Chapter 8: Erosional Forces

1. Chapter 8: Erosional Forces

2. Aim: What is the difference between erosion and deposition?

3. Erosion • is a process that wears away surface materials and moves them from one place to another • Agents of erosion include gravity, water, wind and glaciers • Water and wind need to have large amounts of energy to move materials • Glacial erosion moves sediments trapped in the ice by melting

4. Deposition • Agents of erosion drop sediments they are carrying as they lose energy • Eroded sediments are not lost from earth • They are moved by erosional forces and relocated to other areas

5. Aim: How does gravity act as an erosional force?

6. Mass Movement • Is an type of erosion that happens as gravity moves materials down slope • Some are so slow you can’t see them happening • Other happen rapidly and you can see • Landslides are one of the 5 types or a combination of them • 5 types: slump, creep, rock falls, rock slides and mudflows

7. Slump • Is when a mass of material slips down slope as one large mass • 3 ways this can happen: • when a slope becomes too steep and the base can no longer support the sediments above • When water weakens the base and the slipping sediments move downhill • Strong rock layer lays on top of a weaker layer and it can not support the strong rock

8. Creep • Occurs when sediments slowly shift their positions downhill • Is common in places where there freezing and thawing are common • Evidence of creep includes: leaning trees or human-built structures

9. Rockfalls • Happens when rocks break loose from a steep slope and fall to the ground • As they fall they knock other rocks loose • Falling rocks can cause serious damage to structures on the ground • This is caused by the ice wedging • On roadways signs warn of falling rock areas

10. Rock Slides • Occurs when layers of rocks slide downslope • They move quickly and can be destructive • Occur in areas with steep, mountainous slopes • Mostly happen after heavy rainfall or during earthquakes

11. Mudflows • Occur in places that have thick layers of loose sediments, like soil • Happen after vegetation has been removed because of fires • With the lose of vegetation and heavy rainfalls the loose sediments are carried downhill by gravity • While it moves down slope it gains energy and can destroy anything in its path

12. Aim: How can we slow down erosion on steep slopes?

13. Preventing Erosion on Slopes • Building on steep slopes, you must worry about erosion • By building we quicken the process of erosion by removing vegetation and makes slopes steeper • There are ways to prevent erosion • By planting vegetation on slopes we keep soil in place, which reduces the risk of mass movements • Construction of draining systems prevents water from building up

14. These methods help build a stronger stability of the slope • We can also help stability by building walls to hold soil in place • Large amounts of money are spent on trying to prevent mass movements • Earthquakes and rain make it difficult to completely prevent erosion on slopes • Eventually materials will weaken and gravity takes over causing mass movements

15. Aim: How do glaciers cause erosion?

16. Glaciers • Glaciers form in regions where snow accumulates • As snow builds up the weight of snow compresses the lower layers into ice • With enough pressure the mass of ice and snow begins to move, which are known as glaciers • Are agents of erosion • When moving over land they erode, and change features of earth • While the move the take eroded materials and deposit them somewhere new

17. Plucking • Glaciers weather and erode solid rock • When glaciers melt the water flows into cracks in rocks • Water refreezes , expands and pieces of rock are plucked out by the ice • During this process, boulders, gravel, and sand are added to bottom and sides of the glacier

18. Transporting and Scouring • As the glacier moves large amounts of sediments are transported • Sand and plucked rock fragments scour and scrape the soil and bedrock • Glaciers cause grooves to form along bedrock • Grooves are deep, long, parallel scars • Striations are shallower scars • Grooves and striations show the direction that the glacier is moving

19. Aim: How do glaciers deposit sediments?

20. Types of Deposits • As glaciers melt boulders, sand, clay and silt are left behind • Till Deposits • Mixture of different sized sediments • Can cover huge areas of land • Till areas in the US span from northwestern Iowa and northern Montana, which are wheat fields • Some farmland in Ohio contain till deposits • Rocky pastures of New England have till deposits

21. Moraine Deposits • Form at the end of glaciers • Type of till deposit • This deposit doesn’t cover a large area of land • Materials pile up and form a ridge known as a moraine • Can also form at the sides of glaciers

22. Outwash Deposits • Materials deposited my the meltwater from a glacier is called outwash • Carries sediments and deposits them in layers • Heavier materials drop first • Can form a fan-shaped deposit

23. Eskers • Type of outwash deposit • Looks like a winding ridge • Forms in a melting glacier when meltwater forms a river within the ice • River carries sand and gravel and deposits them within their channel

24. Aim: What are the 2 types of glaciers?

25. Continental Glaciers • Are huge masses of ice and snow • Are thicker than some mountain ranges • In the past covered as much as 28% of earth • Today they cover 10% of the earth • Many of them melted after the last ice age

26. Valley Glaciers • Occur in mountains where the temperature is very low all year round • To find evidence of glaciers look for striations and plucking • They erode bowl-shaped basins called cirques • Arete forms when 2 valley glaciers side by side erode a mountain • Horns form when the glacier erodes the mountain from several directions • Valleys eroded by glaciers have a U-shape

27. Aim: How does wind cause erosion?

28. Wind Erosion • Wind picks up loose sediments and materials • It can’t pick up very large sediments • Wind erodes earth’s surface by deflation and abrasion • Deflation • wind blows across loose sediment, removing small particles like silt and sand • Larger materials are left behind • Occur mostly in deserts, plowed lands and beaches

29. Abrasion • Windblown sediments strike rocks and change the surface and shape of the rocks • The impact of the sand is so strong it breaks off small fragments • Occurs mostly in beaches, deserts and plowed lands

30. Aim: How can we reduce wind erosion?

31. Sandstorms occur when wind blows forcefully in deserts • Dust storms occur when soil dries out • Soil particles weigh less than sand-sized particles and they move higher through the air • Dust storms can cover hundreds of kilometers • Blow soil from places where vegetation has been removed

32. Reducing Wind Erosion • Planting vegetation is one of the best ways to prevent wind erosion • Farmers plant trees along their fields to act like windbreaks • The trees reduce the energy of the wind and it is unable to pick up loose sediments • Tree belts also trap snow, which makes the soil more moist • Moist soil helps prevent erosion

33. Vegetation is planted along seacoasts and deserts too • Grasses, which have fibrous root systems are the best at stopping wind erosion • Their roots are shallow and twist between particles to hold them in place • planting vegetation is a good way to reduce the effects of abrasion and deflation, but can’t complete stop it

34. Aim: What do wind deposits develop into?

35. Loess • Loess is deposits of fine-grained sediments • Sediments settle on hilltop and in valleys • The particles pack together and create a thick, yellowish deposit • Is as fine as talcum powder • Many farmlands have fertile soil that developed from loess deposits

36. Dunes • Dune is a mound of sediments drifted by the wind • Common in desert regions • Sediments being blown by the wind build up against a rock or bundle of vegetation • Side of dune facing the wind has a gentler slope • Side away from the wind is steeper • Shape of a dune depends on the amount of sediment, wind speed and direction and amount of vegetation present

37. Crescent-shaped dune (barchan dune): open side faces the direction that the wind is blowing • This type of dune forms on hard surfaces where the sand supply is limited • Transverse dune: forms where sand is abundant • The direction of this dune is perpendicular to the wind direction • Star dune form in areas where the wind direction changes and form pointed structures