Powerpoint Presentation Physical Geology, 10e

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Mass Movements Natural Disasters, 5th edition,
Chapter 9
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Mass Movements

• El Cajon, California, 2000
• Isolated thunderstorm rained and hailed, eroded
  soil around 200 ton boulder in hillside so that
  it rolled free, eight hours later
• Crashed into house with noise louder than an
  earthquake while owners were on ski trip,
  destroying 60 of house

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Mass Movements

• ? Gravity induced disasters
• Catastrophic mass movements usually triggered by
  some other event, such as earthquake, volcanic
  eruption, major rainstorm
• Mass wasting or Mass Movement refers to the down
  slope movement of rock and soil by gravity.
• The geologic process that follows weathering.
• Slopes are always geologically unstable.
• Down slope movement may be fast or slow.
• Mass movements are a type of geologic hazard.

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The Role of Gravity

• Power behind agents of erosion rainfall, water
  flow, ice gliding, wind blowing, waves breaking
• Geologic time all slopes are inherently unstable
• Can measure pull of gravity, using trigonometry
  to measure downhill force

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Creep

• Slowest, most widespread form of slope failure
• Almost imperceptible downhill movement of soil
  and uppermost bedrock layers, usually seen by
  effect on objects deformed or leaning downhill

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Creep

• Occurs by swelling and shrinking of soil in
  response to
• Freezing and expanding of water in pores
• Absorption of water and expansion of clay
  minerals
• Heating by Sun and increase in volume
• Soil expands perpendicular to ground surface,
  shrinks straight downward in response to gravity

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External Causes of Slope Failures

• Typical Slump (landslide) mass whose center of
  gravity has moved downward and outward, with
  tear-away zone upslope and pile-up zone downslope
• External processes that increase likelihood of
  slope failure
• Steepening slope (fault movements)
• Removing support from low on slope (stream or
  wave erosion)
• Adding mass high on slope (sediment deposition)

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External Causes of Slope Failures

• Can define failure surface (bottom of landslide)
• Can divide material above failure surface into
  driving mass and resisting mass
• Slope is held in place by equilibrium between
  driving mass and resisting mass
• Humans cause landslides by adding mass high on
  slope (view lots), removing mass from base of
  slope (widen road, flatten building lot) or both
  simultaneously

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Internal Causes of Slope Failures

• Inherently Weak Materials
• Clays (most abundant of sedimentary minerals)
  form during chemical weathering of rocks
• Clay crystals are very small, shaped like books
• Chemical composition of clays can change ?
  altering strength, size and water content ?
  altering strength of rock

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Internal Causes of Slope Failures

• Quick Clays
• Most mobile of all deposits fine rock flour
  scoured by glaciers, deposited in seas and later
  exposed above water
• Weak solid loosely packed, house of cards
  structure held together by salt
• When exposed, fresh water dissolves salt and
  house of cards structure can collapse so that
  ground turns to liquid and flows away
• Common in Scandinavia and Canada

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Internal Causes of Slope Failures

• Water in Its Different Roles
• Weakens earth materials by
• Weight water is heavier than air that usually
  fills pore spaces of sedimentary rocks in slopes
• Interplay with clay minerals water is absorbed
  (internally) and adsorbed (externally) by clay
  minerals, decreasing their strength, because
  positive side of water molecule attaches easily
  to negatively charged clay surfaces
• Decreasing cohesion of rocks water flowing
  through rocks can dissolve minerals holding rock
  together (dissolved gypsum and clay cement of St.
  Francis dam in California, 1928)

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Internal Causes of Slope Failures

• Water in Its Different Roles
• Weakens earth materials by
• Subsurface erosion water flowing through rocks
  can physically erode away (remove) loose material
• Pressure in pores of rocks and sediments
  pressure on water in pore spaces of rocks
  increases with increasing weight of sediment
  piled on top of rocks, and if pore space water
  becomes over-pressurized, gives lift to
  overlying sediments making them unstable

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Internal Causes of Slope Failures

• Water in Its Different Roles
• Quicksand
• Occurs if sand grains are supersaturated with
  pressurized water
• Water flowing upward through sand can lift grains
  to cancel pull of gravity ? sand has no strength
  (no ability to hold weight)
• Water-pressurized sand on slope will flow away
• Water-pressurized sand in flat area is quicksand
• Behaves like high-viscosity liquid, denser than
  water
• Anything that floats in water will float even
  more easily in quicksand does not suck objects
  down

