Mass Wasting

Question 1

what is mass wasting?

Answer 1

downslope movement of rock or regolith (loose weathered materials + soil driven by force of gravity)

Question 2

What should we care about mass wasting?

Answer 2

The cost of recovery from mass wasting (all forms combined) is more each year than earthquakes + volcanoes, tornadoes combined

Question 3

What are the controls of mass wasting?

Answer 3

Properties of material involved (rigid, cohesive or loose)

Slope angle

Water content of material

Removal of vegetation

Mass movement triggers

Question 4

what are the properties of material involved in mass wasting?

Answer 4

Rigid, brittle material (e.g. rock)

Soft, cohesive material (e.g. clay-rich soil or rock in which particles stick tgt + form a coherent mass)

Loose particles (e.g. gravel, sand, etc.)

Phys + chem weathering can further weaken geologic materials + make them prone to failure (esp in case of rock)

Question 5

What is the angle of repose?

Answer 5

maximum angle that a given material will sustain

Diff in angle of repose according to grain size, grain shape (round vs angular) + water content

Question 6

What is the slope angle?

Answer 6

Gravity has greater effect on potential downslope movement of material on steeper slopes

Increase the sloped, increase the driving force (mass wasting events tend to be most spectacular on steep mountain slopes)

Question 7

What are the natural slopes prone to mass wasting?

Answer 7

River banks

Mountain slopes

Fault surfaces

Wave-cut shorelines

Glacial valleys

Question 8

What are the Man-made slopes prone to mass wasting?

Answer 8

Road-cuts

Excavation projects

Gravel pits

Quarries

Drainage ditches

Question 9

what can water do to particles

Answer 9

Presence of some water can increase cohesion of loose particles (due to surface tension)

But addition of lots of water can destabilize material

Many cases:

Water along fractures + btwn particles can act as a lubricant (Promoting blocks to slide)

Water can cause particles to remain separated + to move freely (Material behaves like fluid)

Question 10

What can plants do for material?

Answer 10

Roots of plants anchor loose material such as soil in place on a slope

Removal of vegetation removes this stabilizing factor + can promote downslope movement

Question 11

What can trigger mass movement?

Answer 11

vibrations , from shaking + liquefaction (lubrication of particles by invading water)

Weather events

Slightest added stress on a material can trigger mass movement

Question 12

Types of mass wasting classified according to what 3 criteria?

Answer 12

type of movement
type of material moved
rate of movement (velocity)

Question 13

what are the types of movement?

Answer 13

Falling (freefall)

Sliding (along planar or curved surface)

Flowing (material internally deforms like a fluid)

Question 14

what are the types of material moved?

Answer 14

Brittle solid material (most rock)

Cohesive solid material (e.g. clay-rich soil or rock)

Loose particles (e.g. particles of soil, mud, debris of assorted sizes)

Question 15

what are the rates of movement

Answer 15

rapid or slow

Question 16

What is a rockslide?

Answer 16

Blocks of solid, brittle rock slide down a planar surface

Generally very fast + destructive

Question 17

what is a rock avalanche

Answer 17

After a mass of rock is dislodged (often in a rockslide), it can break into many smaller pieces + move downslope as a rock avalanche

Fragments of rock travel downslope on a cushion of air within + beneath the moving mass of rock fragments

Question 18

what is a slump

Answer 18

Material moves as coherent blocks along curved surfaces (little or no internal deformation within blocks)

Commonly occurs in cohesive clay-rich sedimentary rocks (e.g. shales) + soils (clay helps particles stick together)

At surface, slump scarps mark these curved surfaces of sliding

Rate of movement depends on material involved (but usually slow)

Question 19

what is an earthflow

Answer 19

When clay-rich soil becomes saturated w/ water, it behaves like a viscous fluid (deforming internally)

Commonly produced at the foot of a slump (where water is leaking from curved slump surface)

Typically move slowly, but can move quickly if lots of water has soaked into the material and/or the slope above is steep

Question 20

what is a debris flow

Answer 20

if assortment of surface materials, both fine- and coarse-grained

Question 21

what is a mudflow

Answer 21

if predominantly fine grained materials

Question 22

both debris flow + mudflow consist of what?

Answer 22

loose material mixed w/ large amt of water
- less viscous than earthflows + can move extremely quickly (extremely destructive)
- movement of this liquefied material is often confined to channels

Question 23

what are lahars

Answer 23

debris flows + mudflows specifically associated w/ volcanoes

Question 24

how are lahars formed

Answer 24

Formed when large amts of water (e.g. from heavy rainfall, or from melting snow at the summit of an active volcano) mixes w/ loose pyroclastic materials (e.g. ash) + flows downslope

Question 25

what is a creep

Answer 25

Expansion + contraction of surface material leads to very slow + gradual downhill movement of soil

Its effects can be observed in the tilting fences + headstones + curvature of tree trunks + damage to building foundations

Unspectacular but very costly

Question 26

what are the human influences on mass wasting

Answer 26

Natural Processes + increased population density + increased human activity = increased hazard potential

Question 27

list the human influence on mass wasting hazards

Answer 27

Weakening of slope material by saturation by water
-Enhanced due to land use changes such clear cutting + agriculture

Steepening of the slope by artificial or natural undercutting of toe of slope
-For buildings and roads

Addition of weight at top of slope
-Dams, buildings, urbanization, roads

Humans are sculpting the surface of Earth faster than natural processes – no slope is too rugged for development

Question 28

what are some strategies to reduce damage from slope movements

Answer 28

Grading (add material to reduce slope angle)

Surface + subsurface drainage control (devices to permit drainage of excess water from soil + underlying weathered rock)

Retaining walls, and slope supports (stabilize slope + support surfaces of potential weakness such as fault planes + rupture surfaces under potential slump blocks

Anchoring into bedrock (to reduce load + prevent slippage of man-made structures due to deformation of loose materials)