Transcript Erosion

Erosion
•This is the process that moves
weathered sediments from one
location to another.
•Causes the loss of topsoil.
Where is erosion more common?.
• More common on steep slopes than on
gentle slopes.
• More common in areas where there is little
vegetation.
What are the agents of erosion?
• 1. Gravity
• 2. Running water
• 3. Glaciers
• 4. Wind
What do the agents of erosion have
in common?
• They only carry the sediments as long as
they have enough energy of motion.
• They all drop their load when the energy of
motion decreases. – the dropping of
sediments is called deposition.
1. Gravity
• Causes loose material to move down a
slope. This is caused mass movement.
• Mass movement can be slow or rapid
Types of mass movement.
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Slump
Creep
Rockslide
Mudflow
Slump
• Type of mass movement
that takes place on steep
slopes.
• Loose material or rock
layers slip downward as
one large mass.
• Usually leaves a scar
where the material
originally rested.
• Occurs because the
material under the slump
weakens.
Creep
Sediments slowly move down
a hill. Very common is areas
where there is freezing and
thawing. As the ground
freezes small sediments are
pushed up be expanding
water in the soil. When the
soil thaws, the sediments fall
downslope. This happens
slowly about a millimeter
each period.
When creep occurs along a roadway, fenceposts and
telephone poles, on the other hand, don't grow
vertically and merely tilt on creeping slopes.
Rockslide
A rockslide happens
when large blocks of rock
break loose from steep
slopes and tumble quickly
to the bottom
Mudflows
Usually occurs in
relatively dry area.
Have a thick layer of
dry sediments.
When you have heavy
rain, the water mixes
with the soil and forms
mud.
Gravity pulls this
substance downhill.
2. Running Water
• Water moves more sediments than any
other agent of erosion.
• Running water has a great deal of energy.
• Water usually moves down hill under the
influence of gravity.
• When water looses its energy it deposits
its sediments.
What happens to water when it
rains?
• Soaks into the ground.
• Evaporates.
• Runs over the ground. – Runoff
– Eventually enters streams, lakes or the
ocean.
– Affected by the amount of rainfall and the
length of time over which it falls.
– Affected by steepness of land.
– Affected by amount of vegetation present.
Types of water erosion.
• Rill and Gully Erosion
– Groove or small ditch on the side of a slope
left behind by running water.
– Begins when a small stream forms during
heavy rain.
– As stream flows it has enough energy to carry
away some soil
– Groove created is called a channel.
– If the rill channel becomes broader and
deeper it forms a gully.
• Sheet erosion
– Sometimes water erodes without being in a
channel ex. Flowing out from a river, flowing
down a gentle slope, fanning out after flowing
down a mountain.
– Sheet erosion occurs when water that is
flowing like a sheet picks up and carries away
sediments.
– When the water looses its energy the
sediments left behind cover the area like a
sheet.
• Stream erosion
– As water in a stream moves along, it picks up
sediment from the bottom and sides of its
channel.
– This makes the stream deeper and wider.
– Sediment carried along is called the load.
– The lightweight sediments carried along are
the suspended load
– The heavier sediments rolls along the bottom,
called the bed load. Acts like sandpaper,
wears away other rocks by abrasion.
Water deposition.
• Sediments are deposited at different
locations as the stream moves.
– Alluvial fan- shaped like a triangle. If
sediments are deposited when water empties
into a larger water body (ocean, gulf, lake) the
alluvial fan is called a delta.
Here the alluvial fans are called
deltas.
3. Glaciers
• Snows
• Snow does not get a chance to melt so it piles
up.
• Weight of the snow compresses the bottom layer
into ice.
• Pile of snow is so large that ice on the bottom
melts forming a putty-like mass.
• When the whole mass of snow begins to slide on
this putty-like mass and moves downhill- this is
a glacier.
Continental glacier
– huge mass of ice and snow
found near the polar regions
- covers about 10% of Earth’s
surface
- found in Greenland and
Antarctica
-Found in
mountainous
areas where
temperature is
low enough so
that snow does
not melt over
the summer
season.
-Glaciers in the
glacier national
park in
Montana.
Valley glacier
How do glaciers cause erosion?