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Analysis of Slope Stability

• Coulomb-Terzaghi equation s s (p-hw) tan f
• s is shear resistance
• s is cohesion
• p is weight per unit area above slide surface
• hw is height of water column times weight of
  water
• tan f is tangent of angle of internal friction
  (slide surface)
• Strength of hillside comes from cohesion (how
  well it sticks together) plus the weight of all
  its materials under gravity
• Strength is offset by pore-water pressure and
  angle of slide surface
• Failure angle is low (near horizontal) for weak
  materials (clays) and high (near vertical) for
  strong materials (granite)
• If hw p, then shear resistance comes only from
  cohesion

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Decreases in Cohesion

• Rocks that are buried compress into smaller
  volumes
• Rocks that are later uplifted to the surface
  expand in volume, fracture and increase porosity
  ? reduces strength of rock

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Adverse Geologic Structures

• Weaknesses due to pre-existing geologic
  conditions
• Ancient slide surfaces sliding creates a smooth,
  slick layer of ground-up materials that can
  easily slide over and over again, especially when
  wet
• Orientation of layering in hillside can make it
  stronger or weaker
• Layers at flatter angle than hillside ?
  daylighted bedding allows slippage
• Layers at steeper angle than hillside ? difficult
  to slip

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Adverse Geologic Structures

• Rocks with weaknesses
• Not cemented together
• Clay layers
• Soft rock layer on strong layer
• Split apart by joints
• Ancient fault ? slide surface

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Triggers of Mass Movements

• Most failures have complex causes
• Slopes lose strength over time through numerous
  events and near-failures
• Underlying causes push slope to brink of failure
• Finally immediate cause triggers collapse
• Triggers could be heavy rains, earthquakes,
  thawing of frozen ground, construction projects
  and even sonic booms

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Classification of Mass Movements

• Speed of movement (extremely slow to extremely
  rapid) and water content (wet or dry)

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Falls

• Elevated rock mass separates along joint, bedding
  or weakness and falls downward through air in
  free fall until hitting the ground, bouncing and
  rolling

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Falls

• Yosemite National Park, California, 1996
• 162,000 ton mass of granite (in two pieces) slid
  and launched into air, fell 500 m before hitting
  valley floor and being pulverized into cloud of
  dust
• Blast knocked down 1,000 trees
• Magnitude 3 earthquake
• 50 acres covered with inch-thick layer of dust
• Vertical column of dust 1 km high
• One person killed by tree

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Slides

• Movement of block above failure surface
• Rotational slides
• Move downward and outward above curved slip
  surface, with movement rotational about an axis
  parallel to slope
• Head moves downward and rotates backward
• Toe moves upward on top of landscape
• Move short distances

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Slides

• Ensenada, Baja California, 1976
• Slump preceded by arcuate cracks in hillside
• Cracks widened and area slid slowly toward ocean,
  as residents evacuated
• Toe of slide lifted sea floor above sea level

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Slides

• Translational Slides
• Move on planar slip surface such as fault, joint,
  clay-rich layer
• Move as long as on downward-inclined surface, and
  driving mass exists
• Different behaviors
• Remain coherent as block
• Deform and disintegrate to form debris slide
• Underlying material fails so overlying material
  slides
• Point Fermin, California, 1929
• Sandstone block on clay layer slid seaward, with
  no resisting mass

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Slides

• Translational Slide Vaiont, Italy, 1963
• Fractured rock layers dip toward valley on both
  sides
• Rock layers have old slide surfaces, clay layers,
  limestone layers with caverns
• Water filling reservoir saturated rocks in toes
  of slopes and elevated pore-water pressures
• Heavy rains triggered landslide 1.8 km by 1.6
  km mass (240 million m3) slid at up to 30 m/sec
  into reservoir
• Block filled part of reservoir and displaced
  water to crash over dam and into towns at both
  ends of reservoir

