2. BREAKING
NEWS: HEAVY
STUFF FALLS
DOWN!
Mass wasting: the downslope
transport of surface materials in
direct response to gravity.
Everywhere on earth, gravity pulls
objects to earth’s center. We refer to this
downward force as weight.
Gravity is always helping rock, sediment,
and soil to move downhill on sloping
surfaces.
How heavy an object is (the more it
weighs) determines how much gravity is
pulling it to the center of the earth.
Heavier object = greater downward pull
4. GRAVIT
Y TUG-
OF-
WAR
G: gravity (weight of the block)
F: the weight of the block, pulling perpendicular to the
block/earth surface
D: downslope component of gravity (shear stress
pulling block downhill).
f: friction keeping the block from sliding
For the block to move downhill, F has to be <
D
If there is a low slope angle, F > D
The steeper the angle, the higher D value
F >
D
F <
D
5. GRAVITY =
GP + GT
Remember, gravity (for our purposes) just means weight.
Gp: Perpendicular component of gravity
The weight of the block being pulled down perpendicular
to the ground surface
Gt: tangential component of gravity
The weight of the block being pulled down the surface of
the
slope (shear stress-blocks moving past each other)
Here, the weight of the block
pulling downhill (Gt) is more
than the weight of the block
being pulled to the tilted hill
surface. Since the downhill
weight is greater, this block is
more likely to slide.
Here, more of the weight of the block is perpendicular to the
slope, making it unlikely to slide. The weight pulling the block
to the hill surface is more than the weight pulling the block
downhill.
Gt
6. WILD CARD:
FRICTION
Remember, gravity (for our purposes) just means
weight.
Now we have identical blocks on identical slopes.
But the one slope is made up of loose rocks fallen
from the mountain already, and the other has trees
on it.
The small rocks on the left
will act as ball bearings and
decrease the friction by
rolling underneath the rock,
making it more likely to slide.
The trees on the slope on
the right will slow the rock
down, creating more friction
for the rock to overcome on
its way down the mountain.
7. MATERIALS
AND MOTION
Mass wasting events are
categorized by:
�Kinds of Earth materials
involved
�Ways the materials move
�Terminology
(Soil, earth, debris, and
mud)
�Speed
(Slow vs. fast mass wasting)
8. Creep: slow migration of
particles to successfully lower
elevations.
Gradual downslope motion
usually affects a thin top layer of
weathered rock particles.
Most widespread and persistent
form of mass wasting
Heaving process
SLOW MASS
WASTING: CREEP
9. SOIL CREEP: ABOVE
GROUND AND BELOW
While the mechanism of
soil creep happens below
the surface and very
slowly, the effects of soil
creep can be seen in the
landscape if you know
what you’re looking for.
10. SLOW MASS WASTING:
SOLIFLUCTION (SOIL
FLOW)
Solifluction: Slow downslope movement
of water-saturated soil or regolith
Occurs in high-latitude or high-elevation
tundra regions: permafrost conditions
Active layer: part of the soil that thaws in
summer
Because the melted water can’t be
absorbed by the frozen ground beneath
it, the mass of wet muddy soil slides
downhill on top of the permafrost
12. ROCKFALLS
Earth materials
plummeting downward
freely through the air (not
rolling downhill)
�Large slabs break into clasts
at the bottom of the slope
Triggers:
�Steep slopes
�Spring thaws/freezes
�Earthquakes
13. ROCK FALL IN TENNESSEE CAUSED BY UNLOADING/EXFOLIATION OF A
GRANITE CLIFF
https://www.youtube.com/watch?v=39LCzBS8yOM
15. ROCKFALL IN TAROKO GORGE,
TAWIAN
Taroko Gorge: a dashcam video of a
very near-miss with a co-seismic rockfall
in Taiwan. This is a still from the AGU
landslide blog, written by Dave Petley.
