Transcript Wind & Desert Landscapes

WIND & DESERT LANDSCAPES
WHAT IS A DESERT?
Deserts form when yearly precipitation is less
than evaporation
 Tropical deserts have:
 sparse vegetation (lack of rainfall)
 Strong winds (convection currents from sun
heating)
 Flash floods
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FACTORS OF DESERT FORMATION
Latitude:
 Key factor
 Equator is an area of low pressure, heat rises
from ground level and moves to the upper
atmosphere, then away from the equator
 This occurs until about 30 degrees North and
South of the equator
 Deserts are common in this region
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FACTORS OF DESERT FORMATION
Mountain barriers:
 As air is forced to rise over a mountain barrier,
the rising air cools and condenses, causing
heavy rainfall on the windward side of the
mountain
 On the leeward side, the drier air warms as it
sinks
 The higher the mountain range, the drier the
descending air – it can be so dry that it pulls
moisture from the ground
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FACTORS OF DESERT FORMATION
Cold ocean currents:
 As warm air crosses cold water, the air cools
rapidly causing water vapour to condense and
rain clouds/fog form offshore
 Air drops moisture over the water
 As the air crosses onto warm land, air
temperatures rise – warm air can hold more
moisture
 Because of this, there is no rain
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FACTORS OF DESERT FORMATION
Ocean influence:
 Oceans are the main source of atmospheric
moisture, so areas far way from ocean
influence are very dry
 These areas can be classified as deserts
 Usually, the longer an air mass is over land, the
more moisture it will lose

WIND EROSION IN DESERTS
Trees and vegetation
protect the soil, acting as
a windbreak, holding the
soil in place with their
roots, and slowing heavy
rain with leaves
 Because of this, areas
with sparse vegetation
experience strong winds
that pick up surface
materials (silt, clay, sand)
and carry them in
suspension

WIND EROSION IN DESERTS
Sand can only be carried in suspension in
stronger winds, and remains closer to the
ground than silt and clay
 The movement of sand is largely by saltation
 Strong winds in deserts can generate dust
clouds that are carried long distances by air
currents

WIND EROSION IN DESERTS
Silt and clay are too
soft to have significant
abrasive power
 Sand easily
grinds/scours rock
surfaces
 Sand abrasion can
cause mushroomshaped rocks called
toadstools

WIND EROSION IN DESERTS
Deflation is when wind
blows away sand, silt
and clay in deserts so
that underlying rock is
exposed
 These stony
landscapes are called
Badlands
 Deflation can lead to
deep hollows called
blowouts
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WIND EROSION IN DESERTS
Blowouts may be deep
enough to reach the water
table and contain water
for at least part of the
year
 Many oases form this way
 A desert oasis fed by a
spring line from an
underground aquifer will
never run dry unless
pressure in the aquifer
drops enough to stop the
water flow

WATER EROSION IN DESERTS
Rain is rare but often comes as a sudden
downpour in mountains surrounding deserts
 With no vegetation to anchor soil, rainwater
rushes down valleys carrying sediment toward
the valley floor
 When water reaches the flat land, the flow
slows and debris are deposited
 Water is no longer confined to a valley and
spreads to create an alluvial fan of deposits

DESERT SAND DUNES
As wind blows across
the desert, deposits of
sand can collect on an
obstruction (rock, plant,
etc.) and over a time a
sand dune forms
 As the dune grows, it
acts as a windbreak,
slowing the velocity of
wind and allowing for
more deposition

DESERT SAND DUNES
Sand dunes have a long, gentle slope on the
windward side and a steep slope on the
leeward side
 Sand particles move by saltation up the
windward slope, and roll/slide down the
leeward slope (slip face)
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DESERT SAND DUNES
In areas where wind predominantly blows in the
same direction, dune migration can take place
 This can be a problem in areas were roads,
buildings, and parking lots are located
 Vegetation is often planted to slow wind

FOUR TYPES OF DUNES
The type of dune is determined by the strength
and direction of the wind, vegetation, and local
landforms
 If wind blows consistently from one direction,
dunes will have a long, gentle windward slope
and a steep leeward slop

FOUR TYPES OF DUNES
1. transverse dune:
 These dunes form in
regions where abundant
sand is available for
dune formation
 Continuous sand ridge
formed at right angles
to the wind
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FOUR TYPES OF DUNES
2. Barchan dune:
 Crescent-shaped dune
 Forms over
obstruction
 Horns point downwind
and migrate slowly in
the direction of the
wind
 Can be incorporated
into transverse dunes
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FOUR TYPES OF DUNES
3. Parabolic dune:
 Crescent shaped, but
forms around a blowout
 Shaped like a barchan
dune, but horns point
upwind
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FOUR TYPES OF DUNES
4. Longitudinal dune:
 Found in deserts where
only a thin layer of sand
is present and winds
blow continuously in the
same direction
 Long and straight, form
parallel to the wind
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DESERT LANDFORMS
Alluvial fans may
become so large they
join together to form
bajadas
 Wide, dry ditches that
channel water out to
the desert floor and
contain water only
during flash floods are
called wadis
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DESERT LANDFORMS
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Wadis extend to a
depression in the desert
basin, where water
accumulates after
floods
The result is the
formation of playa lakes
The extreme heat of the
desert evaporates the
water quickly and only
salt deposits remain
DESERT LANDFORMS
Streams and rivers
erode through
plateaus of
sedimentary rock to
form deep, steep
sided canyons
 Because of the lack
of rainfall, almost no
lateral erosion occurs
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DESERT LANDFORMS
When a large area of
plateau is separated as
a result of erosion from
the main plateau, a flat
topped feature called a
mesa is formed
 Erosion can break
apart mesas into
smaller flat topped
features called buttes
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DESERT LANDFORMS
Wind can remove all sand from a basin
leaving behind only the underlying rock layer
over time
 These rock surfaces are called hamadas
 Sometimes eroded sediment of upland
areas is carried off by wind and deposited in
lower areas. A desert basin filling with debris
in this way is called a bolson
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DESERT LANDFORMS
When sand is
abundant, sand seas
are common
 These sand seas are
called ergs
 They migrate in the
direction of the wind
 Difficult to stabilize

DESERT LANDFORMS
All deserts, regardless of amount of sand, have
some amount of windblown dust
 Windblown dust is called loess, and can be
found in thick deposits on the leeward side of
deserts where prevailing winds subside and the
dust settles
 Loess deposits are very fertile