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Slides

• Translational Slide Gros Ventre, Wyoming, 1925
• Sedimentary rock layers daylight into valley on
  both sides
• Weakened rocks include clay layers
• 38.2 million m3 mass slid down 640 m into valley,
  damming river
• Lake formed but seepage through landslide dam was
  greater than flow into lake ? apparently stable
  situation
• Heavy snowmelt overtopped dam and caused flood
  downstream

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Slides

• Translational Slide Turnagain Heights,
  Anchorage, Alaska, 1964
• Magnitude 9.2 earthquake triggered numerous mass
  movements
• Rocks composed of glacially ground, clay-rich
  sediments
• Sliding began after 90 seconds of shaking
  liquefied clays at depth
• Rotational slides trapped clay layer at depth so
  it deformed internally, moving block on top of it

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Flows

• Mass movements that behave like fluids internal
  movements dominate, slip surfaces absent or
  short-lived
• Range of
• All sizes of materials
• Wet to dry
• Barely moving to gt 200 mph
• Gradation from movement on slip surface, to no
  slip surface
• Many names loess flow, earthflow, mudflow,
  debris flow, debris avalanche

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Flows

• Loess Flow Gansu Province, China, 1920
• Large earthquake triggered rapid, dry flow of
  hills of loess, burying villages and killing
  200,000 people
• Earthflow Portuguese Bend, California, 1950s
• Rock layers dip seaward, contain clay, and ocean
  waves erode toe and keep ancient earthflow moving
  seaward
• Unstable land used for farming until residential
  development built in 1950s

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Long-Runout Debris Flows

• Most spectacular, complex movement massive rock
  falls that convert into highly fluid, rapid
  debris flows that travel far (up to 25 times
  vertical distance)
• Blackhawk Event, California, 17,000 years ago
• Huge rock fall in San Bernardino Mountains flowed
  out into Mojave Desert flowed 7.5 times farther
  than fell, at speeds estimated up to 120 km/hr

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Long-Runout Debris Flows

• Elm Event, Switzerland, 1881
• Farmers quarried slate from base of mountain
  until cracks opened up in hillside above
• Fall, jump, surge
• Mass of mountain began to disintegrate as it fell
• Hit floor of quarry and disintegrated completely
• Rebounded with huge jump out from ledge
• Shot out from mountainside, flowed 2,230 m into
  valley

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Long-Runout Debris Flows

• Turtle Mountain, Alberta, Canada, 1903
• 90 million ton mass of dipping limestone slid
  down daylighted bedding surface 3,000 feet into
  valley
• Shattered, flowed 3 km across valley, 130 m up
  opposite side
• Buried southern end of town, killing about 70
  people
• Nevados Huascaran Event, Peru, 1962
• No perceptible trigger
• Mass of glacial ice and rock fell ? 13 million m3
  debris flow
• Debris flowed up to 170 km/hr down river valleys,
  killing 4,000 people

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Long-Runout Debris Flows

• Nevados Huascaran Event, Peru, 1970
• 45 seconds of shaking from magnitude 7.7
  earthquake triggered fall
• 100 million m3 of granite, ice, glacial
  sediments, water
• Speeds up to 335 km/hr
• Sequence of events
• 400 to 900 m vertical fall
• Mass landed on glacier and slid along surface
• Raced up side of hill, launched debris into air
• Boulders rained down on houses, people, animals
• Flow (up to 335 km/hr) buried Yungay (18,000
  people) in 30 m of debris
• Swept across Rio Santa and 83 m up opposite
  slope, buried Matacoto

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Movement of Highly Fluidized Rock Flows
(Sturzstroms)

• Hypotheses for fast and far movement
• Water provides lubrication and fluidlike flow
• Some observed flows were dry
• Steam liquefies and fluidizes moving mass
• Frictional melting fluidizes moving mass
• Some deposits contain blocks of ice, lichen ? no
  significant heat or friction
• Falling mass traps air beneath and rides trapped
  air
• Elm sturzstrom was in contact with ground
• Identical flow features on ocean floor, Moon,
  Mars (no atmosphere)

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Movement of Highly Fluidized Rock Flows
(Sturzstroms)

• Most likely hypothesis for fast and far movement
• Blocks in moving mass hit blocks in front of
  them, imparting kinetic energy ? vibrational or
  acoustical energy propagates as internal waves,
  fluidizing rock debris (acoustic fluidization)

