas where sand is abundant and no vegetation
  occurs appear wavelike, with crests as high as
  200 m (sand seas ergs)

14
Types of Dunes

• Longitudinal dunes long parallel ridges of sand
  aligned generally parallel to the prevailing
  winds, which converge from slightly different
  directions

15
Types of Dunes

• Parabolic dunes form where vegetation occurs,
  often along coastal zones, crescent shaped but
  with tips upwind the body of the dune is
  anchored by vegetation

16
Types of Dunes

• Star dunes large star shaped dunes form where
  winds are variable and sand is abundant

17
Name that Dune!
18
Desert Landforms

• Landforms found in deserts include-
• Playas shallow desert lakes
• Alluvial fans delta-shaped mounds of sediment
  deposited as streams flow from highlands onto
  plains
• Inselbergs isolated erosion-resistant rock
  bodies
• Mesas Flat topped mountains capped by
  erosion-resistant rock layers, wider than tall
• Buttes Like mesas, but taller than wide

19
Playas shallow desert lakes that often evaporate
between rainy seasons and create evaporite
mineral deposits
20
Alluvial fans delta-shaped mounds of sediment
formed as streams flowing from mountains spread
out and deposit their sediments onto the plains
below
21
Inselbergs isolated erosion-resistant rock
bodies that stand alone on the desert floor
22
Mesas Flat topped mountains capped by
erosion-resistant rock layers, wider than tall
23
Buttes Like mesas, but taller than wide