u5l2 - mass movements...mass movements • mass movement - downslope movement of soil and weathered...
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Mass Movements
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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
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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
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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
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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
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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
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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
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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