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Lecture

13-Mass Wasting


Department
Earth Sciences
Course Code
EARTHSC 1G03
Professor
Maureen Padden

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13- MASS WASTING
Mass Wasting: movement of mass bedrock, rock debris, or soil downslope b/c of
gravity; important in erosion and weathering.
ANGLE OF REPOSE
Steepest angle of descent/dip of granular material when material on the slope
face is on the verge of sliding.
Varies 35 degrees to 45 degrees (fine sand<course sand<angular pebbles)
Classification:
Rate of Movement
Type of Material
Type of movement:
Flow descending is moving downslope as viscous fluid
Slide relatively intact, moving along one or more well-defined surfaces
o Translational side: descending mass move along a plane parallel to
surface slope
o Rotational slide or “slump”: movement along curved surface upper part
moves downward, lower part moves outward
Triggered by undercut slopes or earth quake
Fall free-falls or bounces down a cliff
Controlling Factors
Contributing Factors
Most Stable Situation
Most Unstable Situation
Slope Angle
Gentle slopes or horizontal surf.
Steep or vertical surface
Local relief (vertical
distance from valley
floor and mountain
summit)
Low
High
Thickness of debris
over bedrock
Thin debris
Thick debris
Bedrock type
Massive, un-jointed
Layered, stratified, jointed
Orientation of planes
of weakness in bed
rock
Planes at right angles to hillside
slopes
Planes parallel to hillside
slopes
Climate factors:
1. Ice
2. Water in soil/debris
3. Precipitation
4. Vegetation
1. Temperature above freezing
2. Film of water around fine
particles
3. Frequent but light
rainfall/snow
4. Heavily vegetated (roots
anchor into ground)
1. Repeated
freezing/thawing (frost
wedging in joints)
2. Episodic saturation of
debris w/ water
3. Long periods of drought
w/ rare episodes of
heavy precipitation,
snow melt
4. Sparsely vegetated
Triggering mechanisms: (1) Earthquakes; (2) Weight added to upper part of a slope; (3)
Undercuttng of bottom of slope; (3) heavy rainfall
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