Mass Wasting

The downhill movement of soil and loose unconsolidated sediments is due to the downslope component of the force of gravity and is resisted by the force of friction. The downslope driving forces (shear forces) and frictional resistive forces are in balance at the angle of repose which is the maximum slope angle that unconsolidated materials can maintain. At angles steeper than the angle of repose friction is not sufficient to counter the shear force and mass wasting occurs. At angles less than the angle of repose the shear forces cannot overcome friction and sediments may accumulate to form steeper slopes.

Water plays an important role in mass wasting. Dry sediments have no cohesion. Damp sediments are cohesive as water coats the sedimentary grains and holds them together with its surface tension (surface tension is the result of the dipolar nature of water). The angle of repose is greater in damp sediment than in dry (think of a sand castle). In water-saturated sediments all the pore spaces are filled with water. Water pressure in the pore spaces partly counters the weight of grain on grain thereby decreasing friction and the angle of repose, and possibly causing mass wasting.

Where Ff = sum of the frictional resisting forces
and Fg = sum of the gravitational driving forces

I) Processes that increase shear stress (driving forces)
     a) loading the slope (increasing its weight) by
         - increasing the water content
          - building on it
     b) earthquake shaking
     c) undercutting (oversteepening) by
          - stream erosion
          - road or building excavation
     d) subsurface collapse
          - sinkhole
          - mine collapse

II) Processes that decrease the shear strength (frictional resisting forces)
     a) increased pore pressure (water pressure in void spaces)
          - increasing load of water and saturation
          - compaction (from vibration)
     b) fissuring - fracturing (frost wedging)
     c) dissolution of cements in sedimentary rocks
     d) removal of vegetation and roots that hold the soil
          - by fire
          - by clearing for building

Flows
Slow	Creep		creep can be caused by differential expansion & contraction displacement by organisms (e.g., tree roots) ice heave earthquake shocks or other vibrations release of weathering products
		soil creep
		rock creep
		talus creep
		rock glaciers	ice-charged breccia; top moves faster than bottom slope of the front is at angle of repose
		solifluction	viscous flowage of wet, plastic mud, especially over frozen substrate; caused by freeze thaw of saturated mud

	Flows	earthflows	imperceptible to 10 mph (17 km/h)
		debris flows	1 to 15 meters per second sandy/gravelly matrix mixed with water consistency of wet concrete they form steep lobate fronts and margins stop flowing when water drains out
Fast		mudflows	mud matrix from stiff mud to muddy water consistencies but denser & more viscous than water can carry large boulders 1) upper erosion zone 2) middle sub-linear chanel btween steep mud or boulder levees 3) lower depositional zone of spreading lobes lahars are volcanic mudflows: rain or snowmelt mixes with ash from volcanic eruptions

Slides
slumps	downhill sliding along a concave-upward curving basal surface toe of the slump often terminates in earthflow or mudflow
rockslides	typically, blocks slide along bedding or jointing (fractures)
deris slides	similar to above but aggregate sliding on basal slope
rock falls	rock breaks loose along jointing or by toppling over talus slope found at base of cliff
sturzstroms	disintegrating rock slide entrapped air or fluid at base makes it behave as a flow
debris avalanches	a rapid flow of rock or snow and debris