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

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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

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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.