Transcript Compared to the desolate surface of the Moon, Earth must

Mass movements
Mass movement
The process that transports
Earth material (bedrock,
sediment, soil) down slopes
by the pull of gravity
Follows after weathering
May be fast or slow
Can occur on steep slopes or
shallow dipping slopes
Occur when factors that drive
materials downslope
overcome the factors that
resist downslope movement
Mass movements
In 1970, an earthquake-induced rock
and snow avalanche on Mt.
Huascaran, Peru, buried the towns
of Yungay and Ranrahirca.
The death toll from the earthquake
and landslide was 66,700.
The avalanche started as a sliding
mass of glacial ice and rock.
The avalanche swept about 16.5 km
to Yungay at a speed of 200 km/hr.
The fast-moving mass picked up
glacial deposits and by the time it
reached Yungay, it is estimated to
have consisted of about 50100
million m3 of water, mud, and
rocks.
Mass movements
Yungay, Peru, May 31, 1970
Mass movements
Mass movements
Mass movement
I. Causes
A. Gravity, Friction, and Slope
1. Gravity: The principle force
tending to pull materials
downslope
2. Friction: resists the
downslope movement of
material
3. Slope: (all else being equal)
the steeper the slope, the
more likely that the materials
will move downslope (mass
movement)
Mass movements
Mass movement
I. Causes
B. Composition of Materials
1. Solid bedrock: generally stable
= prohibits mass movements.
Tectonic deformation produce
fractures and joints = unstable
a. Sedimentary bedding planes
may slide along
Highway 20 east of Newhalem.
b. Mechanical weathering has
created cracks (exfoliation,
freeze-thaw)
c. Metamorphic rock with foliation,
if foliation lies parallel to the
slope then the rock can slide
along it
Mass movements
The United States Geological Survey will install
an instrument on the hillside to collect data on
vibration and movement.
Unstable rocks fill Afternoon
Creek. Compare the rocks with
the full grown trees on the left
and right.
Mass movements
Mass movement
I. Causes
B. Composition of Materials
1. Solid bedrock
Mass movements
Mass movement
I. Causes
B. Composition of Materials
2. Unconsolidated (loose)
sediment
a. Slopes of loose material can
only get so steep before the
particles on their slopes start
sliding down
b. Angle of repose—maximum
angle at which unconsolidated
material is stable
Sand = 30 to 35
Cinder = 30 to 40
Mass movements
Mass movement
I. Causes
B. Composition of Materials
2. Unconsolidated (loose)
sediment
a. Slopes of loose material can
only get so steep before the
particles on their slopes start
sliding down
b. Angle of repose—maximum
angle at which unconsolidated
material is stable
Sand = 30 to 35
Cinder = 30 to 40
Mass movements
Mass movement
I. Causes
C. Vegetation
Vegetation, especially those
with deep root networks,
helps hold material on a
slope
Replanting efforts along
roadcuts susceptible to
mass movements)
Vegetation removed by forest
fires, clear cuts, people
(farmers)
Mass movements
Mass movement
I. Causes
D. Water
 Most important factor causing
previously stable slopes to slide
 A small amount can increase
cohesiveness = binds by
surface tension
 Excessive water promotes
slope failure
 Reduces friction between
surface material and underlying
rocks
 Reduces cohesiveness/friction
between individual grains =
easier to flow
Mass movements
Mass movement
I. Causes
D. Water
Most important factor causing
previously stable slopes to slide
A small amount can increase
cohesiveness = binds by
surface tension
Excessive water promotes slope
failure
Reduces friction between surface
material and underlying rocks
Reduces cohesiveness/friction
between individual grains =
easier to flow
Mass movements
Mass movements
II. Types of Mass Movements
Classified based on the speed and
manner in which they travel
downslope
A. Slow
1. Creep
 Slowest form (cm or mm per
year)
 Moves unconsolidated material
down-slope
Caused by: freeze-thaw, or wetting-drying
events
or: rain drop splashes, gravity, animals
burrowing, wind
 Occurs everywhere, even on
gentle slopes
 Affects only the top few meters;
top moves faster than bottom
(due to friction) so objects in the
soil start to lean over
Mass movements- Creep
Mass movements
Mass movements
II. Types of Mass Movements
2. Solifluction (soil flow)
 Unconsolidated material
 Occurs in very cold environments
where surface is frozen to
hundreds of meters
 In summer, warm sun melts top
meter or so of frozen ground
 Water can’t percolate downward
(because of permafrost), so soil
gets water-saturated and flows
downslope
 Same process can occur where
there’s impermeable bedrock or
a clay layer
 2-6” per year
Mass movements
II. Types of Mass Movements
2. Solifluction (soil flow)
Unconsolidated material
Occurs in very cold environments
where surface is frozen to
hundreds of meters
