What is mass wasting?
Mass Wasting
Spontaneous Gravitational
Movements
Slope material
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Regolith is the layer of mineral
matter overlying bedrock and is
formed from the weathering of
the underlying bedrock or can
be transported from elsewhere
Regolith formed on the spot (in
situ weathering of bedrock) is
referred to as residual regolith
and those brought in from
elsewhere through any agent
(wind, streams, or glaciers) as
transported regolith
• Mass wasting can be defined as the
spontaneous downhill movement of soil,
regolith and rock under the influence of
gravity
• This implies it will normally occur on a
slope where loose unconsolidated material
becomes unstable
Slope material (cont.)
• Accumulations of regolith at the foot of a
slope are called colluvium
• Alluvium is the transported regolith
deposited by streams in valley bottoms
From: Strahler, A., 2002: Physical Geography.
John Wiley & Sons, New York.
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Mass wasting processes
Processes and forms of mass wasting
•
Can occur through four actions
1. Free fall
2. Slides – caused by differential internal
cohesion
3. Flow – caused by weak internal cohesion as
well as exceeding the threshold angle of
repose
4. Swelling – caused by expansion of clay or
through frost action
Types of mass wasting
• Soil creep – extremely
slow downhill movement
of soil and regolith
• Evidence that the process
is active
– Large blocks at the foot of
slopes that did not roll there
– Edges of strata bending in
downhill direction
– Fence posts and telephone
poles leaning over in a
downslope direction
– Retaining walls leaning
over and breaking outwards
From: Strahler, A., 2002: Physical Geography.
John Wiley & Sons, New York.
Causes for soil creep
• Heating and cooling of
soils
• Growth of frost needles
• Drying and wetting of soil
material (particularly the
clays)
• Trampling and burrowing
of animals
• Earthquakes
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Types of mass wasting (cont.)
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•
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Earthflows – the downslope
movement of a mass of watersaturated soil, regolith or clay
within a few hours
Earthflows are often associated
with spontaneous liquefaction
Spontaneous liquefaction is
when a super-saturated silt or
clay body takes on the
characteristics of a fluid
(becoming a quick clay) due to
vibration caused by an
earthquake
Earthflow (British Columbia)
Quebec, Canada, (1955)
From: Strahler, A., 2002: Physical Geography.
John Wiley & Sons, New York.
Types of mass wasting (cont.)
Periglacial features (Colorado, USA)
• Solifluction is a slow moving earthflow in
arctic regions with permafrost
• The upper part of the permafrost starts
thawing, leaving the lower part supersaturated with water that causes the
movement
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Solifluction lobes (NW Territory,
Canada)
Types of mass wasting (cont.)
• Mudflows – are literally rivers of mud where a
river mobilizes such an amount of debris that it
“chokes” itself
• Mudflows are common in desert areas. WHY?
• Mudflows that occur on the slopes of an erupting
volcano are referred to as lahars
• Watery types of mudflows are called debris flows
or debris floods when they consist of rock debris
(angular blocks or rock fragments)
Debris flood deposit (French Alps)
Systems to break down debris flows
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Debris flood damage after flash
floods (Venezuela, 1999)
Types of mass wasting (cont.)
• Landslides are the
rapid sliding of large
masses of bedrock and
regolith beginning
their descent as a unit
block without internal
flowage
Types of landslide
• Rockslide – is a
bedrock mass slipping
on a sloping rock
plane
• Slumping - a bedrock
mass moving
downslope down a
curved slip surface
and at the same time
rotating backward
Types of mass wasting (cont.)
• Rockfalls are free
falling or rolling of
single masses of rock
down a steep cliff
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