Mass Wasting: The Work of Gravity, Chapter 8
Essentials of Geology, 8e Stan Hatfield and Ken PinzkeSouthwestern Illinois College
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
Mass Wasting and landform development
For mass wasting to occur, there must be a slope angle
Most rapid events occur in areas of rugged, geologically young mountainsAs a landscape ages, less dramatic downslope movements occur
Controls and triggers of mass wasting
Gravity is the controlling forceImportant triggers include
Saturation of the material with water– Diminishes particle cohesion–Water adds weight
Oversteepening of slopes– Stable slope angle (angle of repose) is
different for various materials
Controls and triggers of mass wasting
Important triggers includeOversteepening of slopes – Oversteepened slopes are unstable
Removal of anchoring vegetationGround vibrations from earthquakes–May cause expensive property damage– Can cause liquefaction – water saturated
surface materials behave as fluid-like masses that flow
Controls and triggers of mass wasting
Landslides without triggersSlope materials weaken over timeRandom events that are unpredictable
Classification of mass wasting events
Generally each event is classified byType of material involved– Debris–Mud– Earth– Rock
Classification of mass wasting events
Generally each event is classified byType of motion– Fall (free-falling pieces)– Slide (material moves along a surface as a
coherent mass)– Flow (material moves as a chaotic mixture)
The velocity of the movement– Fast– Slow
Forms of mass wastingSlump
Movement of a mass of rock or uncon-solidated material as a unit along a curved surfaceOccurs along oversteepened slopes
Forms 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
Forms 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
Debris-flow Deposition Resulting in Widespread Destruction on theCaraballeda Fan of the Quebrada San Julián
Venezuela, 1999
Forms of mass wasting
EarthflowForm on hillsides in humid regionsWater saturates the soilCommonly involve materials rich in clay and silt
Forms of mass wasting
Slow movementsCreep– Gradual movement of soil and regolith
downhill– Aided by the alternate expansion and
contraction of the surface material
Forms of mass wasting
Slow movementsSolifluction– Promoted by a dense clay hardpan or
impermeable bedrock layer– Common in regions underlain by permafrost– Can occur on gentle slopes