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Erosion

Link to the La Conchita Landslide in California January 2005

Standards: Major Understandings

2.1t Natural agents of erosion, generally driven by gravity, remove, transport, and deposit weathered rock particles. Each agent of erosion produces distinctive changes in the material that it transports and creates characteristic surface features and land-scapes. In certain erosional situations, loss of property, personal injury, and loss of life can be reduced by effective emergency preparedness.

2.1u The natural agents of erosion include:

  • Streams (running water): Gradient, discharge, and channel shape influence a stream's velocity and the erosion and deposition of sediments.
  • Sediments transported by streams tend to become rounded as a result of abrasion. Stream features include V-shaped valleys, deltas, flood plains, and meanders.
  • A watershed is the area drained by a stream and its tributaries.
  • Glaciers (moving ice): Glacial erosional processes include the formation of U-shaped valleys, parallel scratches, and grooves in bedrock. Glacial features include moraines, drumlins, kettle lakes, finger lakes, and outwash plains.
  • Wave Action: Erosion and deposition cause changes in shoreline features, including beaches, sandbars, and barrier islands. Wave action rounds sediments as a result of abrasion. Waves approaching a shoreline move sand parallel to the shore within the zone of breaking waves.
  • Wind: Erosion of sediments by wind is most common in arid climates and along shorelines. Wind-generated features include dunes and sand-blasted bedrock.
    Mass Movement: Earth materials move down-slope under the influence of gravity.

2.1v Patterns of deposition result from a loss of energy within the transporting system and are influenced by the size, shape, and density of the transported particles. Sediment deposits may be sorted or unsorted.

Haiti-a country in crisis how does this compare with Easter Island?
When Easter Island was discovered, it was covered with trees.The 20-30 original settlers brought sweet potatoes and chickens with them. They developed an elaborate society; focused on ritual sites which were embellished with gigantic stone statues. These statues were moved from the quarry on rollers made from trees. At its height; the population of Easter Island numbered about 7,000.

When Europeans first came to the Island in 1722, they found about 3,000 people living in caves and tiny straw shacks. There were no trees and people were perpetually at war. Their chickens were kept in fortified stone coops, which were the object of inter-clan raids - and at times they resorted to cannibalism to supplement this meager source of food. They no longer remembered how their ancestors had transported the mysterious stone statues.

What had happened? Their collapse began with the destruction of the trees. They could no longer build boats, or houses, or move their statutes - lots of which were found stranded in the quarry. Without tree cover, the soil leached its nutrients.

EROSION- the transporting of weathered material

August 11, 2004 Huge Japanese landslide is caught on video VIDEO KINKI, JAPAN - A huge landslide was caught on camera in Japans central Nara region on Tuesday after heavy rains in the region. The landslide was recorded as it happened by staff from the Japanese Ministry of Land, who were inspecting the region for instability. The landslide completely blocked a major mountain roadway, but nobody was injured in the incident. Landslides are common in Japan where around 70 percent of the landscape is mountainous or hilly.

Mass Movements: Mass of earth material moving down slope.

  • Gravity is the driving force behind all mass movements. 

Mass Movements include: Rockfalls, avalanches, mudflows, and landslides are all types of rapid mass movements.

  • These mass movements are often triggered by earthquakes or volcanoes.

Here is the city of Armero in South America before and after a mud flow (Lahar).


Roll over image with your mouse.
Over 25,000 People Dead!

  • Talus Slope: piles of rock fragments at the base of a cliff.
  • Debris flow: A rapid flow of sediment mixed with water (some are also called mudflows).  Lahars are a type of debris flow that occurs on a volcano.
  • Here is a landslide that occurred in New York leaving a large scarp on the side of the mountain.
  • Earthflow is when the regolith moves downslope like a liquid.  Liquidification of soil can be triggered by earthquakes.
  • Slumps are slow movement of material downslope. Slumps often leave crescent shaped scarps and are commonly triggered by earthquakes.

Double click on image to view.

  • Creep is a very slow downhill movement of soil and mixed sediment at rates of cm/yr.  Creep causes fence posts, poles and other objects to lean downhill.  Trees bend in order to grow upward.

Soil is what gives us life! 

  • Soil erosion is a serious problem facing our world today. 
  • Wind alone blows away 1 billion metric tons of topsoil per year. 
  • Without topsoil crops will not produce food. 
  • Contour farming, windbreaks, terraces, strip cropping and no till are methods that farmers have devised to reduce the affects of erosion.

The Hydrologic Cycle

By looking at the above diagram define the following words: evaporation, condensation, precipitation, transpiration, infiltration, runoff, advection, sublimation.

Name four places that water can be stored.

1. ___________ 2. ____________ 3. ____________ 4. ____________

Download a water cycle movie

Permeability and Porosity Lab

Which Bedrock lets water permeate faster?

