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.

• 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.

http://www.soils.umn.edu/academics/classes/soil2125/img/10shet2.jpg

• 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.

http://www.geo.vu.nl/~rondeel/Murweb/delta3-alluvial-fan.JPG

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

Valley glacier-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.

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.

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. 

Striations

  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

horn

Aretes – sharp, thin ridges

U-shaped valley

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

Desert pavement

How deflation leads to the formation of a 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?

• 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.

Terracing as a Means of Soil Conservation

No-till farming of wheat

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.