Erosion by Gravity

1. Chapter 8, Section 1 Erosion by Gravity

2. Erosion and Deposition • Erosion • Process that wears away surface materials and moves them from one place to another. • Erosion only occurs when the eroding agent has enough energy to move sediments. • Eroded sediments are not lost, but simply recycled.

3. Erosion and Deposition • What wears away sediments? • Agents of erosion: • Gravity • Force of attraction that pulls objects towards the center of the Earth. • The greater the mass of an object, the greater its gravitational pull. • Water • Wind • Glaciers

4. Erosion and Deposition • Dropping Sediments • Deposition • Dropping of sediments by eroding agents when they lose energy.

5. Mass Movement • Mass movement • Any type of erosion that happens when gravity moves materials downslope. • Can be very slow or very quick. • Usually occurs on steep slopes and after heavy rains. • Water adds mass and creates a fluid pressure between the grains and layers of sediment causing the sediment to expand and weaken. • Examples: • Slump • Creep • Rockfalls • Rock slides • Mud flows

6. Mass Movement • Slump • Occurs when a mass of material slips down along a curved surface. • Leaves a curved scar where the slumped material originated. • Often occurs when: • The slope becomes so steep that the base material cannot support the material above it. • The base material is weaker than the material above it (ex. clay) • Water moves to the base of a slipping mass of sediment.

7. Mass Movement • Creep • Occurs when sediments slowly shift their positions downhill. • Common in areas of freezing and thawing. • Causes structures and trees to “lean” uphill.

8. Mass Movement • Rockfalls • Occur when blocks of rock break loose from a steep slope and tumble through the air. • As the rocks move down the slope, they break other rocks loose.

9. Mass Movement • Rock Slides • Occur when layers of rock slip downslope suddenly. • Commonly occur on steep slopes in mountainous areas or areas with steep cliffs. • Happen most often after heavy rains or earthquakes.

10. Mass Movement • Mudflows • Occur when heavy rains mix with sediment causing the sediment to become thick and pasty and then move downhill under the force of gravity. • The materials of the mudflow are deposited at the base of the slope…often in a fan shape. • Usually occur in areas that have thick layers of loose sediments. • Often happen after vegetation has been removed by fire.

11. Consequences of Erosion • Building on Steep Slopes • Building on slopes can speed up erosion by making the slope steeper or removing vegetation.

12. Consequences of Erosion • Making Steep Slopes Safe • Plant vegetation to anchor the soil and absorb water. • Use drainage pipes or ceramic tiles inserted in slopes to prevent the build up of water. • Use walls of concrete or boulders to hold soil in place.