Water Erosion: Running water is the #1 agent of erosion.

Logrithmic Scales in Earth Science by Charles Burrows (pdf file)

  • The amount of sediment a stream can carry is determined by the speed and size of the stream.
  • Note: the faster the stream is traveling, the more and larger the sediment it can carry.  
  • What is the largest particle size that can be carried by a stream that travels at: 

100 cm/sec, 200 cm/sec, 300 cm/sec, 400 cm/sec?

Settling Rate Flash!

Streams produce V-shaped valleys because they cut through the center of the valley.

  • Dendritic pattern of erosion as a river erodes a plateau.
     
  • Water will wind its way down-slope and find its way to the lowest point (the ocean).
  • Gradient - the steeper the slope then the faster the stream will travel.  Steeper gradient increases the erosional effects of the stream.
  • Rivers carry sediment in solution (dissolved), suspension and saltation (bouncing along the bottom of the river bed).  
  • Note: The amount of material that a stream is carrying is called the stream's Load.
  • Discharge - greater amount of water is able to travel faster down a slope, and carry more sediment. 
  • Bigger, faster rivers erode more material than smaller slower ones.

Meandering Streams: streams that curve or bend (draw in notes)

Shape of the river determines the amount of erosion and deposition of sediment.

  • Water on the outside of a curve speeds up while on the inside water slows down.
  • Inside Turn - stream slows down and deposits some of its load (deposition).
  • Outside Turn - stream speeds up and can carry more sediment (greater erosion).
  • Creates meanders that get cut off (oxbow lakes).

How many oxbow lakes can you see?

Which side is eroding? depositing?

Flood Plain

Water in streams, rivers, and lakes, will eventually make its way to the ocean and sometimes form deltas.

This is called DEPOSITION!
View the Animation

Flash Demos

Beach Erosion

  • Why do you think that these lines are on the side of the mountains? 
  • Rip currents a very dangerous currents that flow away from the beach.  Make sure you swim parallel to the beach if caught in a rip current.  Wave Motions!  Breaking Waves!
  • This is a picture of the beach swash zone. 
  • Longshore current moves sediment along the shore in the direction that the waves hit the beach.

Glacial Erosion

See our glacier retreat!

Theories for Glaciation!

Glaciers - The movement of sediments by large ice sheets.  Sediments transported by glaciers will be angular compared to those transported by water.

Valley Glaciers-formed in the mountains snow turned to ice makes rivers of ice that flow downhill.

  • As the rivers of ice flow downhill they gouge away and erode the sides of the mountains away.  These dark lines are called lateral moraines. 
  • Like a river, glaciers move faster in the middle.
  • Cirques-bowl shaped formations high in the mountains where valley glaciers begin to form.  This is a cirque lake formed from the melting of glacial ice.
  • Aretes and cirques-these three valley glaciers are separated by ridges called aretes.  Aretes, or knife shaped ridges, are formed between two valley glaciers.
  • When three or more cirques cut into the same peak a sharp peak or "horn" or matterhorn is left. 

This is a picture of the famous Matterhorn in Switzerland.

  • Valley glaciers make mountain peaks sharp while continental glaciers round off mountain peaks.
  • Here is a valley glacier that is retreating. 
  • Notice how the valley has been rounded off and now forms a nice U-shaped valley or glacial trough.
  • A glacier retreats by melting faster than it moves down slope.  
  • Glaciers can not move uphill.

Where a smaller valley glacier meets a larger valley glacier a hanging valley trough forms.

  • Notice the U-shaped form of the valley walls.
  • Many times rivers formed from the melting glacier plunge over the cliffs of a hanging trough forming a hanging trough waterfall.
  • Unlike the V-shaped valley of water erosion the glacial erosion produces a U-shaped valley.

Continental Glaciers cover large areas of land and smooth off mountain tops and deepens and widens valleys. 

  • Glaciers may form crevasses on the surface as the ice cracks and moves across the earth's surface.

Glacial Deposition - because a glacier simply drops all the sediment it was carrying as it melts, we get a layer of unsorted and slightly angular fragments. 

  • This is called Glacial Till.
  • This glacial till may look a lot like the soils in this area. 
    Notice the mixed sediment of all different sizes and shapes.
  • As the glacier melted it dropped all this sediment.
  • Striations-scratches and grooves caused by rocks dragged across the bedrock by glaciers.
  • Drumlins- long, smooth, canoe-shaped hills made of glacial till formed as a glacier runs over an earlier glacier moraine.
  • Erratics - Large boulders that have been transported by glaciers
  • Chatter marks: marks left on the bedrock as a glacier scrapes rocks over its surface.
  • Esker: long narrow piles of sediment deposited by a river running under a glacier.
  • Eskers are formed from rivers of meltwater in  pipe-like tunnels beneath the glacier.
  • Roches mountonnees-rocks that are smoothed out on one side because of being run over by a glacier.
  • Icebergs- large pieces of ice broken off a glacier into the ocean.  This process is called calving.

