PART 1

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 motionsto sudden slides, falls, and flowsMaterials moved: fine mud tolarge boulders

Variables that influence Mass Movements:

Material’s weight resulting from gravityMaterial’s resistance to sliding or flowing

(friction b/w material and slope)Trigger: such as an earthquakeErosion and undermining of soil increase

potential for earth’s materials to move downhill

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

Types of Mass Movements: Creep

Creep: slow, steady downhill flow of loose, weathered materials (soils)

Move few cm per yearEffects seen over long periods of timeCan 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)

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 soilMudflows: 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

Types of Mass Movement: Slides

Landslide: rapid downslope movement of materials that occurs when thin block of soil, rock, and debris separates from underlying bedrockRock Slide: type of landslide that occurs when sheet of rock moves downhill on sliding surfaceVery destructive form of mass movement!

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

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 USSun melts surface snow, then refreezes at

night, snow falls on top builds up heavy, slip, slide down slope as avalanche

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

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

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.