Lesson 12: Mass Wasting

Question 1

What is the driving factor of mass wasting?

Answer 1

Gravity

Question 2

What factors control/trigger mass wasting? (6)

Answer 2

1. Strength of Slope Materials
2. Saturation of Material with Water
3. Vibrations from Earthquakes
4. Oversteepening of slopes by undercutting
5. Removal of vegetation
6. Alternating freezing and thawing

Question 3

How does the strength of slope materials control slope stability?

Answer 3

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

Question 4

How does saturation of material with water control slope stability?

Answer 4

Completely dry sediments have weak cohesion
Moist sediments are the strongest
Completely saturated sediments are the weakest of all

Question 5

How does shaking control slope stability?

Answer 5

shaking can weaken a body of rock/sediment

Question 6

How do oversteepened slopes control slope stability?

Answer 6

they are unstable

Question 7

How does the removal of vegetation control slope stability?

Answer 7

Plants bind to soil and regolith together and shield the soil surface from rain

Question 8

How does thawing control slope stability?

Answer 8

By releasing a block of rock that was attached to a slope by a film of ice

Question 9

How is mass wasting classified? (2)

Answer 9

1. Slope failures
2. Sediment flows

Question 10

Define slope failures.

Answer 10

A sudden failure of the slope results in the transport of debris downhill by falling/rolling/slumping/sliding

Question 11

Define sediment flows.

Answer 11

Debris mixed with water/air flows downhill like a fluid.

Question 12

What are the different types of slope failures? (3)

Answer 12

1. Falls - when material drops through the air, vertically or nearly vertically
2. Slumps - a rotational slide as a result of movement along a curved surface
3. Slides - the result of rock/debris slide down a pre-existing surface

Question 13

What are the different types of falls? (2)

Answer 13

1. Rock falls - rock on a steep slope becomes dislodged and falls down the slope
2. Debris Falls - like rock falls, involve a mixture of soil, regolith, vegetation, and rocks

Question 14

How are falls characterized?

Answer 14

Very fast
Talus form at the base of the cliff

Question 15

How are slumps characterized?

Answer 15

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

Question 16

How are slides characterized?

Answer 16

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

Question 17

What are the 2 types of sediment flows?

Answer 17

1. Slurry flows - sediment flows that contain between 20 and 40% water
2. Granular Flows - sediment flows that contain between 0 and 20% water

Question 18

What are the different types of slurry flows? (4)

Answer 18

1. Solifluction - downslope movement of soil over a permafrost
2. 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
3. Mudflows - the result of heavy rains
4. Lahars - volcanic mud/debris flows

Question 19

What are the different types of granular flows? (4)

Answer 19

1. Creep - a slow earthflow
2. Earth flow - slope material liquefied and runs out forming a bowl/depression at the head
3. Debris avalanches - large volume mixture of rock and regolith with a very high velocity.
4. Snow Avalanches - similar to debris avalanches, but only involving snow

Question 20

The driving force behind all mass wasting processes:

Answer 20

Gravity

Question 21

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?

Answer 21

It will decrease as the slope angle is lessened/reduced.

Question 22

Which type of mass movement involves a downward sliding mass of rock/unconsolidated material moving along a curved surface?

Answer 22

Slump

Question 23

Unconsolidated granular particles assume a stable slope at the angle of ____.

Answer 23

repose

Question 24

At the base of most cliffs is an accumulation of fallen material called ___.

Answer 24

talus

Question 25

___ denotes the exposed, crescent-shaped rupture surface at the head of a slump.

Answer 25

scarp

Question 26

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

Answer 26

parallel

Question 27

_____ is the single most common mass-wasting trigger.

Answer 27

An increase in water content.

Question 28

A ____ involves the free fall of material, usually down a steep cliff.

Answer 28

Rockfall

Question 29

Solifluction is common ______.

Answer 29

on mountainous slopes of southern Alaska (where it is very cold)

Question 30

What is the main difference between debris flow and mudflow?

Answer 30

Debris flows contain less than 50% of fines
Mudflows contain more than 50% of fines

Question 31

___ are volcanic mudflows/debris flows.

Answer 31

Lahars