Chapter 07: Mass Wasting

Question 1

Mass wasting

Answer 1

General process of Earth materials moving downslope due only to gravity

Question 2

Trigger mechanisms (5)

Answer 2

Processes or events that reduce friction and/or increase effect of gravity

1. Nature of material (some are more cohesive/stables, others not)
2. Oversteepened slopes
3. Water content (unsatured v. saturated)
4. Loss of vegetation (e.g. wildfires)
5. Earthquakes/volcanic activity (vibrations)

Question 3

Angle of repose

Answer 3

Steepest stable slope allowed by a loose material

35° is generally the steepest stable slope for loose material

Question 4

Planar surfaces and stability

Answer 4

Planar surfaces: bedding planes, faults, fractures, foliation

Such surfaces represent weaknesses within rocks that can greatly reduce slope stability (dangerous when inclined in same direction as slope)

Question 5

Fragment shapes and stability

Answer 5

Angular fragments have more frictional resistance than round fragments

Question 6

Oversteepened slopes

Answer 6

Naturally:
Formed by meandering/migrating streams that form the cutback/overhang combination; overhang eventually mass wastes

Manmade:
Leveling terrain for human activity (e.g. roads)

Question 7

Poor pressure

Answer 7

Pressure of groundwater held within a soil or rock; pressure increases as pores retain more water (from increased rainfall or snow melt)

Question 8

Adhesive force

Answer 8

Force keeping material together; in underground material, small amounts of water (water film) keep material together, but being saturated with water causes a loss of friction

Question 9

Frost wedging

Answer 9

Liquid water freezes, expands gaps in rock’s planar weakness (e.g. fractures, foliation, bedding), causing material to fall

Question 10

Risk management of mass wasting (6)

Answer 10

1. Build retaining walls
2. Drain excess moisture
3. Terracing (building terrace steps/benches for gradual fall of material
4. Covering steep slopes
5. Re-grade slopes
6. Plant anchor vegetation

Question 11

Land subsidence

Answer 11

Process of land either gradually lowering in elevation or suddenly falling after gradual removal of subsurface material

Subsurface ground failure

Natural: e.g. dissolution

Man-made:

1. Withdrawal of fluids (water, petroleum)
2. Withdrawal (mining) of solids (salts, minerals, rocks)

Question 12

Karst topography

Answer 12

Land with pock marks (sink holes, sinking streams) with caves underneath and collapsed sinks

Question 13

Sinkholes

Answer 13

Collapse of land due to subsurface failure with unusually large void spaces

Subsurface cavitites (caves) formed by groundwater dissolving rock; caves were filled with water, helped support weight of surface land. Once cavities are empty, sudden collapse occurs, forming sinkholes.