Regents Glacier Images compiled by Charles Burrows

Wind Erosion and Deposition

What is Loess?

  • Dunes: wind blown mounds of sediment. 
  • Windward side: gentle slope facing the wind
  • Leeward side: steeper slope on other side.  Dunes move with the wind
  • Sand is brought up the windward side of the dune and slips down the leeward side. 
  • Dunes move slowly but surely across the landscape.
  • Barchan Dunes: dunes formed when there is a limited amount of sand.  The ends point downwind.
  • Here is a desert filled with barchan dunes.
  • Deflation: wind blowing away loose particles.
  • Desert Pavement: hard surface of pebbles and boulders left when all the fine sediment is blown away.
  • Ventifact: caused by wind blown abrasion.
    Sand blown into the sides of this rock smoothes the surface.
      Arches!
  • Cross Bedding - when wind changes direction beds of sand cross one another at different angles looking like this.

Review Questions:  In order to be prepared for the next exam you should know the answers to the following questions:
1. What is the difference between weathering and erosion?
2. What are the by-products of weathering?
3. What role do these play in the formation of sedimentary rocks.
4. What is karst topography?
5. How does weathering affect man?
6. How does man affect weathering?
7. How does mechanical weather prepare a rock for chemical weathering.
8. Why does climate so strongly influence rates of chemical weathering.
9. What is soil? How is it produced?
10. Why is soil important?

Review Questions:
1. What is it called when rocks are broken up and moved away?
2. Which factor has the most influence on the development of soil? 
a. longitude  b. climate  c. amount of sediment  d. age of bedrock
3. Chemical weathering will occur most rapidly when rocks are exposed to the
a. hydrosphere and lithosphere
b. mesosphere and thermosphere
c. hydrosphere and atmosphere
d. lithosphere and atmosphere
4. Which change would cause the topsoil in New York State to increase in thickness?
a. increase in slope
b. increase in biologic activity
c. decrease in rainfall
d. decrease in air temperature
5. Which soil horizon contains the greatest amount of material formed by biological activity?
a. A-horizon
b. B-horizon
c. C-horizon
d. D-horizon
6. Which erosional force acts alone to produce avalanches and landslides?
a. winds  b. running water  c. sea waves  d. gravity
7. What are 3 ways that sediments may be eroded?
8. The area drained by a river and its tributaries is called its
a. mouth  b. source  c. watershed  d. divide
9.
The primary force responsible for most of the transportation of rock material on the surface of the earth is
a. gravity  b. wind  c. running water  d. glaciers
10.
Which is the largest sediment type that could be carried by a stream flowing at a velocity of 50 cm/sec?
a. small boulders  b. fine sand  c. coarse sand  d. clay
11.
Which material would most easily be carried in suspension by a slow-moving stream?
a. silt  b. clay  c. sand  d. gravel
12.
How are dissolved particles of sediment carried in a river?
a. by bouncing/rolling 
b. by precipitation 
c. in suspension 
d. in solution
13.
Where is deposition occurring in a river meander?
a. on the outside of a curve
b. on the inside of a curve
c. on the river bed
d. in the middle of the river
14.
What particles will settle first at the mouth of a river?
a. cobbles  b. sand  c. pebbles  d. silt
15.
Where is water flowing fastest in a straight river?
a. along the bottom
b. on the sides
c. on the top
d. just below the top in the center
16.
Which landscape characteristic best indicates the action of glaciers?
a. few lakes
b. deposits of well-sorted sediments
c. residual soil covering large areas
d. polished and scratched bedrock
17.
Which feature is more likely to be formed by a valley glacier than by a continental glacier?
a. drumlin
b. esker
c. lateral moraine
d. till plain
18.
Which force is responsible for the movement of a glacier?
a. ground water
b. running water
c. wind
d. gravity
19.
The direction of movement of a glacier is best indicated by the
a. elevation of erratics
b. alignment of grooves in bedrock
c. size of kettle lakes
d. amount of deposited sediments
20.
The general direction of continental glacial advance in New York was from
a. south to north
b. north to south
c. west to east
d. east to west
21.
Which natural resource of economic value would most likely be found in this region?
a. rock salt
b. petroleum
c. natural gas
d. sand and gravel
22.
If the front of an active glacier is observed to be stationary, it is correct to infer that the ice in the glacier is
a. not advancing at all
b. melting as fast as it advances
c. advancing faster than it melts
d. melting faster than it advances