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Mass Movements. GLY 2010 – Summer 2012 - Lecture 17. Mass Movement - Definition. Transporting of earth materials downslope due to gravity, without the aid of a transporting medium such as water or ice. Mass Movement Triggers. Gravity Vibration Water. Gravity.
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Mass Movements GLY 2010 – Summer 2012 - Lecture 17
Mass Movement - Definition • Transporting of earth materials downslope due to gravity, without the aid of a transporting medium such as water or ice
Mass Movement Triggers • Gravity • Vibration • Water
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
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
Earthquake Triggered Slide • Madison Canyon Earthquake of August 1959 • An estimated 19 people are still beneath the quake rubble
Water • The weight of water soaking into soil, increases the force from gravity • This may be enough to overcome friction and start mass movement
Water’s Roles • Water acts as “glue” • Water acts as a “lubricant”
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
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
Angle of Repose • The maximum angle at which loose sediment can form a stable slope • Different materials will have different angles of repose
Angle of Repose - Examples • Typical angles: • Fine sediment - 30-35° • Coarse sediment - to >40° (talus slopes)
Angle of Repose, Snow Avalanches form when the angle of repose is exceeded
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
Types of Slow Mass Movements • Creep • Solifluction
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
Creep Mechanism • Expansion and contraction leads to creep • May be due to freeze/thaw or rain/evaporation cycles
Effect of Creep • The tilted headstone is the result of creep
Bent Curbs Due To Creep • Hollister, California – Left, 1966; Right, 1992
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
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
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
Solifluction Terraces • Solifluction near Fairbanks, Alaska
Effects of Permafrost • Thawing of Permafrost leads to slow subsidence of structures built on it • Becoming a very common problem at high northern latitudes
Effects of Permafrost • When a rail line was built across this permafrost landscape in Alaska, the ground subsided.
Rapid Mass Movements • Slides • Falls • Flows • Debris avalanche
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
Slides • A single intact mass (rock, soil, or unconsolidated material) moves downslope along a slip plane • Slip planes are planes of weakness
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.
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
Failure Along Metamorphic Foliation Surface • Slaty cleavage seen here is at a high angle • Failure has occurred along the foliation surface
Slump Failure, McClure Pass, Colorado • Note half buried automobile
Falls • Fastest type of mass movement • Occurs when material on a near vertical cliff breaks free and falls freely to the surface below
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
Recent Rock Fall • Recent rock fall, Zion National Park, Utah
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
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
Types of Flows • Flows are broken into categories: • Earthflow • Mudflow • Debris flow
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
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
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
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
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
Debris Flow Animation • Computer simulation depicting the Sourgrass debris flow (Sierra Nevada, North Fork of the Stanislaus River), of January 1, 1997
Debris Avalanche • Very steep, unvegetated slopes may produce an avalanche
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