1 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 gravity Role of mass wasting Geologic process that often follows weathering Combined effects of mass wasting and running water produce stream valleys
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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
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