1. Chapter 8, Section 1
Erosion by Gravity

2. Erosion and Deposition
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.