Download UNIT 8 (Chapter 11) SLOPE SYSTEMS Study Guide

UNIT 8 SLOPE SYSTEMS (Chapter 11 - Mass Wasting) Study Guide
(Revised 7/12)
UNIT 8 HOMEWORK
WEB HIT HOMEWORK - part 1: ONE WRITTEN PARAGRAPH from any selected unit web hit site
VIDEO WEB HIT HOMEWORK – part 2: ONE WRITTEN PARAGRAPH from any selected unit video
site
For any Unit Web Hits and Unit Web Videos, go to the “DMC HOME” website; in Search box –type “Geology”,
select “Vernon Kramer”, scroll down to GEOL 1303, select “Syllabus”, select “Web Hit Links”, later then the select
“Video Web Hits”, click on icon of interest for web sites
OR: go to DMC Home website, select “Degrees, Certificates, Courses”, scroll down to Natural Sciences and select
“Geology”, select “Faculty Listings”, select “Walter Vernon Kramer”, find “Geol 1303”, select “Syllabus”, and there
you can find the” web hit links” click on icon of interest for web sites and then to “Web video hits”, click on icon of
interest for video site
[IF NONE OF THE WEB SITES COME UP, YOUR COMPUTER PROBABLY NEEDS TO BE REBOOTED (RESTARTED)
Generalities
- Most of the Earth’s surface has some degree of a slope (ground having an inclined surface).
- Slopes are created as the result of erosional processes, igneous activity or tectonic activity.
- Slope systems can also be active within meteor craters, even on other planets.
- Gravity is the main energy force affecting slope systems.
Slope System Terms
Mass movement (slope failure): This is the movement of regolith and masses of rock downhill. Mass
movement is a major geologic process operating on all slopes. The associated movement speeds can
vary from millimeters per year to more than 200 mph.
Talus: Rock and soil fragments that accumulate in a pile at the base of a ridge or cliff
Talus slope: An inclined surface (hillside or mountain side) of talus
Angle of repose: The steepest angle at which loose grains will remain stable without sliding
downslope, generally 30° for dry sand. Everything that is stacked has an angle of repose.
Gravity can move material downslope only when the force of gravity exceeds the cohesive strength of
the surface material (see below for the factors affecting cohesion).
Cohesive force: The resistance of an object to move downhill.
Factors That Strengthen Cohesive Force (The factors that strengthen the resistance of an object to
move downhill by an accumulation of forces.)
1) Friction (probably most important) can prevent rock and soil from moving downhill.
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2) Stickiness of particles can hold the soil grains together. “Moist” is the most sticky phase that can
hold sediments together. (Being too wet or too dry can reduce cohesive strength.)
3) Cementation of grains by calcite or silica deposition can solidify earth materials into strong rocks.
4) Roots can bind material together.
5) Others such as man-made walls or pavement can prevent some movement of material downhill.
- But at all slopes there are always “some” downhill movements.
Factors That Weaken Cohesive Strength (The factors that weaken the resistance of an object to
move downhill.)
1) Water content
2) Alternating expansion and contraction
3) Removal of vegetation
4) Undercutting
5) Overloading
6) Vibrations
1) High water content can weaken cohesion because abundant water both lubricates (overcoming
friction) and adds weight to a mass.
2) Alternating expansion by wetting and contraction by drying of water reduces strength of cohesion,
just like alternating expansion by freezing and contraction by thawing.
- This repeated expansion perpendicular to the surface and contraction vertically by gravity overcomes
cohesion resulting with the rock and sediment moving slowly downhill. (Always on exams)
3) Removing vegetation whose roots can no long help bind sediments together reduces cohesion.
4) Undercutting by rivers, streams, as well as massive amounts of animal burrows, can severely tax the
cohesive strength of a slope.
5) Overloading or the addition of too much weight can exceed the cohesive strength of a sediment,
often with undesired consequences.
6. Vibrations from earthquakes, sonic booms, explosions and highway traffic have created vibrations
that have overcome cohesion and caused mass movement.
If you think education is expensive, try ignorance. Derek Bok
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Profiles of a Slope
A slope in equilibrium generally has flat lying bedrock, such as hard limestone that will form gentile
cliffs and softer shales that form gentle slopes.
