1. Mass Movements at Earth’s Surface
Part 1

2. Mass Movements
Mass movement: downslope movement of loose sediments and weathered rock resulting from the force of gravity
All occur on slopes; since very few places on Earth are flat, almost all Earth’s surface undergoes mass movement
Range from extremely slow motions
to sudden slides, falls, and flows
Materials moved: fine mud to
large boulders

3. Variables that influence Mass Movements:
Material’s weight resulting from gravity
Material’s resistance to sliding or flowing (friction b/w material and slope)
Trigger: such as an earthquake
Erosion and undermining of soil increase potential for earth’s materials to move downhill

4. Other Variable: Water
Too little water: prevent sediment from holding together at all
Too much water: can make slope unstable increasing weight of soils and sediment
In this case: water not involved as transport agent; moves along with materials vs. materials moving along with water

5. Types of Mass Movements: Creep
Creep: slow, steady downhill flow of loose, weathered materials (soils)
Move few cm per year
Effects seen over long periods of time
Can cause tilting of utility poles, fences, bending of trees, cracking of walls, breaking of underground pipelines
Solifluction: slow, downhill movement of loose, water-logged materials in regions of permafrost (mudlike liquid)

6. Types of Mass Movement: Flows
Flows: can move slowly as a few cm per year or hundreds of km per hour
Earth flows: slow movements of soil
Mudflows: swiftly moving mixtures of mud and water triggered by earthquakes (common in volcanic regions and sloped semi-arid regions)
Los Angeles Basin (southern CA) slopes left with little vegetation, heavy rains fall, cause mudflows

7. Types of Mass Movement: Slides
Landslide: rapid downslope movement of materials that occurs when thin block of soil, rock, and debris separates from underlying bedrock Rock Slide: type of landslide that occurs when sheet of rock moves downhill on sliding surface Very destructive form of mass movement!

8. Types of Mass Movement: Slumps
Slump: when mass material in landslide rotates and slides along a curved surface
Occur in areas that have thick soils and on moderate to steep slopes
Common after rains bc water reduces frictional contact between grains of soil; weight of additional water pulls it downhill
Triggered by earthquakes

9. Types of Mass Movement: Avalanches
Avalanches: landslides occur in mountainous areas with thick accumulations of snow
Usually occur on slope
10,000 avalanches each year in mountains of western US
Sun melts surface snow, then refreezes at night, snow falls on top builds up heavy, slip, slide down slope as avalanche

11. Rock Falls
Occur at high elevations, steep road cuts, on rocky shorelines
Rocks loosened by physical weathering and plant growth
Rocks break up and fall downward

12. Mass Movements Affect People
Dangerous Mudflows: widespread loss of human life primarily result of location of villages both high up and in steep terrain and at foot of unstable, saturated slopes
Reducing the Risks: avoid building structures on such steep and unstable slopes
Trenches used to divert water from a slope and control drainage
Covering steep slopes with steel nets and protective fences

13. When you are done with the notes…
Complete the Mass Movement Flipbook.
For Each type of mass movement you need to include the following information
- definition
- Where they typically occur
- Risk associated with it
- a Visual
This will be checked tomorrow at the beginning of class and you will have a quiz on it.