1. Mass Wasting is the movement of regolith (unconsolidated rock material) down slope solely as a result of gravity.

2. Factors that influence mass wasting processes include:

(a) nature of slope material

(b) amount of water in the slope material

(c) steepness of the slope.

Slopes may be composed of:

(a) large masses of bedrock

(b) debris piles of loose rocks and soil

(c) sediment.

2. Loose material is referred to as unconsolidated.

3. Material that is compacted and held together by mineral cements is considered consolidated.

1. Figure 11.1: For unconsolidated material, the angle of the slope determines whether the material will remain stationary or move.

2. The maximum angle at which loose material can be inclined without sliding down is called the angle of repose.

3. A slope that is steeper than the angle of repose is unstable and will cause material to slide downhill.

4. The angle of repose depends on the material and it’s water content. For most dry sands, the angle is 35^o.

5. The angle of repose is greater for particles that are larger, flatter, and more angular.

6. Moisture also increases the angle of repose of sediments. A small amount of moisture between sand grains will bind them together due to surface tension. Surface tension is the attractive force between molecules at a surface.

7. Too much water, however, results in particles moving freely over one another and therefore dramatically reduces the angle of repose.

1. Consolidated materials, on the other hand, are held together by cements or certain attractive forces.

(a) Cohesive forces cause particles of the same kind (clays) to bind together as a result of mutual attraction.

(b) Adhesion refers to the attractive force between materials of different kinds (e.g. sand and water).

(c) Mineral cements can consolidate material.

(d) The binding action of plant roots can also consolidate material.

2. Consolidated material will therefore move as a unit rather than as individual grains and therefore are not specifically governed by an angle of repose.

1. Whether consolidated or unconsolidated, the susceptibility of material to mass wasting processes is strongly determined by the steepness and instability of the slope.

2. In any body of rock or rock debris located on a slope, two opposing forces determine whether the body will remain stationary or will move. These two forces are shear stress and shear strength.

3. Shear Stress is the force acting to cause movement of a body parallel to the slope.

4. There are two components of gravity:

(a) Perpendicular component (acts at right angles to the slope)

(b) Tangental component (acts parallel to the slope)

5. As the slope becomes steeper, the tangental component of gravity increases relative to the perpendicular component and the shear stress becomes larger.

6. Shear Strength is the internal resistance of the body to movement. This internal resistance includes:

(a) frictional resistance

(b) cohesion between particles

(c) binding action of plant roots

7. The safety factor determines whether a particular material undergoes slope failure.

Fs (Safety Factor) = Shear Strength/Shear Stress

(a) Fs >1 (Slope is Stable)

(b) Fs < 1 (Slope Failure)

8. Factors that increase the shear stress include:

(a) increase in the slope angle

(b) jolting (e.g. earthquake)

9. Factors that decrease the shear strength include:

(a) weathering

(b) decay of plant roots

(c) saturation by rain

1. Dry sand is unstable and difficult or impossible to mold. When some water is added, the sand grains can be shaped into vertical walls such as for sand castles. The water and sand grains are drawn together by a force called Capillary Attraction that results from surface tension of the liquid and tends to hold wet sand together in a cohesive mass.

2. As mentioned before, however, too much water reduces cohesion between grains and produces a slurry that flows.

3. Increased water pressure at base of a large rock mass can also increase the buoyancy of rock and therefore promote slope failure.

4. In summary, small amounts of water can aid in holding grains together, but excess water actually reduces the shear strength and promotes slope failure.

1. Figure 11.7: A Landslide refers to the overall down-slope movement of a mass of bedrock, unconsolidated material and/or vegetation.

2. There are two types of landslides:

(a) Slope Failure: Sudden failure of a slope comprised of a relatively coherent mass of rock or rock debris. The mass moves as a coherent unit during slope failure.

(b) Sediment Flow: Down-slope flow of mixtures of sediment, water and air involving internal, non-coherent motion of the material.

1. Slides constitute one type of slope failure involving the movement of rocks and other debris down an inclined surface.

2. The body of debris resting at the base of a slope or cliff after a slide is referred to as talus.

3. There are several different types of slides:

(a) Figure 11.16: A slump is a slow slide of unconsolidated material that moves as a coherent unit. A slump involves downward and outward rotational movement of regolith and is associated with heavy rains or sudden shocks.

(b) Figure 11.9: A rockslide involves a sudden down-slope movement of bedrock along an inclined surface.

(c) Figure 11.17: A debris slide involves a sudden down-slope movement of debris consisting of a mixture of sediments, rocks, soil and/or vegetation.

Falls involve the free falling of detached material from a cliff or steep slope.

(a) Figure 11.8: A rockfall is the free falling of detached bodies of bedrock.

(b) A debris fall is like a rockfall except that the material consists of mixture of rocks, sediments, soils, vegetation etc.

(c) A soil fall involves the free falling of soil from a cliff or steep slope.

1. Flows move as a plastic or viscous fluid composed of mixtures of sediment, water and sometimes air under the force of gravity. Flows are characterized by internal motion of the moving material.

2. There are two types of sediment flow:

(a) Slurry flows are moving masses of water-saturated sediment.

(b) Granular flows are not water-saturated, but rather are supported by grain to grain contact.

1. Figure 11.18: Solifluction is a type of slurry flow that occurs in cold regions and is only detectable over long periods of time. During periods of thaw, the upper part of the regolith becomes waterlogged and oozes downhill as a slurry of broken rocks and other debris over underlying frozen material.

2. Debris Flows are fluid mass movements of rock fragments supported by a muddy matrix. Debris flows appear tongue-like and are associated with heavy rainfalls.

3. Figure 11.14: Mudflows are flowing masses of material mostly finer than sand. Mudflows are highly fluid, very mobile and have a consistency ranging from that of fresh cement to soup depending on the water content.

1. Figure 11.11: Creep represents the slowest of the unconsolidated mass movement. Creep involves the slow, downslope movement of regolith at rates of 1-10mm/yr and can only be visually detected over time spans of many years.

2. Figure 11.12: Earthflows are fluid movements of relatively fine-grained materials such as soils, weathered shales and clays. Earthflows may be active for days, months or years. An earthflow generally has a long, narrow tongue-like shape with a rounded, bulging front. Earthflows typically range in size from several meters long to over 1 km long.

3. Liquifaction is a special type of earthflow occurring in highly porous sediments (clay to sand-sized) where a shock causes abrupt fluidization and failure of the material.

4. Grain Flow involves movement of dry or nearly dry granular sediment. Pore spaces between grains are filled with air. Grain flow occurs naturally when the slope of the granular sediment exceeds its angle of repose.

5. Figure 11.15: Debris Avalanches are the fastest of all unconsolidated flows. Debris avalanches can be extremely destructive involving huge masses of falling rock and other debris that break up and pulverize on impact. Debris avalanches often travel down slope over great distances.

(a) Shocks such as earthquakes, man-made explosions, etc.

(b) Volcanic eruptions, especially stratovolcanoes with steep slopes, snowfields and summit glaciers.

(c) Modification of slopes by human activity such as highway cuts and stripping of vegetation.

(d) Undercutting action of stream or coastal waves against sea cliffs.

(e) Heavy rainfall.