1. Chapter 3: Erosional Forces
Section 1 – Erosion by Gravity

2. Key Vocabulary for Section 1
Erosion - a process that wears away surface materials and moves them from one place to another.
Deposition - agents of erosion drop the sediments they are carrying as they lose energy. (Dropping sediments as they are being carried)

3. Key Vocabulary (continued)
Mass movement - any type of erosion that happens as gravity moves materials downslope.
Slump – a type of mass movement; when a mass of material slips down along a curved surface.
Creep - occurs when sediments slowly shift their positions downhill.

4. What causes erosion? *These are called agents of erosion
GRAVITY! (The force of attraction that pulls all objects toward Earth’s center.)
Water/Wind
Water and wind erode materials only when they are strong enough.
Glaciers
Glacial erosion works by slowly moving sediment that is trapped in solid ice. As the ice melts, sediment is deposited, or dropped.

5. Types of Mass Movement Slump (slips down a curved surface) Causes:
The slope becomes too steep and the base material can no longer hold the rock and sediment above it
when water moves to the base of a slipping mass of sediment
strong rock layer lies on top of weaker rock layer

6. Types of Mass Movement (continued)
Creep
Causes:
Slowly shifting sediments
Common in areas of frequent freezing and thawing

7. Types of Mass Movement (continued)
Rockfalls
Causes:
When blocks of rock break loose from a steep slope and tumble through the air
Rock slides (steep rock layers suddenly slip)

8. Types of Mass Movement (continued)
Mudflows (a thick mixture of sediments and water flowing down a slope)
Causes:
occur in areas that have thick layers of loose sediments
When heavy rains fall, water mixes with sediment, causing it to become thick and pasty.
At the bottom of a slope, it loses its energy of motion and deposits all the sediment and everything else it has been carrying.

9. Types of Mass Movement (continued)
Landslides are a special kind of mass movement because they can either be one of these types or a combination of these types of mass movement.

10. When/where do these types of mass movement usually occur?
On steep slopes
Where there is strong gravity
After a heavy rain

11. Ways to make steep slopes safer:
Plant vegetation (reduces erosion)
Deep trees roots and fibrous grass roots hold soil together (reduces the risk of mass movement)
Adding drainage pipes into slopes (prevents water from building up)
Building walls made of concrete/boulders (reduce erosion by holding soil in place)