MASS MOVEMENTS AT EARTH’S SURFACE
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
VARIABLES THAT INFLUENCE MASS MOVEMENTS:
Material’s weight resulting from gravity
Material’s resistance to sliding or flowing (friction between material and slope)
Trigger: such as an earthquake
Erosion and undermining of soil increase potential for earth’s materials to move downhill
OTHER VARIABLE: WATER
Too little water: prevent sediment from holding together
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
TYPES OF MASS MOVEMENTS 1: CREEPCreep: slow, steady downhill flow of loose, weathered materials (soils)
Causes: gravity, low grade slope
Moves a few centimeters 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 (mud-like liquid)
Occurs most in colder regions that have permafrost
TYPES OF MASS MOVEMENT 2: FLOWSFlows: can move slowly as a few centimeters per year or hundreds of kilometers per hour
Causes: gravity, excess water, slope
Earth flows: slow movements of soil
Mudflows: swiftly moving mixtures of mud and water
Trigger: earthquakes (common in volcanic regions and sloped semi-arid regions)
Occurs most in flood prone and coastal areas
Los Angeles Basin (southern CA) slopes left with little vegetation, heavy rains fall, causing mudflows
TYPES OF MASS MOVEMENT 3: SLIDES
Landslide: rapid downslope movement of materials that occurs when thin block of soil, rock, and debris separates from underlying bedrock
Causes: gravity, earthquake activity, slope, excess water
Rock Slide: type of landslide that occurs when sheet of rock moves downhill on a sliding surface
Trigger: earthquake, floods, ice wedging
Occurs in earthquake and flood prone areas
Very destructive form of mass movement!
TYPES OF MASS MOVEMENT 4: SLUMPS
Slump: when mass material in a landslide rotates and slides along a curved surface
Causes: earthquake activity, excess water, slope, frost wedging, and undercutting.
Occur in areas that have thick soils and on moderate to steep slopes
Common after rains because water reduces frictional contact between grains of soil; weight of additional water pulls it downhill
Triggered by earthquakes
TYPES OF MASS MOVEMENT 5: AVALANCHESAvalanches: landslides occur in mountainous areas with thick accumulations of snow
Causes: earthquake activity, loose snow build up
Usually occur on slope; in regions that are cold and have a lot of snow
10,000 avalanches each year in mountains of western US
Trigger: many things such as earthquakes, animal movement, heavy snowfall, sound waves
Sun melts surface snow, then it refreezes at night, new snow falls on top and builds up. The snow is heavy, so it slips, and slides down slope as an avalanche
1
2
3
4
5
TYPES OF MASS MOVEMENT 6: 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
Rocky areas and roads cut close to, or through a mountain
6
MASS MOVEMENTS EFFECT 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
Billions of dollars worth of property damage every year
MASS MOVEMENT FLIPBOOK
Make a flipbook foldable with 4 sheets of paper. Title on the first flap, and then 6 flaps for each type of mass movement, and 1 flap for the effects of mass movements.
Directions ~ Make a foldable as directed, then use your notes and this presentation to draw and fill in the following information:
Top half of a section: Picture of the mass movement scene
Bottom half: definition, effects, trigger(s), what causes it, and, where it occurs most often.