ErosionErosionGEOLOGY TODAY - Chapter 7

Barbara W. MurckBrian J. Skinner

HILLSIDE CREEP

N. Lindsley-Griffin, 1999

N. Lindsley-Griffin, 1999

Erosion by WaterErosion by Water

Erosion begins as soon as rain hits the surface and begins to run downhill.

Turbulent flow packs more energy than slower laminar flow.

Particles are carried as dissolved load, suspended load, bed load. (Fig. 7.19, p. 207)

(Fig. 7.18, p. 206)

Erosion by WaterErosion by Water

N. Lindsley-Griffin, 1999

Fine sediment carried in suspension gives the Huang He River of China its yellow color

Erosion by WaterErosion by Water

Fig. 7.20, p. 207

N. Lindsley-Griffin, 2000

Erosion by WindErosion by WindWind can move only very small particles: sand by saltation, dust by suspension. (Fig. 7.21, p. 208)

N. Lindsley-Griffin, 1999

Erosion by IceErosion by Ice

N. Lindsley-Griffin, 1999

Ice flows slowly downhill.

Density gives it laminar flow, ability to carry very large fragments. (Figs. 7.22A, C; p. 209)

Till, Matanuska Glacier, AK

Kaskawulsh Glacier, Yukon

Erosion by IceErosion by Ice

N. Lindsley-Griffin, 1999

Because of its density, ice has great power to erode and shape the landscape.

Polished and grooved surface made by Findelen Glacier, Swiss Alps.

(Fig. 7.22B, p. 209)

Erosion by Mass WastingErosion by Mass Wasting

N. Lindsley-Griffin, 1999

Rock fragments loosened by weathering move downhill under the pull of gravity.

Angle of repose:

Maximum angle at which loose material remains stable

TalusCrater Lake N. P., OR

Angular fragments

Poorly sorted

Locally derived

No layering

N. Lindsley-Griffin

Characteristics of landslide depositsCharacteristics of landslide deposits

Balance of Forces on a SlopeBalance of Forces on a Slope

N. Lindsley-Griffin, 1999

Slopes are stable ifdriving force (DF) of slope material is equal to, or less than, the resisting force (RF)

CAUSES OFINSTABILITY:Adding weightReducing frictionIncreasing slope

Slope stable: DF RFSlope unstable: DF = RF

Slope Failures: SlumpsSlope Failures: Slumps

N. Lindsley-Griffin, 1999

Fairly coherent blocks slip down on curved planes

Blocks rotate backwards at top.

May have mudflow at base.

Shear strength:In solid rock depends on atomic forces

In loose material depends on friction between material particles

(Tab. 7.2, p. 210)

Slumps: Turnagain Heights, AK

Slumps: Turnagain Heights, AK

N. Lindsley-Griffin, 1999

THE SETTING

City built on gravel layer

over a thick, water-soaked clay layer

THE TRIGGER

Anchorage, Alaska,

earthquake of 1964

THE RESULT

Liquefaction of clay layer

City slides towards the sea

Slope Failures: RockfallsSlope Failures: Rockfalls

N. Lindsley-Griffin, 1999

Sudden and rapid

Very steep slopes

Consist of loose rock

Not water-saturated

(Tab. 7.2, p. 210)

Slope Failures: TalusSlope Failures: Talus

N. Lindsley-Griffin, 1999

Small fragments accumulate at base of cliff Forms talus apron or talus slope

Canadian Rockies

Slope Failures: Debris FallSlope Failures: Debris Fall

N. Lindsley-Griffin, 1999

Debris is a mixture of rock, soil, trees, rock climbers…..

(Tab. 7.2, p. 210)

Slope Failures: RockslideSlope Failures: Rockslide

N. Lindsley-Griffin, 1999

Rocks slide down a steep inclined plane

(Tab. 7.2, p. 210)

Slope Failures: Debris SlidesSlope Failures: Debris Slides

N. Lindsley-Griffin, 1999

A mixture of soil, regolith, rock, and other debris sliding on inclined planar surface

(Tab. 7.2, p. 210)

N. Lindsley-Griffin, 1999

Slope stability and water:

Small amounts of water -

strengthen material

Large amounts of water -

increase weight

Material loses surface

tension

Sand liquefies

Clay swells

Solifluction - very slow movement of water-saturated slurry

(Tab. 7.2, p. 211)

Sediment Flows: may be wet or drySediment Flows: may be wet or dry

Wet Sediment Flows: Debris FlowsWet Sediment Flows: Debris Flows

N. Lindsley-Griffin, 1999

Water-saturated slurry flow with particles larger than sand, moves rapidly

(Tab. 7.2, p. 211)

Wet Sediment Flows: MudflowsWet Sediment Flows: Mudflows

N. Lindsley-Griffin, 1999

Lahar, 1985, Nevada del Ruiz, Colombia

Buried 25,000 people, 15 meters thick, traveled 70 km/hour

Rapid slurry flows consisting mostly of fine particles

Lahars - hot volcanic mudflows

(Tab. 7.2, p. 211)

N. Lindsley-Griffin, 1999

Imperceptible, slow, downslope movement of regolith

Dry Sediment Flows: CreepDry Sediment Flows: Creep

(Tab. 7.2, p. 211)

Dry Sediment Flows: EarthflowsDry Sediment Flows: Earthflows

N. Lindsley-Griffin, 1999

Relatively rapid granular flow of soil and regolith that is not water-saturated.

(Tab. 7.2, p. 211)

Dry Sediment Flows: Debris Avalanches

Dry Sediment Flows: Debris Avalanches

N. Lindsley-Griffin, 1999

Very rapid movement of rock and regolith.

Rare, but extremely dangerous - several hundred mph.

(Tab. 7.2, p. 211)