Chapter 15 Mass Wasting

1
Chapter 15 Mass Wasting

• Mass wasting the downslope movement of rock,
  regolith, and soil due to the influence of
  gravity. Does not require presence of water to
  move this material- but water may facilitate
  movement.
• Landslides and mudslides are among the most
  spectacular and deadly of geologic events.

2
Peru, 1970 Offshore earthquake (remember
subduction zone), triggered rockfall on vertical
face of Nevado Huascaran. After falling 1 km,
the rock mass shattered and pulverized, tons of
rock and ice raced down mountainside, into a
valley, killing 20,000 people. In addition to
the original rock material, as the avalanche
progressed, it picked up additional debris. A
portion of the debris cleared a 650 to 1000 ft.
high ridge.
3
3
Landforms are shaped when exposed rocks are
weathered and the weathered products are removed.
Erosion by water is usually gradual, except
during and following intense rainfall events,
when mass-wasting events may play a role in
landform development. Usually, rugged,
geologically young areas are most prone to
spectacular mass-wasting events. In a
Continental Arc setting, inland from a subduction
zone, the Andes Mts. are continually growing.
4
4
Over time, as the landscape matures, the slopes
become less steep (approaching equilibrium) and
the terrain becomes more subdued. Larger
mass-wasting events are replaced by smaller, more
local events. Western US, geologically younger,
more active more prone to mass-wasting
events. Eastern US mass-wasting usually is
associated with slope disturbances in mountainous
areas. Example Ellijay, GA 2003.
5
5
Controlling Forces of Mass-Wasting
Gravity Friction - Cohesion
When Gravity overcomes Friction Cohesion,
movement can either be slow and gradual or rapid,
when there is a triggering event.
Triggering Events can include
Over-steepened slopes
Removal of Vegetation
6
6
More triggering events
Earthquakes/other vibrations
No apparent reason
Water is heavy and it acts as a lubricant. When
soil becomes saturated, the weight overcomes the
internal friction holding the mass on the slope.
Landforms over time become stabilized. Slopes
are often held up by resistance at the base of
the slope. When base is undercut, support is
lost.
7
Natural slope that has reached equilibrium Roadcu
t at base of slope removes support, slope needs
to regain equilibrium by mass-wasting.
Roadcut
8
8
A stable slope of unconsolidated, particles,
the slope angle is called the angle of repose.
Angle of repose varies from 25 to 40 degrees.
Larger, angular particles support the steepest
slopes. If slope becomes destabilized, debris
will move downslope to re-establish stability.
9
9
An example of oversteepening Valley-fill
material deposited during slope stabilization
(erosion and redeposition). Flash floods due to
22 of rainfall scoured the valley.
10
10
When vegetation is removed by logging, fire,
clearing for cropland, etc., anchoring effect of
roots is lost. Or, conversely, on a modified
slope (construction area, etc.), that is newly
grassed, immature grass may not have deep enough
roots to stabilize slope. Earthquakes or other
vibrations Can disrupt internal friction of
locked rock fragments. Gentle, long-term
vibrations caused by trucks or cars could also
play a role, near highways.
11
11

• Earthquakes can also cause liquefaction (on
  slopes and flat areas), i.e., the loss of the
  soils structural integrity.
• Sometimes landslides happen with no apparent
  warning.
• Old Man of the Mountain, New Hampshire
• http//www.cs.dartmouth.edu/whites/old_man.html
• Mass Wasting Processes are classified based on
  the
• Type of material
• The kind of motion displayed
• The Velocity of movement

12

• Materials
• Unconsolidated soil (regolith) Debris flow,
  mudflow, etc.
• Bedrock Rock slide, Rock fall
• Type of motion
• Fall freefall, material leaves the slope
• Slide mass remains coherent and moves along a
  definable slope
• Flow mass moves downslope as a viscous flow

13
Rate of movement Rock avalanches are generally
the fastest, reaching up to 125 miles per hour.
When mass starts moving down steep slope, air
becomes trapped beneath avalanche, serves as a
cushion, lessens friction with ground. Most
mass-wasting events are generally slow, sometimes
as slow as downslope creep which may be
measured in millimeters per year.
14
14
Slump - mass of rock or unconsolidated soil
moves as a unit along a curved surface (glide
plane). Rupture is often spoon-shaped and
occurs because of over-steep-ening (perhaps with
water and/or vibrations acting as the trigger).
Figures 15.11 and 15.12 show slumps. 15.12 shows
backward tilt of upper surface.
15
15

• Rockslide/Debris slide usually takes place
  where strata are tilted or joints and fractures
  parallel surface. More common after rain or
  during spring, when water lubricates slide
  surface. If material is unconsolidated, debris
  slide is used instead. Examples rockslides
  after earthquakes, etc.. When strata are tilted,
  weakness of shale or clay may contribute to
  slippage (Fig. 15.14, pg. 458)

16
16
Outline map of Franklin Mts., El Paso, TX
showing major faults and landslide blocks. The
sedimentary rocks dip (tilt) about 35 degrees to
the west. The landslide blocks are just huge rock
slides. Most of the western slides likely slid
upon shale layers.
17
Debris Flows/Mud Flows contain large amounts of
water, generally tend to follow canyons and
valleys. Debris Flows in Semi-Arid regions soil
regolith are washed into stream
channels. Consistency ranges from wet concrete to
a soupy mixture. When flow leaves confinement of
steep, narrow canyon, the sediment is rapidly
deposited in a fanlike manner.
18
Miniature version of an alluvial fan
below. Alluvial fan deposits are usually very
poorly sorted, i.e., there is a wide range of
grain sizes.
19
19

• Lahars Volcanic mud flows.
• Triggers
• Water saturation of unstable ash along flanks of
  large, composite volcanoes.
• Sudden melting of tons of ice and snow during
  eruption.
• Examples Toutle River (Mt. St. Helens), Mt.
  Pinatubo (after eruption), Nevado del Ruiz,
  Colombia destruction of town of Armero 25,000
  dead.

20
20

• Earthflows more common to hillsides in humid
  areas. Breakaway of heavily saturated soil
  leaves a rounded scar and produces a flow
  downslope from the slumped area. In Figure
  15.17, the earthflow is associated with an
  upslope slump. These are generally slower than
  earlier described mudflows.
• Earthflows likely lose more of their coherence
  than a slump.

21
Slow mass-wasting events Creep may be caused
by alternating events freezing/thawing or
wetting/drying. Creep may be promoted also by
saturation of the soil by water. Solifluction
Soil flow when underlying conditions (tight
clay, frozen permafrost) do not allow the
downward drainage of water in saturated soils.
22

• Underwater landslides flanks of large,
  underwater volcanoes and volcanic islands. Also
  occur at the margins of continental shelves.
• May occur as simple slumps, or may become
  turbidity flows that build broad, underwater
  sediment fans.
• In areas subject to repeated turbidity flows,
  distal deposits may build up as a series of thin,
  graded bedding sequences, called turbidite
  sequences.