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Mass Movements

Mass Movements

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Mass Movements

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  1. Mass Movements GLY 2010 – Summer 2012 - Lecture 17

  2. Mass Movement - Definition • Transporting of earth materials downslope due to gravity, without the aid of a transporting medium such as water or ice

  3. Mass Movement Triggers • Gravity • Vibration • Water

  4. Gravity • Gravity constantly pulls materials downslope, opposed by friction • If gravity overcomes friction, the material moves • The steeper the slope, the greater the proportion of full gravity works on the material

  5. Vibration • Earthquakes, explosions, or other seismic energy sources energize particles on the slope, and they begin to move around - some of them may slide downhill • If enough slides at once, an “avalanching effect” is generated, pushing the material downhill ahead and triggering mass movement • A blast of air, approaching hurricane force, may precede the front of a large mass movement

  6. Earthquake Triggered Slide • Madison Canyon Earthquake of August 1959 • An estimated 19 people are still beneath the quake rubble

  7. Water • The weight of water soaking into soil, increases the force from gravity • This may be enough to overcome friction and start mass movement

  8. Water’s Roles • Water acts as “glue” • Water acts as a “lubricant”

  9. Water As Glue - Surface Tension • Water has a very high surface tension • Small amounts of water in sediment, which is essentially all surface, act to hold the sediment together

  10. Water As Lubricant • As more water is added, it forms layers with greater thickness between particles • Surface tension is no longer important, because the water layer is thicker • Water acts as a lubricant, cutting frictional forces, and making mass movement easier

  11. Angle of Repose • The maximum angle at which loose sediment can form a stable slope • Different materials will have different angles of repose

  12. Angle of Repose - Examples • Typical angles: • Fine sediment - 30-35° • Coarse sediment - to >40° (talus slopes)

  13. Angle of Repose, Snow Avalanches form when the angle of repose is exceeded

  14. Avalanche Video

  15. Artificial Oversteepening • Man activities may create unstable slopes • Examples: • Road building • Mining • Blasting narrow roads through mountains often leads to slope failure • In mountainous states, particularly those that get a lot of rain, road failure is a fact of life

  16. Natural Oversteepening

  17. Types of Slow Mass Movements • Creep • Solifluction

  18. Creep • Creep is the imperceptibly slow down-slope movement of soil and near-surface rock materials • Process is generally not directly observed - instead, creep is best discerned through the movement or response of objects affected by the process • Typical movement = mm’s to cm’s per year • Extremely common on sloping terrain

  19. Creep Mechanism • Expansion and contraction leads to creep • May be due to freeze/thaw or rain/evaporation cycles

  20. Effect of Creep • The tilted headstone is the result of creep

  21. Bent Curbs Due To Creep • Hollister, California – Left, 1966; Right, 1992

  22. Leaning Fence Posts – Barnes County, N.D. • Fenceposts, being shallowly-seated objects, are particularly prone to creep, and have tilted down-slope under creep almost to a horizontal position • Small trees on this slope likewise show the influence of creep: as their trunks are progressively tilted over by creep, their new growth responds phototropically back upwards towards the sun, leading to a curved appearance of the trunk Photo by D.P. Schwert, North Dakota State University

  23. CREEP Cass County, N.D. • Photo under an I-94 overpass at Fargo • Concrete pads have shifted down-slope on soils affected by creep • Upward squeezing of the pads is induced by differential down-slope flow velocities of soil and "rock" materials. Photo by D.P. Schwert, North Dakota State University

  24. Solifluction • A type of “fast” creep • Occurs in cold regions where the soil remains frozen most of the year (permafrost) • May occur in other boggy soils, especially after vegetation removal

  25. Solifluction Terraces • Solifluction near Fairbanks, Alaska

  26. Effects of Permafrost • Thawing of Permafrost leads to slow subsidence of structures built on it • Becoming a very common problem at high northern latitudes

  27. Effects of Permafrost • When a rail line was built across this permafrost landscape in Alaska, the ground subsided.

  28. Rapid Mass Movements • Slides • Falls • Flows • Debris avalanche

  29. Rapid Mass Movement Rates • Rapid mass movements are much faster than creep or solifluction • They may move within seconds to a minute or so, at rates of meters per second, or more slowly, or days and weeks, at rates of meters per day

  30. Slides • A single intact mass (rock, soil, or unconsolidated material) moves downslope along a slip plane • Slip planes are planes of weakness

  31. Slip Plane Geometry • Slip planes are usually flat • If they are curved, the slide moving along a concave slip plane is known as a slump • Some rotation of the material is involved in slumping As the photo illustrates, even though the Gros Ventre rockslide occurred in 1925, the scar left on the side of Sheep Mountain is still a prominent feature.

  32. Slump Animation • Computer simulation of a deep-seated "slump“ type landslide in San Mateo County, California • Over 250,000 tons of rock and soil moved in this landslide

  33. Failure Along Metamorphic Foliation Surface • Slaty cleavage seen here is at a high angle • Failure has occurred along the foliation surface

  34. Slump Failure, McClure Pass, Colorado • Note half buried automobile

  35. Falls • Fastest type of mass movement • Occurs when material on a near vertical cliff breaks free and falls freely to the surface below

  36. Picture of Fall • A fall has left an obvious scar, and a talus pile • Failure was caused by artificial oversteepening of the surface to make a road

  37. Recent Rock Fall • Recent rock fall, Zion National Park, Utah

  38. Fall Video

  39. Ocoee River Valley – US 64 • On November 10, 2009, a series of rockfalls preceded a landslide, which would have killed people had geologist Vanessa Bateman not cleared the area prior to the slide

  40. Flows • Mass of mostly unconsolidated material moving downslope as a viscous liquid • May be dry or wet • Move rapidly, especially when wet • Considerable mixing may occur during the movement

  41. Mass Movement Animation

  42. Types of Flows • Flows are broken into categories: • Earthflow • Mudflow • Debris flow

  43. Earthflow • Dry masses of clay or silt regolith • They have high viscosity, and typically move relatively slowly (meters/hour to meters/minute) • The slow movement usually precludes loss of life, but they do cause substantial property damage

  44. Earthflow Effect • This small, tongue-shaped earthflow occurred on a newly formed slope along a recently constructed highway • It formed in clay-rich material following a period of heavy rain • Notice the small slump at the head of the earthflow

  45. Mudflow • Composed of wet mixtures of mud and water, they move swiftly • Mudflows often develop after heavy rains (cloudbursts) in semi-arid regions, where sparsely vegetated slopes have masses of loose regolith • Canyons in semi-arid deserts are prone to mudflows

  46. Lahars • Lahars are a special type of mudflow produced on the slopes of a volcano • Volcanic ash and hot gases melt accumulated snow and glacial ice, producing large quantities of mud

  47. Similar to mudflows, but consisting of particles larger then sand-sized, often with some boulders of a meter or more Because of the larger particles size, they require steep slopes Debris Flow

  48. Debris Flow Animation • Computer simulation depicting the Sourgrass debris flow (Sierra Nevada, North Fork of the Stanislaus River), of January 1, 1997

  49. Debris Avalanche • Very steep, unvegetated slopes may produce an avalanche

  50. Wildfires and Debris Flows • Wildfires can lead to destructive debris-flow activity • In July 1994, a severe wildfire swept Storm King Mountain west of Glenwood Springs, Colorado, denuding the slopes of vegetation • Heavy rains on the mountain in September resulted in numerous debris flows, one of which blocked Interstate 70 and threatened to dam the Colorado River • A 3-mile length of the highway was inundated with tons of rock, mud, and burned trees