• As they move they act like a bulldozer
pushing loose material out of their path.
This material can be added to the mass of
the glacier or piled up along the side.
• They weather and erode rock and soil that
was not loose.
Glacial erosion
Plucking – When the
ice melts, water flows
into rocks. Later the
water refreezes in the
cracks, expands, and
breaks the rock. The
boulders, gravel, and
sand that is plucked
can then be added to
the bottom or the
sides of the glacier.
Abrasion - the material plucked
by the glacier grinds into the
bedrock like sandpaper across
wood.
-If the sediments are large they
leave grooves in the bedrock
smaller grooves are called
striations
-if the sediments are fine they
polish the rock.
Striations
This picture depicts
a boulder that has
been subjected to
abrasion by the
bottom of a glacier
riding over the top
of it. The orientation
of the striations
suggest ice
movement from the
top of the picture to
the bottom.
This picture shows
the bedrock of a
glaciated valley that
has been subjected to
abrasion by the base
section of the
ice. Fine materials
such as silt and clay
abrade very small
striations to create
"Polish".
Polish has created a
smooth surface that is
seen here.
Glacial deposition
• When glaciers begin to melt they are
unable to carry much sediment. The
sediment is deposited on the land.
• When glaciers melt and begin to shrink
back – retreat.
• As the glacier retreats a mixture of
boulders, sand, clay and silt is left behind.
This is called the till.
Glacial till
Till can also be deposited
when the glacier stops
moving forward. This does
not cover as wide an area.
Made up of the rocks and
soil that the glacier had
been pushing along.
Looks like a ridge of
material that a bulldozer left
behind.
Smaller ridges can be
found along the sides of the
glacier
These mounds of material
are called moraines.
Another form of glacial
deposition is called
outwash.
This is deposited from
the glacier’s melted ice.
Usually found beyond the
end of the glacier.
Outwash carries
sediments like a river.
-Alluvial fan- fan shaped
deposit
- Esker – looks like a
winding road.
How do you know if erosion was
caused by a glacier?
• Valley glaciers erode bowl shaped basins called
cirques into the sides of the mountain
• If two valley glaciers side by side erode a
mountain peak a long ridge (arete) forms
between them
• If the valley glacier erodes the mountain from
several direction , get a sharpened peak called a
horn.
• Valleys eroded by glaciers have a U - shape,
those eroded by a stream are V - shaped.
cirque
U-shaped
valley
horn
Aretes –
sharp,
thin
ridges
4. Wind
• When air moves it picks up loose material
and transports it to other places.
• Usually cannot pick up heavy sediments
• Carries sediments over large areas.
• Sandstorms
• Dust storms
How does wind cause erosion?
• Deflation - Small particles are picked up
by the wind and the heavier ones are left
behind.
• Abrasion - When wind blown sediments
strike rock and the surface gets scraped
and worn away.
Wind removes
smaller soil
particles and
leaves rocks
exposed on the
surface.
Dust storm
How deflation
leads to the
formation of a
desert
pavement
Desert
pavement
Wind deposition.
• Loess - fine (can be
fine like talcum
powder) grained
sediments deposited
by the wind.
• Dunes – a mound of
sediments drifted by
the wind
What can cause erosion?
• 1. Agricultural cultivation.
Increase use of farmland because of the
increase in the population
• 2. Deforestation.
Forests are constantly being cleared for
lumber, farming, and grazing.
• 3. Overgrazing
• 4. Urban Construction
How to prevent erosion?
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1. Shelter belts of trees.
2. Grazing management.
3. No-till farming.
4. Contour farming
5. Terracing
6. Special steps at construction sites
7. Special steps when building on steep
slopes.
Field
shelterbelts
reduce wind
erosion and
conserve soil
moisture.
During
construction
covering
exposed soil
with plastic to
reduce erosion
Contour
Farming
Farming with
row patterns
nearly level
around the
hill—not up
and down hill.
No-till farming of
wheat
Terracing as a Means of
Soil Conservation
Crop Rotation
Changing the crops
grown in a field,
from year to year.
Grazing management - farmers
must make sure that cattle do not
over graze a field, grass must be
given time to recover.