“A magnitude 6.1 earthquake struck eastern
Taiwan on Thursday 18th April 2019 at at 13:01
local time. This shallow earthquake shook the
extraordinarily beautiful Taroko Gorge in eastern
Taiwan, a major tourist attraction with very tall,
very steep slopes above a major highway. A
dashcam video has been posted to Youtube,
shot by a motorist on the road at the time of the
earthquake. Unsurprisingly, the earthquake
triggered large numbers of rockfalls. The
dashcam video captures a near-miss event that
was extraordinarily close to being a tragedy”
https://blogs.agu.org/landslideblog/2019/04/20/taroko-gorge-1/
21. LANDSLIDES
Landslide: Cohesive or semicohesive unit of Earth
material that slips
downslope in continuous contact with the land surface
They are categorized (if possible) by the material or kind of motion
involved:
Rockslides
Debris slides
Mudslides
Slumps
Landslide
22. ROCKSLIDE/
AVALANCHE
IN NEW
ZEALAND,
2013
Top photos are
where the rock
broke off the cliff
face. You can see
the extreme
weathering in the
picture on the
right.
https://blogs.agu.org/landslideblog/2013/01/23/an-analysis-based-on-images-and-video-of-the-mount-cook-national-park-landslide-on-monday/
23. THE MAY 1971
SAINT JEAN
VIANNEY
LANDSLIDE
DISASTER
https://blogs.agu.org/landslideblog/2020/03/19/saint-jean-vianney-landslide/
For scale, look at
the housing
development in
the picture on the
left. That is here.
Vertical aerial image of the
Saint Jean Vianney landslide,
from Tavernas et al. (1971) Flow
runout
(end)
Flow direction and path
Red circles show some of the destroyed
houses
24. FLOWS:
4 MAIN
TYPES
Flows carry water in moving sediments vs. rivers which carry sediments in
moving water.
Earthflow Debris flow
Mudflow Lahars
25. Earthflows can be
independent events
or a
compound feature:
slump–earthflow
EARTHFLOWS
A slump– earthflow
moves as a cohesive
unit along a concave
surface in the middle
and upper reaches
of the failure.
Downslope of the
failure plane, the mass
continues to move but
in the more fluid-like,
less cohesive manner
of an earthflow
Earthflow- thick unit of fine-grained,
unconsolidated hillside sediment or shale
becomes saturated and mixes and
tumbles as it moves.
29. DEBRIS AND MUD
FLOWS
Faster than earthflows
Debris flow: smaller grain size
sediment
Mudflow: larger grain size
Flow levees –Higher edges of
debris left along the sides of the
flow
Flow
chann
el
Levee
s
31. STILLS FROM ILLGRABEN DEBRIS
FLOW, SWITZERLAND, 2016 (research
site)
https://blogs.agu.org/landslideblog/2016/07/28/illgraben-debris-flow-video/
1. The front of the flow
consists of large number of
(in this case) huge boulders
tumbling chaotically.
2. The main, more fluid
portion of the Illgraben
debris flow
3. As the discharge
starts to decline the
debris flow creates a
narrower channel,
leaving deposits
33. VOLCANIC MUDFLOWS: LAHARS
Dangerous, unstable,
steep mountain
slopes +
fine grained sediment
+
melted snow and ice
from eruption =
extreme damage
34. LAHARS
Lahars—specific flows of
material from the sides of a
volcano
Lahars move rapidly down valleys like
rivers of concrete
Lahars can occur with or without a
volcanic eruption
Lahars and excess sediment cause
serious economic and environmental
damage to river valleys and flood plains
lahars commonly occur as seasonal
debris flows on active volcanos topped by
glaciers.
Image: Lahar devastation after the eruption of Mount
Pinatubo, Philippines. Source: USGS
36. WEATHERING, MASS
WASTING,
AND THE LANDSCAPE
Exogenic processes of weathering and mass
movement are:
�Critical to soil formation
�Significant factors in shaping the landscape
�Slopes are evidence of local weathering and mass wasting, and the
processes that formed the landscape
�Often accelerated and induced by human actions
