UPCOMING EXAM • BRING PENCIL And A SCANTRON! • Let me know if you are interested in taking the optional comprehensive final. • No programmable calculators or other electronic devices. No dictionaries. • Covers Chapters Geologic Time, Glaciers, Shorelines, Mass Movement, Streams • Bring blank paper in case you need to work out a problem (NO WRITING IN THE TEST BOOKLET).
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Lec 13 MassWasting - Napa Valley College · Process of Erosion Erosion: materials transported by wind, water, or gravityand deposited somewhere else. With mass wasting we will be
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UPCOMING EXAM• BRING PENCIL And A SCANTRON!
• Let me know if you are interested in taking the optional comprehensive final.
• No programmable calculators or other electronic devices. No dictionaries.
• Covers Chapters Geologic Time, Glaciers, Shorelines, Mass Movement, Streams
• Bring blank paper in case you need to work out a problem (NO WRITING IN THE TEST BOOKLET).
The 1995 La Conchita landslide, southern California, U.S.A.
Mass MovementLandslides,
Rockfalls, Debris Flows, avalanches and sinkholes.
1999 Rockfall
Newfoundland
Landslide destroyed road near Hwy 17 Scott’s Valley, CA
The Toll
Landslide deaths in the U.S. have been estimated at 25-50 people per year.
Every year around the world hundreds of people are killed by
mass wasting events (landslides, debris flows, and rockfalls).
Infrastructure/property damage is a global occurrence, but a large
portion of events are around the Pacific rim---that includes California.
Every year California suffers property damage and/or loss of life due to landslides. Landslides are California’s most common thus costliest natural disaster.
Before the Yungay Avalanche
After the Yungay Avalanche ~25,000 killed 1970 Peru
Process of Erosion Erosion: materials transported by wind, water, or gravity and
deposited somewhere else. With mass wasting we will be focusing
generally on gravity being the transportation force and water
(liquid or ice) facilitating the process.
Mass movement (also known as mass wasting) is the down slope movement of
earth materials under the influence of gravity. The detachment and movement of
earth materials occurs if the stress imposed is greater than the strength of the
material to hold it in place.
Landslides, rockfalls, debris flows and avalanches all have one thing in common,
they occur on slopes. The angle of the slope can vary greatly and in some cases
the slope angle can be very shallow.
Terms you need to know.
• Rockfall
• Rockslide
• Slump
• Debris Flow
• Creep
External triggers
• Earthquake shaking, especially in steep
topography, with saturated slopes, or in
saturated sand (liquefaction)
• Volcanic eruptions create piles of loose
material (e.g., ash), destroy vegetation,
melt snow/ice, cause mountain collapse
• Heavy rainfall and melting snow,
especially if continuous over days/weeks
Moisture and rate of motion
characteristics.
The slower events (creep) generally result only in property damage.
Why are there so
few landslides in
the Sierras?
A. There are few steep
slopes there.
B. The trees hold the land
in place.
C. Soil was removed by
glaciers.
D. There is no rain, only
snow.
Soil Formation
Rock chemically breaks down through the process called weathering.
California Coast Range--Factors that
create slope instability.
• Slope—Many steep slopes!
• Water—A lot of wet weather and storms
(especially in Northern California)!
• Geologic materials, Geologic structures, and
Weathering—Much of the rock is weak due to
being highly fractured, folded and faulted, and
weathered and some are just weak and slippery
(e.g. serpentinite). Soils or unconsolidated
sediments present on slopes.
• Vegetation?
Serpentinite mélange. Tectonically-rounded
blocks of serpentinite, surrounded by a
sheared-out, foliated paste of crushed
serpentinite.
Factors that determine slope stability
• Slope?
• Water?
• Geologic materials?
• Geologic structures?
• Weathering?
• Vegetation?
The California Coast Range (and coast) regularly experiences mass wasting events.
The majority are rotational slumps but debris flows are common as well.
