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

A slump with an earthflow at the base

Pacific Palisades 2/4/56

Forms of mass wasting

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

A lahar from the 1980 Mt. St. Helens eruption

Valtellina Debris Flow-Dolomite Mountains, Italy

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

An earthflow on a newly formed slope

Forms of mass wasting

Slow movementsCreep– Gradual movement of soil and regolith

downhill– Aided by the alternate expansion and

contraction of the surface material

Creep

Some visible effects of creep

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

Permafrost regions in the Northern Hemisphere

Ground subsidence in Alaska due to permafrost

Thistle Slide, Utah 1983

Yungay, Peru1970