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# Chapter 8 section 1 (erosion by gravity)

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### Chapter 8 section 1 (erosion by gravity)

1. 1. Section I: Erosion by Gravity <br />Chapter 8: Erosion Forces<br />
2. 2. I. Erosion and Deposition<br />Erosion<br />The process of erosion wears away surface materials and moves them from one place to another. <br />The mud sediments laying on a road following a flood or landslide comes from material that was further up the hillside or upstream. <br />
3. 3. I. Erosion and Deposition<br />What Wears Away Sediments?<br />Gravity: The force of attraction that pulls objects toward Earth’s center. <br />Other causes of erosion, also called agents of erosion, are water, wind, and glaciers. <br />Water and Wind erode materials only when they have enough energy of motion to do the work. <br />Air cannot move sediments on a calm day. <br />Strong wind can move dust and even larger particles. <br />Glacial Erosion<br />Works differently by slowly moving sediment that is trapped in solid ice. <br />As the ice melts, sediment is deposited, or dropped. <br />Sometimes the sediment is carried even further by moving melted water. <br />
4. 4. I. Erosion and Deposition<br />Dropping Sediments<br />Deposition: Agents of erosion drop the sediments they are carrying as they lose energy. <br />When sediments are eroded, they are not lost from Earth, they are just relocated. <br />
5. 5. II. Mass Movement<br />Mass Movement is any type of erosion that happens as gravity moves materials downslope. <br />Some mass movements occur so slowly they are hardly notices. <br />Some occur so quickly they cause catastrophes. <br />The greater the an object’s mass is, the greater its gravitational force is. <br />Common types of mass movements include slump, creep, rock falls, rock slides, and mudflows. <br />Landslides are mass movements that can be one of these types of combination of these types of mass movements. <br />
6. 6. II. Mass Movement<br />Slump<br />When a mass of material slips down along a curved surface , the mass movement is called slump. <br />
7. 7. II. Mass Movement<br />B. Slump<br />Occurs when a slope becomes too steep, the base no longer can support the rock and sediment above it. <br />The soil and rock slip down slope as one large mass or break into several sections. <br />
8. 8. II. Mass Movement<br />Slump<br />Sometimes a slump happens when water moves to the base of a slipping mass of sediment. <br />Water weakens the slipping mass and can cause material downhill. <br />Sometimes a strong rock layer lies on top of a weaker layer (commonly clay or mud); the clay can weaken under weight of the rock and cannot support the rock causing slump to occur. <br />
9. 9. II. Mass Movement<br />Creep<br />Creep occurs when sediments slowly shift their positions downhill, as figure 3 illustrates. <br />Creep is common if areas of frequent freezing and thawing. <br />Trees and fence posts leaning downhill are examples of creep. <br />
10. 10. II. Mass Movement<br />Creep<br />Stages of Creep<br />The ground freezes.<br />Expanding the ice in the soil pushes sediments up. <br />The frozen soil thaws. <br />Sediments slowly fall down slope. <br />
11. 11. II. Mass Movement<br />Rock Falls<br />Rock falls happen when blocks of rock break loose from a steep slope and tumble through the air. <br />As they fall these rocks hit more and more rocks and knock them loose. <br />Ice wedging plays part in rock falls occurring. <br />When large rocks fall; serious damage can occur at the bottom of the slope. <br />
12. 12. II. Mass Movement<br />Rock Slides<br />Rock Slides occur when layers of rock (usually steep layers), slip down slope suddenly. <br />Rock Slides, like Rock Falls, are fast and can be destructive in populated areas. <br />Commonly occur in mountainous areas or in areas with steep cliffs. <br />Happen most often after heavy rains or during earthquakes , but can happen on any rocky slope without warning. <br />
13. 13. II. Mass Movement<br />Mudflows<br />Thick mixture of water and sediments flowing down a slope. <br />Usually occur in dry areas that have layers of loose sediments. <br />Often happen after vegetation has been removed or after fire. <br />Gravity causes this thick, pasty mixture flow downhill. <br />When mudflow reaches bottom it loses energy and deposits sediment and everything else it has carried. <br />These deposits often form a mass that spreads out in a fan/cone shape. <br />
14. 14. II. Mass Movement<br />Mass Movement Summary<br />Mudflows, rock slides, rock falls, creep, and slump are similar in some ways. <br />Most likely to occur on steep slopes and all require gravity to occur. <br />All types of mass movements are more likely to occur after heavy rain. <br />
15. 15. Developing Land Prone to Erosion<br />Chapter 8 Section 1B: <br />
16. 16. I. Consequences of Erosion<br />Building on Steep Slopes<br />When people build on steep slopes, they constantly must battle naturally occurring erosion. <br />Sometimes builders make erosion worse by removing vegetation. <br />This speeds up the erosion process and creates additional problems.<br />Some steep slopes are prone to slumps because of weak sediment layers below. <br />May increase the chance of mass movement due to change in vegetation, more traffic, and change in natural landscape. <br />
17. 17. I. Consequences of Erosion<br />Making Steep Slopes Safe<br />One of the best ways to reduce erosion is to plant vegetation. <br />Plants can be beautiful or weed like, but they all have root structures that hold soil in place. <br />Deep tree roots and fibrous grass roots bind soil together, reducing the risk of mass movement. <br />Plants also absorb large amounts of water. <br />Drainage pipes can prevent water from building up.<br />These materials help increase the stability of the slope by allowing excess water to flow out of a hillside more easily. <br />
18. 18. I. Consequences of Erosion<br />Making Steep Slopes Safe<br />Retaining walls made of concrete or boulders can reduce erosion by keeping soil in place. <br />Terraces can also slow down soil erosion by causing the water to lose its energy as it flows down the hillside. <br />
19. 19. I. Consequences of Erosion<br />Summary<br />People who live in areas of erosion problems spend a lot of time and money trying to preserve their land. <br />They can never eliminate erosion and the danger of mass movement. <br />Earthquakes, floods, and mudslides are unpredictable. <br />Eventually Gravity wins!!<br />Sediment moves from place to place, constantly shaping reducing elevation and changing the shape of the land. <br />