Gravity is a major force that causes erosion. It causes sediments to move downslope through processes like slumping, creeping, rock falls, rock slides, and mudflows. These mass movements are more likely after heavy rain and on steep slopes. When people build on steep, erosion-prone slopes it can increase erosion risks, but planting vegetation, using drainage pipes and retaining walls, and creating terraces can help stabilize slopes and reduce erosion. Ultimately, gravity ensures erosion will continue shaping the landscape over time.

1. Section I: Erosion by Gravity Chapter 7: Erosion Forces

2. I. Erosion and Deposition Erosion The process of erosion wears away surface materials and moves them from one place to another. The mud sediments laying on a road following a flood or landslide comes from material that was further up the hillside or upstream.

3. I. Erosion and Deposition What Wears Away Sediments? Gravity: The force of attraction that pulls objects toward Earth’s center. Other causes of erosion, also called agents of erosion, are water, wind, and glaciers. Water and Wind erode materials only when they have enough energy of motion to do the work. Air cannot move sediments on a calm day. Strong wind can move dust and even larger particles. Glacial Erosion Works differently by slowly moving sediment that is trapped in solid ice. As the ice melts, sediment is deposited, or dropped. Sometimes the sediment is carried even further by moving melted water.

4. I. Erosion and Deposition Dropping Sediments Deposition: Agents of erosion drop the sediments they are carrying as they lose energy. When sediments are eroded, they are not lost from Earth, they are just relocated.

5. II. Mass Movement Mass Movement is any type of erosion that happens as gravity moves materials downslope. Some mass movements occur so slowly they are hardly notices. Some occur so quickly they cause catastrophes. The greater the an object’s mass is, the greater its gravitational force is. Common types of mass movements include slump, creep, rock falls, rock slides, and mudflows. Landslides are mass movements that can be one of these types of combination of these types of mass movements.

6. II. Mass Movement Slump When a mass of material slips down along a curved surface , the mass movement is called slump.

7. II. Mass Movement B. Slump Occurs when a slope becomes too steep, the base no longer can support the rock and sediment above it. The soil and rock slip down slope as one large mass or break into several sections.

8. II. Mass Movement Slump Sometimes a slump happens when water moves to the base of a slipping mass of sediment. Water weakens the slipping mass and can cause material downhill. 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.

9. II. Mass Movement Creep Creep occurs when sediments slowly shift their positions downhill, as figure 3 illustrates. Creep is common if areas of frequent freezing and thawing. Trees and fence posts leaning downhill are examples of creep.

10. II. Mass Movement Creep Stages of Creep The ground freezes. Expanding the ice in the soil pushes sediments up. The frozen soil thaws. Sediments slowly fall down slope.

11. II. Mass Movement Rock Falls Rock falls happen when blocks of rock break loose from a steep slope and tumble through the air. As they fall these rocks hit more and more rocks and knock them loose. Ice wedging plays part in rock falls occurring. When large rocks fall; serious damage can occur at the bottom of the slope.

12. II. Mass Movement Rock Slides Rock Slides occur when layers of rock (usually steep layers), slip down slope suddenly. Rock Slides, like Rock Falls, are fast and can be destructive in populated areas. Commonly occur in mountainous areas or in areas with steep cliffs. Happen most often after heavy rains or during earthquakes , but can happen on any rocky slope without warning.

13. II. Mass Movement Mudflows Thick mixture of water and sediments flowing down a slope. Usually occur in dry areas that have layers of loose sediments. Often happen after vegetation has been removed or after fire. Gravity causes this thick, pasty mixture flow downhill. When mudflow reaches bottom it loses energy and deposits sediment and everything else it has carried. These deposits often form a mass that spreads out in a fan/cone shape.

14. II. Mass Movement Mass Movement Summary Mudflows, rock slides, rock falls, creep, and slump are similar in some ways. Most likely to occur on steep slopes and all require gravity to occur. All types of mass movements are more likely to occur after heavy rain.

15. Developing Land Prone to Erosion Chapter 7 Section 2:

16. I. Consequences of Erosion Building on Steep Slopes When people build on steep slopes, they constantly must battle naturally occurring erosion. Sometimes builders make erosion worse by removing vegetation. This speeds up the erosion process and creates additional problems. Some steep slopes are prone to slumps because of weak sediment layers below. May increase the chance of mass movement due to change in vegetation, more traffic, and change in natural landscape.

17. I. Consequences of Erosion Making Steep Slopes Safe One of the best ways to reduce erosion is to plant vegetation. Plants can be beautiful or weed like, but they all have root structures that hold soil in place. Deep tree roots and fibrous grass roots bind soil together, reducing the risk of mass movement. Plants also absorb large amounts of water. Drainage pipes can prevent water from building up. These materials help increase the stability of the slope by allowing excess water to flow out of a hillside more easily.

18. I. Consequences of Erosion Making Steep Slopes Safe Retaining walls made of concrete or boulders can reduce erosion by keeping soil in place. Terraces can also slow down soil erosion by causing the water to lose its energy as it flows down the hillside.

19. I. Consequences of Erosion Summary People who live in areas of erosion problems spend a lot of time and money trying to preserve their land. They can never eliminate erosion and the danger of mass movement. Earthquakes, floods, and mudslides are unpredictable. Eventually Gravity wins!! Sediment moves from place to place, constantly shaping reducing elevation and changing the shape of the land.