Lesson 12: Mass Wasting Flashcards
What is the driving factor of mass wasting?
Gravity
What factors control/trigger mass wasting? (6)
- Strength of Slope Materials
- Saturation of Material with Water
- Vibrations from Earthquakes
- Oversteepening of slopes by undercutting
- Removal of vegetation
- Alternating freezing and thawing
How does the strength of slope materials control slope stability?
If planes of weakness are perpendicular to the slope the risk of mass-wasting is less
If planes of weakness are parallel to the slope, the risk of mass-wasting is more
How does saturation of material with water control slope stability?
Completely dry sediments have weak cohesion
Moist sediments are the strongest
Completely saturated sediments are the weakest of all
How does shaking control slope stability?
shaking can weaken a body of rock/sediment
How do oversteepened slopes control slope stability?
they are unstable
How does the removal of vegetation control slope stability?
Plants bind to soil and regolith together and shield the soil surface from rain
How does thawing control slope stability?
By releasing a block of rock that was attached to a slope by a film of ice
How is mass wasting classified? (2)
- Slope failures
- Sediment flows
Define slope failures.
A sudden failure of the slope results in the transport of debris downhill by falling/rolling/slumping/sliding
Define sediment flows.
Debris mixed with water/air flows downhill like a fluid.
What are the different types of slope failures? (3)
- Falls - when material drops through the air, vertically or nearly vertically
- Slumps - a rotational slide as a result of movement along a curved surface
- Slides - the result of rock/debris slide down a pre-existing surface
What are the different types of falls? (2)
- Rock falls - rock on a steep slope becomes dislodged and falls down the slope
- Debris Falls - like rock falls, involve a mixture of soil, regolith, vegetation, and rocks
How are falls characterized?
Very fast
Talus form at the base of the cliff
How are slumps characterized?
Slow
Individual blocks and the upper surface of each slump block remain relatively undisturbed
Leaves scars/depressions on the hill slope
Can be triggered by heavy rains and earthquakes
How are slides characterized?
Piles of talus are common at the base
No rotation of sliding rock along a curved surface; motion along a planar surface
The mass above a slippery non-curved surface slides intact
What are the 2 types of sediment flows?
- Slurry flows - sediment flows that contain between 20 and 40% water
- Granular Flows - sediment flows that contain between 0 and 20% water
What are the different types of slurry flows? (4)
- Solifluction - downslope movement of soil over a permafrost
- Debris flows - a form of rapid mass movement in which a combination of loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope
- Mudflows - the result of heavy rains
- Lahars - volcanic mud/debris flows
What are the different types of granular flows? (4)
- Creep - a slow earthflow
- Earth flow - slope material liquefied and runs out forming a bowl/depression at the head
- Debris avalanches - large volume mixture of rock and regolith with a very high velocity.
- Snow Avalanches - similar to debris avalanches, but only involving snow
The driving force behind all mass wasting processes:
Gravity
Consider a weathered rock or soil particle lying on a slope. How will the gravitational force pulling the particle downward along the land surface vary with the inclination of the slope?
It will decrease as the slope angle is lessened/reduced.
Which type of mass movement involves a downward sliding mass of rock/unconsolidated material moving along a curved surface?
Slump
Unconsolidated granular particles assume a stable slope at the angle of ____.
repose
At the base of most cliffs is an accumulation of fallen material called ___.
talus
___ denotes the exposed, crescent-shaped rupture surface at the head of a slump.
scarp
Fractures, metamorphic foliation, or bedding can significantly reduce the strength of a body of rock. In the context of mass wasting, this is most critical if the places of weakness are ____ to the slope
parallel
_____ is the single most common mass-wasting trigger.
An increase in water content.
A ____ involves the free fall of material, usually down a steep cliff.
Rockfall
Solifluction is common ______.
on mountainous slopes of southern Alaska (where it is very cold)
What is the main difference between debris flow and mudflow?
Debris flows contain less than 50% of fines
Mudflows contain more than 50% of fines
___ are volcanic mudflows/debris flows.
Lahars
