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Mass Wasting (Shaping Earth’s Surface,

Part 3)

Science 330 Summer 2005

Mass wasting and landform development

Mass wasting refers to the downslope movement of rock, regolith, and soil under the direct influence of gravityRole of mass wasting

Geologic process that often follows weatheringCombined effects of mass wasting and running water produce stream valleys

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Mass wasting and landform development

Slopes change through timeNo minimum angle is required for mass wasting to occurMost rapid and spectacular mass-wasting events occur in areas of rugged, geologically young mountainsMass wasting and erosional processes slowly lower the land surface

Controls and triggers of mass wasting

Important factors includeThe role of water

Diminishes particle cohesion (friction)Water adds weight

Oversteepening of slopes – slope angleStable slope angle (angle of repose) is different for various materials

Oversteepened slopes are unstable

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The effect of water on mass wasting

Controls and triggers of mass wasting

Important factorsRemoval of anchoring vegetationEarthquakes as triggers

May cause expensive property damage

Can cause liquefaction – water saturated surface materials behave as fluid-like masses that flow

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Controls and triggers of mass wasting

Landslides without triggersSlope materials weaken over timeRandom events that are unpredictable

Classification of mass wasting processes

Generally each event is classified byType of material involved

Mud EarthRock

Type of motionFall (free-falling pieces)Slide (material moves along a surface as a coherent mass)

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Classification of mass wasting processes

Generally each event is classified byType of motion

Flow (material moves as a chaotic mixture)

The velocity of the movementFastSlow

Types of mass wasting

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Types of mass wasting

SlumpMovement of a mass of rock or unconsolidated material as a unit along a curved surfaceOccurs along oversteepened slopes

A slump with an earthflow at the base

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Types of mass wasting

RockslideBlocks of bedrock slide down a slopeGenerally very fast and destructive

Debris flow (mudflow)Consists of soil and regolith with a large amount of waterOften confined to channels

Types of mass wasting

Debris flowSerious hazard in dry areas with heavy rainsDebris flows composed mostly of volcanic materials on the flanks of volcanoes are called lahars

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A lahar from the 1980 Mt. St. Helens eruption

Types of mass wasting

EarthflowForm on hillsides in humid regionsWater saturates the soilCommonly involve materials rich in clay and silt

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An earthflow on a newly formed slope

Types of mass wasting

Slow movementsCreep

Gradual movement of soil and regolith downhillAided by the alternate expansion and contraction of the surface material

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Some visible effects of creep

Types of mass wasting

Slow movementsSolifluction

Promoted by a dense clay hardpan or impermeable bedrock layerCommon in regions underlain by permafrost

Can occur on gentle slopes

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Submarine landslides

Submarine landslides are common and widespread in occurrenceThe most spectacular underwater landslides occur on the flanks of submarine volcanoes (called seamounts)

Submarine landslides

Large slumps and debris flows scar the continental slopes along the margins of the United States

Triggered by the rapid buildup of unstable sediments, or by forces such as storm waves and earthquakes

Especially active near deltas