MASS WASTING PROCESSES AND LANDFORMS

mass wasting

• gravitational movements of earth materials
• especially unconsolidated sediments and weathered rock
• water contributes to reduced strength of materials but is not involved as a geomorphic agent
• transitional between weathering and erosion, because it is closely related to the declining strength of weathering earth materials, but does not involve a transporting medium (e.g. water, wind, ice)

colluvium

earth materials moved by gravity

gravitation

• tendency for matter at the earth's surface to accelerate towards the center of the earth, given the small mass relative to the earth
• gives all mass above the earth's surface potential energy
  • PE = mgh
  • acceleration due to gravity (g) is constant (9.8 m sec^-2)
  • therefore, for a unit mass PE is proportional to height or relief
  • slope gradient (h/d) is also an energy gradient governing the rate of conversion from potential to kinetic (1/2mv²) energy
  • high geopotential energy is available for mass wasting in landscapes with large relief (h) and steep slope (h/d)
• mass moves over hillslopes when the shear stress exceeds the resistance to shearing (shear strength)

Mass Wasting Processes

Classification

• type of material: rock, sediment, ice, snow, mud, sand
• amount of ice or water involved
• morphology of resulting landform: lobes, levees, talus, slump blocks
• type of movement (mechanism) is the most common and unambiguous criterion:

1. spread (creep)
   • slow, imperceptible, seasonal decline in shear strength
   • decreasing velocity with depth
   • soil creep: slow spread as soil expands and contracts with freezing and thawing or wetting and drying
   • rock creep: slow continuous failure of rock masses, especially in rocks with low yield stress and overlain by stronger rocks; often the precursor to rapid catastrophic mass movements
2. flow
   • rapid failure of earth materials by internal shearing (liquid behavior)
   • usually related to excessive porewater pressure
   • earthflow: flow of unconsolidated materials on an open slope
     • Mink Creek Earthflow, Terrace, B.C.
   • debris flow: confined fluid mass wasting, i.e. in a stream channel, but moving independent of the stream
   • debris flow fans form at the mouths of steep canyons
3. slide
   • slope failure as rock (rockslide) or less consolidated earth materials (landslide) fail at depth by shearing along a distinct sliding plane
   • rotational landslide (slump): curved sliding surface
   • translational landslide: planar sliding surface
4. fall
   • the free fall, bouncing and rolling of rock (rockfall) over steep weathering cliffs to from talus

composite failures

• many movements of earth materials involve a combination of processes
  
  
  • rock avalanche: rapid mass wasting of rock, ice and snow involving sliding and falling
  • solifluction: spread and flow of saturated substrate over an impermeable stratum (e.g. gelifluction over permafrost )
  • landslide is the term most commonly used to refer to mass wasting events, even though sliding may be only one (often the initial) mode of failure

Mass wasting as a natural hazard

Slumping of the bank of the North Saskatchewan River at Edmonton:

Edmonton Journal, October 24, 1999

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