Any Questions?
Geology of the Hawaiian Islands
Class 17
9 March 2004
Mass Wasting
Landslides, Mudflows
Chapter 7
Mass wasting
„ Transportation
of large masses of
rock
„ Downslope movement of rock and
soil debris under the influence of
gravity
„ Very important kind of erosion
1
Mass wasting
Gravity is the driving force of all
mass wasting
Mass wasting
„ Can
range from smallsmall-scale
processes
„
soil creeping down slope
„ To
„
much largerlarger-scale processes
huge landslides
Talus
Mass Wasting Processes
Talus
Accumulation of
rock
debris at
the
bottom
of a
slope
2
Angle of Repose
Talus
Angle of
Repose
Formed
of loose
rubble;
Stands
at the
angle of
repose
(about
30-35°)
„
„
„
Steepest angle at which fragments will
stand without further sliding or rolling
down hill
Controlled by:
„ coarseness and size of fragments
„ uniformity of size of the fragments
The larger and more uniform the size of
the fragments, the steeper the angle of
repose
Talus
3
4
Landslides
Landslides
„ Occur
when a large piece of rock and/or
soil breaks off and slides down hill
„ Often initiated by earthquakes and by very
heavy rainstorms
Steep Headwall
„ Initiated
when rock/soil originally held in
place by internal cohesion suddenly loses
that cohesion
„ Form on slopes that are steep enough for
the weight of the surficial material to
overcome the cohesive force and fail
Jumbled-up
Toes
Spoon-shaped
sliding surface
5
Landslides
Landslides
„ Tendency
to slide increases with
increasing surface slope
„ Addition of water promotes sliding by
adding weight and by reducing cohesion
„ Friction along the slide surface controls
the speed of the downslope movement
„ Addition
of water reduces friction along
the surface and allows the mass to slide
faster
„ Some
slides move as coherent mass
„ Others
break up and the material inside
becomes jumbled and disorganized
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Blackhawk Landslide, Southern California
Slide
moved 8
km down
a slope of
2.5°
Slide is
100 m
thick
Moved on
cushion of
air;
moved
8 km in
about a
minute =
400 km/hr
(225 mph)
Debris flows
„ Occur
when the rock/soil mass loses
coherency and lots of water is involved
„ Debris becomes mixed up completely and
flows as liquid mud
„ Often carry large clasts
„ Can be very destructive
„ More destructive in explosive volcanic
terranes where ash blankets the slopes
and then becomes unstable
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Soil Creep
„ Mass
movement that moves very
slowly
„ Weak soils on steep slopes move
slowly down hill
8
Mass movement in dry regions
„ Example:
leeward sides of the islands
„ Most important processes:
„ Gravity fall
„ Sliding of rock fragments
„ DownDown-slope creep of soil
Mass movement in dry regions
„ Infrequent
large rain storms erode large
amounts of material in a day or two
„ Landslides are not very common
Mass movement in dry regions
„ Chemical
weathering less effective
„ Less soil
„ Fragmented rocks not badly
decomposed
Mass movement in wet regions
„ Weathered
rock held together by plant
roots
„ Dominated by soil avalanches
„ Soil cover on steep slope pulls loose and
slides downward
9
Mass movement in wet regions
„ Most
slides small (less than 100 m long
downslope,
downslope, less than 30m wide, and a few
meters thick)
„ Occur most often after very heavy rains
If bedding
dips downslope, a
landslide is
more likely to
develop
Landslides
„ Certain
areas are more prone than
others
„ Usually because of local geological
factors
Landslides
„ Certain
areas are more prone than
others
„ Usually because of local geological
factors
„ Reducing vegetation that was
stabilizing the soil
„ Over watering (decrease friction)
10
OK to cut
Dangerous
to cut
Cutting a slope during construction
can cause a landslide
Large-scale slumping and sliding
„ Has
affected all of the islands
„ Confirming evidence has been
discovered in the past 10 years
„ Major landslides have removed very
large pieces of the volcanoes
11
Landslides adjacent to the
Hawaiian Ridge
„ Rotational
slumps (non(nondisintegrative)
„ Debris avalanches (disintegrative)
Landslides adjacent to the
Hawaiian Ridge
„ Rotational
slumps
„ Broad (up to 110 km)
„ Short (30(30-35 km long
„ Thick (about 10 km)
„ Steep
Landslides adjacent to the
Hawaiian Ridge
„ Debris
avalanches
„ Narrow
(a few 10’s of km wide)
„ Long (up to 250 km)
„ Happened quickly in one event
12
Landslides adjacent to the
Hawaiian Ridge
„ Slumps
occur during the major shield
building stage
„ Once volcanism stops, the islands are
probably stable
Landslides adjacent to the
Hawaiian Ridge Caused by:
„ Steep
slopes
„ Young slopes of volcanoes very
steep
„ Become even steeper when the
flanks of the volcanoes are inflated
by volcanic activity
Landslides adjacent to the
Hawaiian Ridge Caused by:
„ Volcanic
activity
„ Earthquakes
„ Shake the ground and cause failure
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14
Molokai
Oahu
Tuscaloosa
15
Tuscaloosa
Seamount
1,120
2.5 km Thick
Ko'olau
16 km Wide
Moved
90
km
Volcano
28 km Long
Kilauea
km3
„ Buttressed
on north by Mauna Loa
„ South flank is free surface
Caldera
Hilina Slump
„ Off
south flank of Kilauea
„ About 5200 km2 in area
„ About 100 km wide
„ Headwall is SWRZ of Kilauea on NW;
Kilauea ERZ on the east
16
GPS Measurements
1975 Kalapana
Earthquake
Reference
Magnitude 7.2
3 m of vertical
movement
8 m of
horizontal
displacement
Questions?
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Thursday
„
„
„
„
„
Field trip to Manoa Landslide
Meet at Faculty Housing across from Manoa
Market Place
If you are taking GG 101 Lab you don’t need to
go on this trip
Take Rainbow Shuttle BEFORE 8:45 am
There WILL be questions on Exam 2 about this
trip
Next Tuesday
„
Next Thursday
Exam 2
Local Landslide Hazards
„ Will
cover everything since Exam 1
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Questions?
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