Non-equilibrium slope profiles result from rocks eroding a vastly different speeds; this profile often
forms dangerous cliffs.
equilibrium
non-equilibrium
Slope gradient (steepness of a hill) is the most important factor: The steeper the gradient, the faster
that mass movement can take place.
There are dozens of types of classifications for the types of mass movements (example page 280)
Types of Mass Movement: (Classification that we will use for this class is somewhat different from
that of our textbook)
1) Creep
2) Debris flows (various types)
3) Landslides
4) Avalanches (snow and ice)
- One type of mass movement can change over to another type of mass movement, such as creep
movement becoming a debris flow.
Movement by Creep
Creep: Extremely slow, almost imperceptible downslope movement of soil and rock debris.
- This includes slow movement of large rock blocks with wet/dry cycles and freeze/thaw cycles
propelling this movement.
- Creep properties:
1) Tilted posts and curved fence lines
2) Bulges or wavelike swells in soil
3) Curved tree trunks
4) “Bending” of steep-dipping rocks
5) Deformed roadways and power-line poles
soil creep
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- Swelling and drying clays can cause creep, resulting with cracks in walls of houses, foundations,
driveways, rock fences, etc.
- South Texas has large areas of “expansive clay” that can considerably shrink during dry periods.
- Signs of foundation problems (with swelling or contraction) include large interior wall cracks, large
outside cracks in bricks as well as cracks in sidewalks and driveways.
- In the Corpus Christi area, killer bees have been known to use wall cracks to enter a house.
- Water lines and gas lines also can break as the result of wetting or drying clays.
- Creep is a slow moving process that usually affects only the land’s upper surface, generally less than
ten feet of depth.
- Solifluction: Movement down slope where soil is saturated with water; common in permafrost areas
like Alaska.
-
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Debris Flows
- Debris flows represent a very rapid system that can carry rocky and soil materials long distances.
debris flow
Types of Debris Flows: (As defined by regolith content)
1) Debris flow: Mixture of all types of regolith
2) Debris flow: Lahars; mixture of mostly volcanic rocks and muds
3) Debris flow: Mudflow; mostly silt and clay (mud)
4) Debris flow: Boulder fields; angular and rounded rocks and boulders
5) Debris flow: Rock glaciers; massive quantities of rocks near glacier terrains
“Mixture” Type of Debris Flow
Debris flow: A mixture of mud, water and rock fragments (regolith) of all sizes that flow downslope as
viscous, almost solid liquid
- Water (and sometimes air trapped beneath the flow) can lubricate the debris flow.
- Debris flows can be highly destructive in heavily populated areas.
- Thickness of regolith that creates debris flows can be from inches to tens of feet.
- Debris flows can range from small regolith movements to the movement of an entire mountainside.
- The Slumgullion Debris Flow in SW Colorado was discussed. San Cristobal Lake was created by this
debris flows (the debris acts as a dam).
- Largest debris flow in US is the Blackhawk Slide, CA that reached speeds of 270 mph.
Volcanoes and Lahars (Debris flows)
Lahars: A volcanic debris flow composed mostly of volcanic ash and lava flows
- Lahars can be composed mostly of mud or a mixture of rocks and muds.
- Water source for lubricating lahars include melting glaciers, lakes, rains on volcanoes.
- Lahars can move with speeds up to 90 mph.
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- Lahars are more common with volcanoes that are found along tectonic convergent zones.
- Lahars are responsible for deaths of tens of thousands of people each century.
- An example was given for Amero, Columbia, South America for a major lahar tragedy.
- Mt. Rainier of Washington State is famous for its lahars. Communities near Mt. Rainier often
practice “lahar drills”, for safety. Keep in mind that Mt. Rainier could send lahars upon Tacoma,
WA. After all, Tacoma is built on top of many ancient lahars.
Lahar zones near Tacoma WA
Mudflow (Debris Flow)
Mudflow: A flowing mixture of mud and silt with water content up to 30% that follows a stream bed.
- When a mudflow stops, the consistency of the mud rapidly becomes hard (because of its low water
content).
- Mudflows can travel up to 50 mph and can form deposits more than 300 feet thick.