Most mass wasting events in California are triggered by wet storms.
Driving force (gravity) > resisting strength of rock/regolith
Components of resisting strength? 1.Friction
2.Cohesion
Figure 15.13
View of the mudslide area in the town of La Conchita.
Shear strength is a measure if the resistance of earth materials to be moved.
Angle of repose is the steepest angle from horizontal that an unconsolidated
material can maintain. Grain shape strongly effects the angle as does size
distribution. The angle of repose of dry sand is around 33 degrees.
The material that the slope is composed of is important for several reasons:
-- plays a role in how steep of a slope can be maintained and also how high a cliff
can be.
-- the type of slope failures (mass wasting events) that may occur.
-- the magnitude of these events.
~33 degrees for dry sand
The material that the slope is composed of dictates:
-- how steep of a slope can be maintained.
-- how high a cliff can be.
-- the type of slope failures (mass wasting events) that occur.
Steep slopesvs. shallow slopesis controlled by the rock type.
Grand Canyon formation through the
Colorado River cutting down into the
bedrock. Canyon widening through mass
wasting events.
Consolidated materials:(tends to fall or slide)
• Cemented sediments = sedimentary rock.
• Crystalline materials: igneous, metamorphic, and some sedimentary rocks (evaporites).
Bedding Planes
Safer
Rock Discontinuities
Masses of rock often are full of
joints and fractures. columnar joints
Columnar joints
Devil’s Postpile, CA.
Napa area
Water and Water Ice
Why does ice float on liquid water?
Why is this important?
Water exerts pressureon particles and bedding surfaces which acts a lubricant on frictionalsurfaces.
Water and Water Ice
The density properties of
water are very unique and
have broad ranging
implications in geologic
and ecologic processes.
Mechanical Weathering:
Frost Wedging
Rockfalls
SF Telegraph Hill
1996 Happy Isles rockslide. Photo by David Walter
Happy Isles Rockfall
Granite Slide – Just outside Yosemite, CA
Talus and talus slopes
Talus is rock debris that is piled
up (generally at the angel of
repose) at the base of cliffs.
Talus slopes are rockfall deposits.
Talus cones on the north shore of Ifjorden, Svalbard, Norway.
TALUS SLOPES – ROCKY MOUNTAINS
TALUS SLOPES – ROCKY MOUNTAINS
TALUS SLOPES – DEATH VALLEY
HWY 1 Landslide California Rockslide
Rockslide/Landslide
Ferguson Slide on California State Highway 140
Massive landslide deposit.
A massive landslide scar and deposit.
A massive landslide scar and deposit.
Home destroyed by the April 1998 El Niño-triggered Anzar Road landslide,
San Benito County, California, U.S.A. (Schuster et al., 1998).
Weakly or unconsolidated material:(will slide, slump and often flow)
Factors that determine slope stability
Water: a small amount
increases cohesion but a large
amount reduces friction;
weight of water increases
driving force too.
Water saturation of materials
on a slope is the primary
cause of landslides.
Earthquakes can cause
liquefaction of
unconsolidated materials thus
reducing the friction between
grains.
Roots can mechanically weather bodies of rock, but
roots often hold rocks and soil in place on a slope.
Vegetation often stabilizes slope. Slopes are very susceptible to failure after fires
remove vegetation. Failure generally occurs in the wet winter storms that follow
the fire season.
Paseo Del Mar,
San Pedro, CA
(Southern California)
November 2011
Slump
Parts of a earth slump
Large landslide along
the coast between Fort
Funston and John Daly
Blvd. (Daly City)—what
are the parts?
Old coastal highway
scarp
previous cliff toptoe?
body
Fort FunstonSan Andreas fault
LANDSLIDE – TENESSEE VALLEY BEACH – MARIN, CA
Slumping and Scarps in Dorset, England
The head of a rotational slip in the Black Hills of North Dakota. (Source: Image
provided by the National Geophysical Data Center, NOAA).