Midterm: Mass Wasting

flashcards for the mass wasting chapter
______ is a comprehensive term for any type of downslope movement of earth materials
Mass Wasting
________ are the most common landforms on Earth
What type of slope segment is present on a particular slope depends upon ____ & ______
1) rock type
2) climate
Where is free-face development more common?
on strong hard rocks or in humid environments where thick soil & vegetations are present
Where are convex & concave slopes more common?
on softer rocks or in humid environments where thick soil & vegetation are present
Where do gentle, convex, red-colored slope develop?
formed on weak, easily eroded metamorphic rock in a semiarid climate
_____ are one reason that valleys are usually much wider than the streams they contain
slope processes
In general, movement is considered rapid if ______
it can be discerned with the naked eye
What is the most common driving force?
the downslope component of the weight of the slope material
The most common resisting force is the ____ of the slope material
shear strength
Characteristic of a Landslide with Mechanism: Fall
Mass Movement Type: Rock Fall
individual rocks fall through the air and may accumulate as talus
Characteristic of a Landslide with Mechanism: Fall
Mass Movement Type: Slump
- rotational landslide
- cohesive blocks of soft earth material slide on a curved surface
Characteristic of a Landslide with Mechanism: Slide
Mass Movement Type: Soil Slip
- debris slide or earthslide
- soil & other weathered earth material slide on a titled surface of bedrock or cohesive sediment
Characteristic of a Landslide with Mechanism: Slide
Mass Movement Type: Rock Slide
large blocks of bedrock slide on a planar surface such as layering in sedimentary or metamorphic rock
Characteristic of a Landslide with Mechanism: Slide
Mass Movement Type: Avalanche
granular flow of various combinations of snow, ice, organic debris, loose rocks or soil that moves rapidly downslope
Characteristic of a Landslide with Mechanism: Flow
Mass Movement Type: Creep
very slow, downslope movement of rocks & soil
Characteristic of a Landslide with Mechanism: Flow
Mass Movement Type: Earthflow
wet, partially cohesive internally deformed mass of soil & weathered rock
Characteristic of a Landslide with Mechanism: Flow
Mass Movement Type: Debris Flow
fluid mixture of rocks, sand, mud & water that is intermediate between a landslide & a water flood; includes mudflows and lahars
Flow Landslides
1) creep
2) earthflow
3) debris flow
Slide Landslides
1) soil slip
2) avalanche
3) rock slide
Fall Landslides
1) Rock Fall
2) Slump
If the safety factor is greater than 1, the resisting forces exceed the driving forces, the slope is considered _____
If the safety factor if less than 1, the driving forced exceed the resisting forces, a _____ can be expected
slope failure
What variables determine the driving & resisting forces on slopes?
- type of earth materials
- slope angel & topography
- climate
- vegetation
- water & time
Four Important Material Characteristics
1) mineral composition
2) degree of cementation or consolidation
3) presence of zones of weakness
4) ability of the earth material to transmit water
What are the two basic patterns of slide movement?
1) rotational
2) translational
Rotational slides, or slumps, have ____ slip surfaces
Translational slides generally have _____ slip surfaces
What is the fundamental driving force of mass wasting processes?
How does gravity drive mass wasting processes?
over time, the ever-constance downward pull of gravity exceeds resisting forces & causes slopes to fail, rocks to fall, soils to creep & water-saturated earth materials to flow
_____ occur most commonly in unconsolidated earth materials & in mudstone, shale & other weak rock types
Zones of Weakness for Translational Slides
1) fractures in all rock types
2) surfaces between layers in sedimentary rocks referred to as bedding planes
3) weak clay layers
4) surfaces between layers in metamorphic rocks referred to as foliation planes
A common type of translational slide is a ____
Soil slip [a shallow slide of unconsolidated material over bedrock]
the ability of the earth material to transmit water
Why do soil slips commonly occur because of contrasts in permeability between more permeable soil & less permeable, unweathered bedrock below the surface?
downward percolating rainwater builds in the soil because it cannot easily infiltrate the underlying bedrocks. The accumulation of water can break down cohesion between soil particles and facilitate slippage of the soil, especially on steep hillsides
What two factors are important with slope & topography?
1) steepness of the slope
2) amount of topographic relief
slant or incline of the land surface
Topographic Relief
refers to the height of the hill or mountain above the land below
In general, the steeper the slope, the _____ the driving force
Within the U.S. & Canada, which areas have the greatest frequency of landslides?
- coastal mountains of California, Oregon, Washington & British Columbia
- The Rocky Mountains
- The Appalachian Mountains
- Coastal cliffs & bluffs
What are steep slopes associated with?