In summer, warm sun melts top
meter or so of frozen ground
Water that is created can’t go down
(because of permafrost), so soil
gets water-saturated and flows
downslope
Same process can occur where
there’s impermeable bedrock or
a clay layer
2-6” per year
Mass movements
II. Types of Mass Movements
B. Rapid
km/hour or even meters/second
1. Falls
 Fastest type
 Rock or sediment breaks free
and falls vertically or near
vertically
 Any size of material
 Main way that talus slopes
are built
Mass movements
II. Types of Mass Movements
B. Rapid
2. Slide (rock slide, debris slide)
 Rock or sediment breaks loose
and moves in contact with an
underlying slope along a
preexisting plane of weakness
 Bedding, fault, fracture, foliation
 Moves as a single, intact mass
 Small displacement of soil over
bedrock to whole mountainside
slabs of rock
 Slip plane is generally flat
Mass movements
Mass movements
II. Types of Mass Movements
B. Rapid
3. Slump
 A slide that separates along a
concave up slip surface
 Generally create a slip plane
within unconsolidated material
 Material moves as a coherent
unit—sometimes multiple blocks
 Forms crescent-shaped scar at
the head where the material
detached = scarp
 Does not travel far
 Does not travel very fast
Mass movements
II. Types of Mass Movements
B. Rapid
4. Flow
 Mixture of sediment and soil
moves downslope as a highly
viscous fluid
 Most are water saturated
 Velocity depends on: water
content, transport materials,
underlying material, slope
a. Earthflows:
Relatively dry soil, slow, small
particles, high viscosity, slow
(meters/hour – meters/min)
Mass movements
II. Types of Mass Movements
B. Rapid
4. Flow
b. Mudflows:
saturated with water, fast,
generally particles smaller than
sand, consistency (wet concrete to
muddy water), flow down canyons
Mayflower Mountain debris flow in the Ten Mile
Range near Climax, Colorado, in 1961. Note
the large boulders transported by the flow. Finer
material has been eroded from the top of the
flow.
c. Debris flows:
Similar to mudflows watersaturated, fast (1-25 mph),
composed of debris larger than
sand size-boulders, need steep
slopes
d. Debris or Rock avalanches:
Special kind that moves very fast,
very steep slopes, they “float” on a
cushion of air trapped beneath
Problem Set 3
Due Monday, March 6st at beginning of class
Go to www.geology.cwu.edu and enter 101 in left
navbar search field.
On G101 web page, download Problem Set #3, PDF
file.
Discharge, stream velocity, wetting perimeter
Simple calculations
Need to understand units of calculations (e.g., m/sec or m3/sec)
No late assignments accepted
Sheep Mountain Slide
Sheep Mountain Slide
Turtle Mountain Landslide
Sheep Mountain Slide
This house, which has been the subject of several news stories, is located on
East Boston Terrace in the Capitol Hill area of Seattle. It hangs precariously
over the headscarp of a reactivated old landslide.
This house is in the same drainage as, and a few hundred feet upslope of, a
house destroyed by a landslide in 1942. That landslide killed one resident and
seriously injured another.
One of the larger Seattle landslides along Perkins Lane on the southwest side of the Magnolia
neighborhood. This is an area of continuing large-scale instability. Immediately following the
February 1996 storms, a 1,500 -yd³ landslide slid from the upper portion of the bluff into the
back yard of the home on the right.
Geo-engineers attempted to mitigate the problem by regrading and revegetating the upper
slope. The slide was reactivated in December 1996, damaging at least five houses.
This view to the west over the Magnolia
Bridge, a major artery into downtown
Seattle, shows the landslide that forced
the closure of the bridge and the "redtagging" (condemning or declaring
uninhabitable) of at least five homes
along the headscarp of the slide.
This slide occurred after the rains had
ceased.
Notice the displaced bridge trusses, the
debris on the house at the base of the
slope, and the broken water main just
below the fallen truss and above the
house.
Useless Bay area of Whidbey Island can cause periodic retreat of the bluff edge by
as much as 20 feet or so in seconds. During this recent slide, a portion of the fence
in front of the large house was lost.
Such episodes commonly are preceded and followed by decades of little erosion,
making estimates of average bluff retreat rates potentially meaningless.
Continuing slide activity has made this home near Cape George in Jefferson County
uninhabitable. At this site, the perching layer is a small area of ancient lakebed silt that lies
beneath the house at about mid-bluff level, now covered with grass dropped from the back lawn.
Note the old slide mass at beach level (lower left), now covered by alder trees that are all of
approximately the same age (here, perhaps 25 years old).