Transcript Earth Science 7.3 Landscapes Shaped by Wind

Earth Science 7.3
Landscapes Shaped by Wind
Key Concepts:
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Students will identify today:
 How deflation causes erosion in the desert
 How abrasion shapes desert landscapes
 What types of landforms are deposited by
wind
 How sand dunes differ from one another
Wind Erosion
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Compared with water, wind does
not produce as much erosion in
the desert landscape.
Wind can still be an important
force however and contributes
to shaping the landscape.
Humid areas can resist erosion
by wind because plants hold the
soil together and moisture binds
the soil particles together.
In deserts, the soil is dry and
dusty and there are few plants
with roots to hold the soil
together.
Wind Erosion
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Strong desert winds also pick
up, move and deposit sediment
in great quantities. These are
known as dust storms.
In the Great Depression of
the 30s, farmers in the
central US experienced the
affects of wind erosion when
a severe drought struck.
Wind Erosion
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With all the vegetation
removed to grow crops; the
area became a Dust Bowl when
the strong winds picked up
the soil and blew it in huge
dust storms.
The heavy winds picked up all
the topsoil and blew it away;
leaving behind barren soil
which was unable to grow
anything.
Wind Erosion
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Winds erode in the
desert through two ways
 Deflation
 Abrasion
Wind Erosion
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Deflation is the lifting and
removal of loose particles of clay
and silt by strong winds.
Courser sand particles roll or
skip along the surface in a
process called saltation.
These larger sand particles make
up the “bed load” much as they
do in a stream, rolling along the
bottom.
 In portions of the Dust Bowl,
deflation lowered the land by a
few meters in only a few years.
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Deflation
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Deflation also results in
shallow depressions called
blowouts.
In the Great Plains, you can
find thousands of blowouts
Blowouts range in
size from simple
depressions less than
a meter deep to large
blowouts several
kilometers across
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Desert Pavement
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In sections of the desert, the
surface can be found to have
a course layer of pebbles;
stones too large to be moved
by even the heaviest of winds.
This stony surface is created
by deflation and is called
desert pavement.
Deflation removes all the sand
and particles small enough to
be carried away. What
remains is this layer of desert
pavement.
Abrasion
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Wind can erode by abrasion as
well.
Abrasion is when wind-blown
sand cuts and polishes exposed
rock surfaces.
Blowing sand can grind away at
boulders and smaller rocks,
sometimes sandblasting them
into odd shapes.
Sand rarely travels more than a
meter above the surface so
much of the affect is limited to
cutting close to the ground.
Wind Deposits: Loess
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Loess is windblown silt
that blankets the
landscape
Dust storms pick up this
material, transport it and
deposit it.
The thickest and most
extensive deposits of
loess are in China. The
silt in them comes from
nearby deserts. This fine
buff-colored sediment
gives the Yellow River it’s
name.
Wind Deposits: Sand Dunes
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Like running water, wind
releases it’s load of
sediment when it’s velocity
falls and the energy
available for transport
diminishes.
Sand begins to accumulate
whenever an obstruction
crosses it’s path and the
wind diminishes.
Wind Deposits: Sand Dunes
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Unlike deposits of loess, which
forms blanket-like layers over
broad areas, winds commonly
deposit sand in mounds or
ridges called dunes.
Dunes can occur whenever an
obstruction, such as a rock or
small plant, causes the wind’s
velocity to slow and particles
drop to the ground.
Once the sand starts to mound
it acts as it’s own windbreak
and more and more sand
collects.
Wind Deposits: Sand Dunes
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Dunes are often steeper on the
sheltered side and more gently
inclined on the side facing the
wind.
Wind blows sand up the gentler
windward side. Once the sand
blows over the crest of the
dune, the wind slows and the
sand drops down.
The sheltered side of the dune
gradually becomes steeper and
the sand eventually slides down
the side of the slope.
Wind Deposits: Sand Dunes
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As sand is deposited on the
sheltered side of the dune,
it forms layers inclined in
the direction the wind blows.
These sloping layers are
called cross-beds.
When the dunes are
eventually buried under
sediment and become
sedimentary rock, the cross
beds remain as a record of
their origin.
Wind Deposits: Types of Sand Dunes
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Sand Dunes are not random.
They occur in a variety of
consistent shapes. There are
six major types of sand
dunes.
 Barchan Dunes
 Transverse Dunes
 Barchanoid Dunes
 Longitudinal Dunes
 Parabolic Dunes
 Star Dunes
Wind Deposits: Types of Sand Dunes
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Barchan Dunes: Solitary sand
dunes shaped like crescents
are called barchan dunes.
These form on flat hard
ground where supplies of sand
and vegetation are limited.
Barchan dunes move slowly and
only reach heights of about 30
meters.
If the wind direction remains
consistent, barchan dunes
remain symmetrical. If the
wind direction varies, one tip
of the dune can grow larger
than the other.
Wind Deposits: Types of Sand Dunes
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Transverse dunes: If prevailing
winds are steady, sand is
plentiful, and vegetation is
sparse, dunes form in a series
of long ridges.
These are called transverse
dunes because the ridges are
perpendicular to the direction
of the wind.
Transverse dunes are typical in
many coastal areas. Transverse
dunes in the Sahara and
Arabian deserts create dunes
which reach up to 200 meters
in height.
Wind Deposits: Types of Sand Dunes
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Barchanoid Dunes: A common
dune form that is between a
barchan and transverse dune
is the barchanoid dune.
These scalloped rows of
sand form at right angles to
the wind. The rows resemble
a series of barchans that
have been placed side-byside.
Wind Deposits: Types of Sand Dunes
Longitudinal Dunes:
Longitudinal dunes are long ridges
of sand that form parallel to
the prevailing wind. These
dunes occur where sand
supplies are moderate and the
prevailing wind direction varies
slightly.
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In sections of North Africa,
Arabia, and central Australia,
longitudinal dunes can reach
nearly 100 meters high and
extend for more than 100
kilometers.
Wind Deposits: Types of Sand Dunes
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Parabolic Dunes: Parabolic
dunes look like backwards
barchans. Their tips point
into the wind instead of
away from it.
They form where some
vegetation covers the sand.
Parabolic dunes often form
along the coast where strong
onshore winds and abundant
sand are available.
Wind Deposits: Types of Sand Dunes
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Star Dunes: Star dunes are
isolated hills of sand mostly
found in parts of the Sahara
and Arabian deserts.
Their bases resemble stars
and they usually have three
or four sharp ridges that
meet in the middle.
Star dunes develop in areas
of variable wind direction,
and they sometimes reach
heights of 90 meters.
Key Concepts:
Today we learned:
 Wind erosion is more effective in deserts than in
humid areas.
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There are two types of wind erosion: deflation and
abrasion
The wind can create landforms when it deposits it’s
sediments. Layers of loess and sand dunes are
landscape features created by wind deposits.
A sand dunes form depends on it’s wind direction and
speed, the amount of sand available, and the amount
of vegetation.
Ticket to Go:
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Write down 4 facts that you learned today that
you did not know already in connection to wind
erosion and sand dunes.