- rock falls
- avalanches
- soil slips
For landslides, the type of climate influences ______
- the amount & timing of water that infiltrates or erodes a hillslope
- the type & abundance of hillslope vegetation
____ & _____ are common where erosion resistant bedrock is present
Free faces & talus slope
Landslide Activity in Subhumid to Humid Regions
- deep complex landslides
- earthflows
- soil creep
In what three basic ways does water affect slope stability?
1) many landslides develop during rainstorms when slopes become saturated
2) some landslides occur months or years following the deep infiltration of water into a slope
3) water erosion of the base or toe of a slope decreases its stability
Soil water is often ______ because it reacts with carbon dioxide in the atmosphere and soil to produce weak carbonic acid
What causes much of the chemical weathering of rocks?
caused by the chemical action of water in contact with soil & rock near Earth's surface
Carbonic acid weathering is significant in areas underlain by ______
Most avalanches that kill people are triggered by ______
the victims themselves or by members of their party
What three variables interact to create unstable conditions for snow avalanches?
1) steepness of the slope
2) stability of the snowpack
3) the weather
Snow avalanches generally occur on slopes steeper than about _______
25 degrees
The steepest angle at which snow, or any loose material, is stable is its _______
angle of repose
Most snow avalanches occur on slopes between _____ & ______
35 degrees & 40 degrees
When might snow-covered slopes become unstable?
- when wind piles up snow on the leeward (downwind) side
- rapid precipitation adds weight to the slope
- when temperatures rapidly warm to make the snow very wet
What are the two common types of snow avalanches?
1) loose-snow avalanche
2) slab avalanche
Loose-Snow Avalanche
typically start at a point and widen as they move downslope
Slab Avalanches
- start as cohesive blocks of snow & ice that move downslope
- most dangerous & damaging
- typically triggered by the overloading of a slope or the development of zones of weakness in the snowpack
Avalanches tend to move down tracks, called ______, which have previously produced avalanches
Adjustments to Snow Avalanches
- hazard maps
- clearing excess snow with carefully placed explosives
- constructing buildings & structures to divert or lessen avalanches
- planting trees on slopes in avalanche-prone areas to better anchor the snow
What 3 factors are expected to increase worldwide landslide activity in the 21st century?
1) urbanization & development will expand landslide-prone areas
2) tree cutting
3) changing global climate patterns will result in regional increases in precipitation
In the U.S., on average ____ people are killed each year by landslides
25 people [100 & 150 if collapses of trenches & other excavations are included]
Direct Effects of Landslides
- being hit or buried in falling debris
- damage homes, roads & utilities that have been constructed on the top or side of a hill
- regularly block roads & railroads, delaying travel
- block shipping lanes
Indirect Effects of Landslides
- flooding upstream from a landslide that blocks a river
- transmission of disease from fungal spores disturbed by landslides
What other hazards have been known to cause landslides?
- earthquakes
- volcanoes
- storms
- fires
Natural Service Function of Landslides
- new habitats in forests
- landslide-dammed lakes create new habitat for fish & other organisms
- weathering frees mineral grains from rocks & mass wasting transports these minerals downslope
Human Activities that Increase Landslides
- timber harvesting
- urbanization
- removal of vegetative cover & the construction of roads
Surface Features of Potential Landslides
- crescent-shaped cracks or terraces on hillside
- tongue-shaped area of bare soil or rock on a hillside
- large boulders or talus piles at the base of a cliff
- exposed bedrock with layering that is parallel to the slope
- an irregular land surface at the base of a slope
slow downslope movement of soil & other weakly consolidated earth materials; characterized by slow flowing, sliding or slipping
mixture of weathered rock, soil & other, usually angular, material on a slope; produced by creep, landsliding & other surface processes
Debris Flows
rapid downslope movement of unconsolidated, water-saturated earth material that became unstable because of torrential rain, rapid melting of snow & ice, or sudden drainage of a pond or lakes; sometimes restricted to flows of this type that contain mainly coarse material
Driving Forces
an influence that tends to make earth material move downslope
earth materials such as rocks dropping through the air from steep slopes
Flow [Flowage]
downslope movement of earth materials that deform as a fluid, such as the fluid movement of grains of sand, rock, snow or ice, debris & mud
Resisting Forces
an influence that tends to oppose downslope movement
Safety Factor [SF]
the ratio of resisting to driving forces; a safety factor greater than 1 suggests that a slope is stable
Shear Strength
the internal resistance in earth material to shear stress; on slopes this strength resists failure by sliding or flowing
the deformation or downslope movement of a nearby intact block of earth material along a slip surface
the downslope movement of rock, sediment or soil along a curved slip surface
Snow Avalanche
rapid downslope movement of snow, ice & rock
Talus Slope
pieces of rock that have accumulated at the base of a cliff or steep slope; may form individual piles or a continuous slope composed of blocks of rock that have fallen from above