3.3 - Mass movement UNFINISHED

categories of mass movements + example for each

Falls - rock fall

Heaves - soil creep, talus creep

Slides - rock slide, landslide

Flows - earthflow, mudflow

definition of mass movement

movement of rock or soil downhill due to gravity

3 factors determining classification of mass movements

speed

moisture content

type/size of material

simple, core reason for mass movement

shear stress exceeds shear strength

shear stress definition

force exerted on material downslope due to gravity

shear strength

resistance of material to downslope movement

factors affecting shear strength and explanation

internal friction and cohesion between material particles

• higher moisture → increased pore water pressure → reduced internal friction → higher shear strength

vegetation roots

• more roots → bind the soil → shear strength

factors affecting shear stress and explanation

slope angle

• undercutting/material added to slope → slope angle increase → friction to gravity ratio decreased

weight of material

• added vegetation/saturation → increased weight → more force

slope safety factor definition

ratio of slope material’s shear strength to the shear stress

• >1 = stable

• <1 = mass movement

explain exfoliation (weathering process)

• rock is a poor conductor of heat

• outer layer expands and contracts as it heats and cools at a different rate to inner layer

• outer layer breaks off

briefly explain falls

rock loosened by weathering from very steep slopes/free face

angle of repose

maximum slope angle upon which material can support itself

scree slope

accumulation of rock fragments at the base of a free face

provide evidence of rock fall

briefly explain heaves (aka creeps)

slow movement of small rock or soil particles downhill via repeated cycles of dislocation

what causes heaves

particle pushed perpendicularly away from slope, then falls due to gravity as slope material contracts, repeatedly

• freezing and thawing

  • freeze = expand, thaw = contract

• wetting and drying

  • saturated = expand, dry = contract

evidence for heave

soil buildup on upslope side of walls, trees

tilting of fences and poles downslope

ripples on grassy slopes called terracettes

frost heave (INCLUDES PLACE EXAMPLE)

frost heave

• in tundra environments like ALASKA

• diurnal temperature range causes daily freezing and thawing

• even talus creep can be caused by ice needles pushing them up

solifluction

• surface layer of permafrost thaws in summer

• it is saturated → low internal friction

• slides downslope

in between heave and slide

briefly explain slides

entire mass of material on a slope slips downslope at same rate

occurs along a slip plane (usually bedding plane or fault line)

conditions likely to cause slides

• weak rock with little internal cohesion

• steep slopes - high shear stress

• undercutting

• water - reduces internal cohesion AND lubricates slip plane

• often exfoliation weathering