Transcript Deserts - WordPress.com

PART 1
Introduction to deserts
(not desserts!!!)
What is a desert…
really?
 The
common definition of
“desert” is a region that receives
less than 25cm of rain a
year on average
 Compare
this to Vancouver:
approximately 110 cm a year
 Total
yearly
evaporation > precipitation
 Sparse
vegetation
 Strong
daily winds
 Extreme
heat
 Flash
floods
 Hard
rocky surfaces
Only 15% of the world' s desert surface is
pure sand.
(Parts of the Sahara and Arabia desert)
How do deserts come
form?
 Evaporation
lifts pure water up from the
sea..
 Onshore winds from the oceans carry air
that is rich in moisture over the land.
 Air rises and cools.
 Water vapour condense and falls as
rain.
 It provides vital sustenance before
returning to the sea through ground
water, streams and rivers.
 The
equator is an area of constant low
pressure
 Hot
air at the equator rises and spreads
(up and away) north and south before
cooling, condensing and releasing its
moisture over the tropical zones (high
pressure belts)
 Deserts
occurs in 2
broad belts:
at 20-30 degrees
north and south of the
Equator, along the
Tropics of Cancer and
Capricorn.
 Desert areas develop
under the influence of
the quite permanent
high-pressure areas.



Winds carry moisture-laden air into the
mountains where it is forced to rise
over mountain barriers
Rising air cools, water vapours
condense, and heavy rain falls
Moving past the range, the air
descends, heats up and dries out
 No clouds and sunny skies on the
other side!
 Warm
air over cold water  air cools,
condenses and falls as rain
 Air
drops moisture over the water
 As
it travels to warm land, air
temperature rises
 Warm
air can hold a lot of moisture  no
cloud or rain over desert
Namib Desert
Atacama Desert
 Lands
far away
from oceanic
influence
 The
longer an air
mass is travelling,
the greater its
chances of losing
moisture
 An
example is the
Gobi Desert
How does wind affect
deserts?
 Wind
picks up light surface materials
and carry them in suspension
 Saltation: a
method of transport where
particles roll and bounce along the
ground
 Sparse
vegetation = strong winds
 Lighter
materials: silt, clay
- carried higher up by wind
 Heavier: sand
- carried close to the ground by stronger
wind
- Grinds and scours
What is a toadstool?
What is deflation?
 Wind
blows away sand, silt and clay so
rocks underneath are exposed
 Creates
blowouts in Badlands
 Blowouts
can form oases if it is deep
enough to reach an underground water
source (aquifers)
Desert occupies about one fifth to one
third of the earth' s surface.
How does water affect
deserts?
 Rain
is rare but when it happens it usually
does in the form of sudden, intense
thundershowers
 Flash
floods
 Alluvial
fan (did you get this right?)
PART 2
Desert sand dunes and
landforms
A dune is any
accumulation of
sand-size, windblown
materials.
 Dunes
occur where
there is a large
supply of sand,
wind to move it and
a place where it can
accumulate.
 Dunes are found on
coasts, near rivers
and in desert basins
 Individual
sand
granules (grains of
sand) accumulate
through both water
and wind (eolian)
transportation
 Individual
granules form perpendicularly to
the wind's direction, forming small ripples.
 As
more granules collect, dunes form.
 Dunes
are the most common
depositional landform of the desert
 Windward
slope: long, gentle
(saltation)
 Leeward slope (SLIP FACE): steep
(rolling and sliding rocks)
 Quick
sand is
wet sand held
up by the
water within it
 Usually
not
more than
thigh deep
There are four types
of domes…
 Dunes
are influenced by the strength
and direction of the wind
 The
vegetation
 The
landforms in the area
 Abundant
sand is
available
A
continuous sand ridge
 Right
angles to the wind
 Crescent-shaped
 Over
a small obstruction
 Horns point downward
 Migrate slowly in direction of wind
 May be a part of transverse dunes if they
align together
 Crescent-shaped
 Forms
around a
blowout
 Horns point up
wind
 Vegetation
anchors horns
 Thin
layer of sand
 Winds
blow
continuously oneway
 Long,
straight and
parallel to the
wind



Because of the limited
precipitation, physical
weathering is relatively
more important
Heating-cooling, freezethaw and exfoliation are
common
Produces angular rock
materials
Water is the chief agent of
erosion in deserts
 Heavy rains occur during
intense thunderstorms
 Hard, bare ground is
easily eroded by run-off
 Few plants = cannot
break the fall of raindrops,
slow down water’s
movement, and hold on to
the soil

http://www.youtube.com/watch?v=kV4
aF4AZtY0
Running water is
responsible for
creating many of
these landscapes
Alluvial fan + Alluvial fan = Bajadas
 is
the Arabic term
traditionally referring
to a valley or channel.
A
dry riverbed that
contains water only
during times of heavy
rain
 Shallow
 Heat
lakes

evaporation 
salt deposits
 Present when
playas fill with
water only
rainstorms
causing flash
floods
 Deep, steep-sided
 Sedimentary
valleys
rocks
 Vertical erosion
 E.G. Grand Canyon
 Mesa: an
elevated area of land with a flat top
and steep sides
 table-top shape
 Buttes: isolated hill with steep, vertical sides
and a small, relatively flat top
 PLEATEAU > MESA > BUTTES
 Sometimes
also known as stone or rock
pavement
 Wind removes all sand from the basin
(deflation) leaving behind only the
underlying rock layer
 Semiarid, flat-
floored arid
valley
surrounded by
hills or
mountains
 A large desert
basin that is
slowly filling up
with debris
a
large, relatively
flat area of desert
covered with
wind-swept sand
with little or no
vegetative cover
 Formed
by the accumulation of windblown silt