Mass wasting

Mass Wasting: Refers to the movement of material downslope due to gravity, which does not require a transport medium such as water or wind, distinguishing it from erosion.
- Can occur both slowly and unnoticeably or rapidly and catastrophically (e.g., landslides or rock falls).

Mass wasting is a critical step in the processes of weathering, erosion, and deposition.
- Commonly occurs in river valleys.
- Effects on landscape include:
- Widening of valleys.
- Transformation of rugged landscapes into gentle, rolling terrains.

Gravity vs. Inertia:
- Gravity: Actively pulls material downslope.
- Inertia: A resistance to movement caused mainly by friction resulting from gravity.

Angle of Repose: Defined as the steepest angle at which material can be piled without falling.
- Factors affecting the angle of repose:
- Grain Size: Larger grains generally allow for a larger angle of repose.
- Amount of Water: Water present in the pore spaces affects stability.
- Immediate collapse does not occur and is dependent on various factors.

Low water levels can help grains stick together, thereby increasing the angle of repose.
High saturation (water filling pore spaces):
- Causes grains to become separated.
- Lowers the angle of repose, increasing the risk of mass wasting.

Increased pore-water pressure can lead to instability:
- Pressure on water in pore spaces under increasing weight of sediments can create lift, destabilizing overlying materials.
- Quicksand: A condition created when pore-water pressure equals the weight of sand, leading to a lack of shear strength and causing saturated sand to flow downhill.

Weight: Water is denser than air, filling pore spaces in sedimentary rock slopes.
Absorption and Adsorption:
- Water is absorbed into clay minerals (internally) and adsorbed to negatively charged surfaces (externally), decreasing strength.

Defined as highly mobile deposits resulting from glacial scouring and deposition in seas.
- Characterized by a weak, loosely packed structure held together by salt.
- Fresh water dissolving salts can collapse this structure, turning the material into a liquid that flows away.

Oversteepening: Occurs when a layer is undercut, causing overlaying material to exceed the angle of repose.
- Common causes include:
- Wave action.
- Stream or river action.
- Human activities.

Types of planes that can lead to mass wasting include:
- Saturated sand or clay layers.
- Joints parallel to land surface.
- Weak sedimentary bedding (e.g., shale, evaporites).
- Metamorphic foliation.

Removal of Vegetation: Roots of plants anchor soils; their removal increases susceptibility.
Earthquakes: Many mass wasting events feature unknown triggers.

Creep: Slow downhill movement due to seasonal expansion and contraction; factors include wetting/drying and freezing/thawing cycles.
Slump: Movement of coherent mass of sediments over a short distance, characterized by a head scarp, former slope, toe of slump, and failure surface.
Rockslide: Failure of rock mass falling en masse along bedding planes, fractures, and joints.
Earthflow: Flow of fine grains (sand, clay, silt) that contains more sediments than water.
Debris Flow: A rapid, partially liquefied flow of mud, potentially devastating due to speed and volume.

Use of Engineering Structures:
- Safety structures can stabilize slopes, including:
- Retaining walls, rock bolts, and screening to hold material in place.
Revegetation: Involves planting vegetation to improve soil stability through root anchoring and evapotranspiration.
Regrading: This process redistributes the slope to reduce mass loading and capture debris.
Drainage: Reduces weight and increases strength, especially within weak layers, by lowering the water table.
Reducing Undercutting:
- Utilizing riprap can limit the undercutting effects of waves and stabilize slopes.
Engineering Solutions:
- Building structures like avalanche sheds to redirect snow or barriers to hold rock in place.

What do all of the following types of mass wasting have in common?\n - a) They are all caused by the force of gravity.
- b) They are all results of stream erosion.
- c) They all involve land moving like a thick fluid.
- d) They are all initiated by ground movement caused by earthquakes.
In 2018, a mud flow occurred in California after a wildfire; what was the likely trigger?
- a) Oversteepening.
- b) Removal of vegetation.
- c) An earthquake.
- d) Adding weight to the top of the hill.

What do all of these types of mass wasting have in common?
- a) They are all caused by the force of gravity.
- b) They are all results of stream erosion.
- c) They all involve land moving like a thick fluid.
- d) They are all initiated by ground movement caused by earthquakes.