Mass Movements

Mass Movements

• Mass Movement - downslope movement of soil and weathered rock resulting from force of gravity

• Mass movement occurs when forces pulling material downslope are stronger than material’s resistance to sliding, flowing, or falling

• Alter Earth’s surface over time due to gravity moving sediment and rocks downslope

Factors that Influence Mass Movements

• Material’s weight - works to pull material down slope

• Material’s resistance to sliding or flowing - depends on how cohesive material is and whether anchored to bedrock

• Trigger (earthquake) - shakes material loose

• Water - saturation greatly increases weight and can act like a lubricant

Types of Mass Movements

• Creep - slow, steady, downhill flow of loose, weathered Earth materials, especially soils

• Movement can be few cm/year - effects are noticeable only over long periods of time

• Loose materials on almost all slopes undergo creep

• Example: once-vertical utility poles and fences tilt

Types of Mass Movements

• Mudflows - swiftly moving mixtures of mud and water

• Lahar - type of mudflow that occurs after eruption

• Occurs when snow-topped volcanic mountain erupts and melts snow on top - melted snow mixes with ash and flows downslope

• Mudflows can be triggered by earthquakes

• Common in volcanic regions and in sloped, semi- arid regions with intense, short-lived rainstorms

• Can be extremely destructive

Types of Mass Movements

• Landslide - rapid, down slope movement of Earth materials that occurs when a relatively thin block of soil, rock, and debris separates from underlying bedrock

• Common on steep slopes, especially when soils and weathered bedrock are fully saturated by water

• Can cause immense damage and result in many deaths

Types of Mass Movements

• Slump - when a mass of material in a landslide moves along a curved surface

• Material at top of slump moves downhill, and slightly inward, while material at bottom moves outward

• Occur in areas that have thick soils on moderate-to-steep slopes

• Common after rains and triggered by earthquakes

• Leave crescent-shaped scars on slopes

Types of Mass Movements

• Avalanches - landslides that occur in mountainous areas with thick accumulations of snow

• Radiation from Sun melts surface snow, which then refreezes at night into an icy crust

• Snow that falls on top of this crust can eventually build up, become heavy, slip off, and slide downslope as an avalanche

• Vibrating trigger, even from a single skier, can send this unstable layer sliding down a mountainside