Mass Wasting Wasting Chapter 12. Ch. 12 Mass wasting MASSWASTING 1.Landform development 2.Controls...

Mass Mass WastingWasting

Chapter 12Chapter 12

Ch. 12 Mass wastingCh. 12 Mass wastingMASSMASSWASTINGWASTING

1.1. Landform developmentLandform development

2.2. Controls and triggersControls and triggers

3.3. Mass wasting processesMass wasting processes

4.4. Rock slidesRock slides

5.5. Earth creepEarth creep

6.6. Earth flowEarth flow

7.7. Debris and mud flowDebris and mud flow

8.8. Slump and debris slidesSlump and debris slides

1) Landform development1) Landform developmentMASSMASSWASTINGWASTING

““Mass wasting”Mass wasting”

downslope movement of rock, regolith, soildownslope movement of rock, regolith, soil from force of gravityfrom force of gravity one form of erosion one form of erosion follows weatheringfollows weathering mass-wasting + running water = stream valleysmass-wasting + running water = stream valleys

Mass wasting and streams Mass wasting and streams sculpt Earth’s surfacesculpt Earth’s surface

2) Controls and triggers2) Controls and triggersMASSMASSWASTINGWASTING

controlling force: gravity

other important factors:

water saturation

over-steepened slopes

vegetation removal

earthquakes

Water fills pore spaces between sediments,reduces internal resistance, adds weight

Plants add slope stability byprotection against erosion

Strong ground vibrations

Too steep = unstableangle of repose = steepest stable angle of a slope

Angle of repose: Steepest slope that can remain stable

Sand has a small angle of repose, with increasing grain size and angularity the angle increases

Moisture means larger angle of repose

Talus slope, Wheeler Peak,Great basin Natl Park

Fig. Story 12.1

Normal forceNormal force

Gravitational force which is directed to Earth’s center, it has two components (vectors).

Vector diagram

Friction is a force directed opposite to the downslope force.

Downslope force

Angle of dip

Downslope force

Normal forceNormal force

If down slope force < friction, rock stays put

Vector diagram

Friction

Angle of dip

Downslope force

Normal forceNormal force

If we steepen slope, down slope force > friction, rock starts sliding. The angle where that happens is the angle of repose.

Vector diagram

Angle of dip

Friction

Mass movements occur when the force of gravity (meaning the downslope component) exceeds the strength (meaning frictional adhesion) of the slope materials.

3) Mass wasting processes3) Mass wasting processesMASSMASSWASTINGWASTING

Type of material

Type of motion

Rate of movement

Soil/regolith -or- rock/bedrock

Fall - free-fall of materialSlide - coherent material slides along defined surfaceFlow - material flows as viscous fluid

Fastest - rock avalanche: floats (flows) on trapped airSlowest - creep: inches per year (or less)

Talus slopes

4) Rockslide4) RockslideMASSMASSWASTINGWASTING

Blocks of bedrock break loose, slide down slope

usually where strata are inclined

causes:

Base of slope is undercutRain or melting snow lubricates a sliding surface

5) Earth creep5) Earth creepMASSMASSWASTINGWASTING

Creep

Slow downhill movement of material.

From alternate expansion/contraction of material fromfreezing/thawing or wetting/drying

6) Earthflow6) EarthflowMASSMASSWASTINGWASTING

humid areashumid areas hillsideshillsides rich in clay/siltrich in clay/silt slow ratesslow rates

7) Debris and mud flow7) Debris and mud flowMASSMASSWASTINGWASTING

Flow of soil/regolith containing lots of waterFlow of soil/regolith containing lots of waterFlow of soil/regolith containing lots of waterFlow of soil/regolith containing lots of water

Also called mudflowsAlso called mudflowsAlso called mudflowsAlso called mudflows

Common to semi-arid regions, volcanoes (lahars)Common to semi-arid regions, volcanoes (lahars)Common to semi-arid regions, volcanoes (lahars)Common to semi-arid regions, volcanoes (lahars)

Very hazardousVery hazardousVery hazardousVery hazardous

8) Slump and debris slides8) Slump and debris slidesMASS WASTINGMASS WASTING

downward sliding of materialdownward sliding of material moves as a whole section on curved surfacemoves as a whole section on curved surface usually slow, material doesn’t go far usually slow, material doesn’t go far often involves multiple blocksoften involves multiple blocks commonly due to slope over-steepeningcommonly due to slope over-steepening

Fig. 12.6

Some MC….Some MC….MASSMASSWASTINGWASTING

Which of the following statements is false?

A. Mass movements occur when the force of gravity exceeds the strength of the slope materials.B. Mass movements can be triggered by earthquakes and floods.C. Materials can move down a slope very slowly or as a sudden, catastrophic, large movement.D. Mass movements require wind or running water.

Some MC….Some MC….MASSMASSWASTINGWASTING

Which of the following is not an important factor in causing mass movements?

A. amount of water in materialsB. nature of slope materialsC. steepness and instability of slopesD. temperature of slope materials

Some MC….Some MC….MASSMASSWASTINGWASTING

A hill consisting of loose, dry sand that slopes at the angle of repose and has no vegetation ______.

A. is stable unless oversteepened by excavationB. may flow if it becomes saturated with waterC. will be more stable if vegetation takes root on the hillD. all of the above

Some MC….Some MC….MASSMASSWASTINGWASTING

What is the difference between a rockslide and a rock avalanche?

A. In a rockslide, the rocks move more or less as a unit, whereas in a rock avalanche the rocks move independently like a fluid.B. In a rockslide, the rocks move independently like a fluid, whereas in a rock avalanche the rocks move more or less as a unit.C. A rockslide is faster than a rock avalanche.D. A rockslide involves more debris than a rock avalanche.

Some MC….Some MC….MASSMASSWASTINGWASTING

Which of the following statements about mudflows is false?

A. Mudflows tend to move slower than debris flows.B. Mudflows are most common in semi-arid regions.C. Mudflows contain large amounts of water.D. Mudflows can carry large boulders.

Some MC….Some MC….MASSMASSWASTINGWASTING

Telephone poles that lean slightly downhill are a likely result of which of the following processes?

A. creepB. debris slideC. earthflowD. rock avalanche

Some MC….Some MC….MASSMASSWASTINGWASTING

A slump is __________.

A. a rock flowB. a rock slideC. a flow of unconsolidated materialD. a slide of unconsolidated material