Other Debris Flows (Boulder Fields and Rock Glaciers)
Boulder fields: Long, relatively narrow masses of angular rock debris that moves very slowly
Rock glacier: Long, tongue-like masses of angular rock debris that resemble glacier in outline and
moves at rates of inches to feet per year
Education is not the filling of a pail but the lighting of a fire – W. B. Yates
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rock glacier
Landslides (a vague term applied to almost any type of slope failure)
True landslides involve movement along defined slippage planes.
- “Landslide blocks” move as a single detached unit or a series of detached units, along a definite
fracture plane.
landslides and slumps
Types of Landslides:
1) Rockfall
2) Rockslides
3) Slumps
4) Subaqueous
Rockfall Landslides
Rockfall: Freefall of fragment ranging from a single grain to a huge block. Rockfalls make up most of
the talus slopes. Rockfalls are also common along some highways. We saw examples of dangerous
rock falls that entered houses
rock falls
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Rock Slide Landslides
Rockslide: Rapid movement of a large block of rock along a bedding plane, joint or plane of structural
weakness.
rock slide
- Rockslides may begin movement as a sliding single block or may disintegrate while moving
downslope and create a debris flow. Example given for Franks, Alberta, Canada (1903)
- Rockslides are also common along many highways.
- Rockslides are responsible for many major tragedies including the Vaiont Dam landslide in Italy.
- Ignoring local geology can cost money and many lives.
Slump Landslides
Slump: A mass-movement “block” that leaves a distinct curved (spoon-shaped) incision. (On exams)
- A hillside that has suffered a slump commonly displays a series of “steps” or terraces that tilt
backwards.
slumps
- The lower part of a slump may move as a debris flow with speeds that vary from very slow to very
fast.
- Central Texas has a belt of low to moderate landslide hazards.
- Showed an example for La Conchita, CA as an example for dangerous landslides (slumps)
- Earthquake vibrations can initiate landslides and can cause more damage than the earthquake itself.
Subaqueous Slumps
Subaqueous slumps are mass movements that slide into the water (lakes or oceans).
- Subaqueous slumps are usually created by wave erosion; we have many local examples.
- Tsunamis can be created by subaqueous slumps.
- Largest slumps usually occur in alongside oceanic, volcanic islands.
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subaqueous slumps Hawaii
- Hawaii has had 15 major slumps. These major slumps occur every 100,000 – 200,000 years; and
could create tsunamis that could be 1,000 feet high. (common exam question about this types of
event)
- A minor movement of a large block of rock in Hawaii occurred in the year 2000.
- A potential major slide could occur from La Palma, Canary Islands, off the coast of Africa and
endanger the entire North Atlantic with a large tsunami.
Factors that can create landslides
1) Water and clay can get into fractures which then can “lubricate” a fracture between the bedrock and
the adjacent rock, resulting in a rockfall.
2) Freezing and thawing of water in the cracks can widen fractures to the point of slope failure.
3) A tree root can be growing in the fracture and finally “push” the rock over.
4) Small rocks eroding under the larger rock could have undercut the larger fractured rocks, resulting
in slope failure.
5) Vibrations from the earthquakes and traffic can weaken the cohesive fragments holding the rock in
the cliff.
6) Dissolution by groundwater or karsting (collapsing caves) could be included here.
Avalanche: A large flowing mass of ice, snow and sometimes rock, that can reach speeds of 350 mph.
- Heat is generated between ice particles during an avalanche. Ice particle boundaries will be partially
melted during the freefall. When the movement stops, the ice instantly freezes into a rock-hard, solid
mass.
avalanche into a house
- Avalanche safety is a major concern to all ski lodges and highway departments.
- From 25 - 50 people a year (US) die from avalanches.
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Preventing Mass Movements
- Methods to contain mass movement are numerous. These would include:
1. Planting vegetation (but this is subject to burning)
2. Wall with drain pipes into the hillside to prevent water-pressure buildup
3. Cutting benches (stair-step fashion) into the hillsides rather than just one slope
4. Using woven baskets or rocks and/or steel walls alongside slopes
5. Roof bolts with “wire netting” over an exposure of loose rocks (long bolts drilled into the rocks that
holds the wire fencing)
benching
walls
rock bolts/wire
wire baskets/ rocks
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