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Snow Avalanches

• Behave like earth mass movements creep, fall,
  slide, flow
• Small to large, barely moving to 370 km/hr, few
  meters to several kilometers
• Small avalanches typically fail at one steep
  point, in loose, powdery snow, which triggers
  more and more snow moving downhill
• Usually begin when snow reaches 0.5 to 1.5 m deep
• Snow depth can reach 2 to 5 m before big
  avalanches occur, if snowflakes become rounded
  and packed

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Snow Avalanches

• Snow depth can reach 2 to 5 m before big
  avalanches occur, if snowflakes become rounded
  and packed
• Large avalanches are slabs of snow that break
  free from base like translational slides, turning
  into flows on way down
• Snow mass composed of layers with different ice,
  snow characteristics ? different strength
• Numerous potential failure surfaces
• Dry snow forms faster avalanches than wet snow

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Submarine Mass Movements

• Same mass movements occur below sea rotational
  slumps in delta deposits complex failures at
  subduction zones debris flows down submarine
  volcano slopes

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Submarine Mass Movements

• Hawaii in the Pacific Ocean
• Largest submarine mass movements, covering more
  than five times land area of islands
• Catastrophic flank collapses side of volcano
  breaks off and falls into sea (70 in lasts 20
  million years)
• Create tsunami which ravage Hawaii and affect
  entire Pacific Ocean basin
• Large block at Kilauea (active volcano on Big
  Island) is moving up to 6 cm/day

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Submarine Mass Movements

• The Canary Islands in the Atlantic Ocean
• Three of the Canary Islands have had major
  flank collapses (Tenerife, La Palma, Hierro
  15,000 years ago)
• Next collapse could create powerful tsunami to
  hit west coasts of Africa and Europe and east
  coasts of North and South America

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Subsidence

• Ground surface sags gently or drops
  catastrophically as voids in rocks close
• Slow compaction of loose, water-saturated
  sediments or rapid collapse into caves
• Slow Subsidence
• Ground surface slowly sinks as fluids (water or
  oil) are removed below surface (squeezed out or
  pumped)
• Removal of fluid volume and decrease in
  pore-fluid pressure compacts rock, lowering
  ground above

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Delta Compaction, Mississippi River, Louisiana

• Delta loose pile of water-saturated sand and mud
  ? compacts and sinks down
• Mississippi River delta underlain by 6 km thick
  sediments deposited in last 20 million years
• Current river position constant for last 20,000
  years, but shifts frequently and held in place
  now by human action
• New Orleans and region sinking by sediment
  compaction, dewatering, isostatic adjustment
  about 45 of city below sea level, prone to
  high-water surges in hurricanes

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Oil Withdrawal, Houston-Galveston Region, Texas

• Pumping of water, gas, oil began in 1917
• Houston-Galveston relies on groundwater
  withdrawals
• Area has sunk up to 2.7 m, renewing movement on
  old faults that act as landslide surfaces

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Groundwater Withdrawal, Mexico City

• Extraction of groundwater through wells began in
  1846
• Withdrew water faster than it is replenished,
  causing land subsidence
• Groundwater withdrawal is now banned, but
  subsidence can not be reversed

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Long-Term Subsidence, Venice, Italy

• Venice is built on soft sediments that compact
  under weight of city itself, as global sea level
  rises
• Venetians have been building up islands with
  imported sand for centuries
• 20th century pumping of groundwater ? rate of sea
  level rise in Venice doubled
• Sea level projected to rise 50 cm in 21st century
• Movable floodgates across entrances to lagoon
• Would disrupt shipping, prevent outward flow of
  contaminants
• More sediment to raise ground level
• Pump seawater or carbon dioxide into sand below
  city to pump up region

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Catastrophic Subsidence

• Limestone Sinkholes, Southeastern United States
• Limestone forms from CaCO3 shells of marine
  organisms, dissolves in naturally acidic
  groundwater flowing through ? forms extensive
  water-filled caverns
• When groundwater levels drop, caverns are empty
  and buoyant support of water holding up cavern
  roofs is removed ? roofs collapse, forming
  sinkholes

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How To Create a Cave

• Caves usually occur in limestone
• Equilibrium equation to create or dissolve
  limestone
• Ca 2HCO3 ?CaCO3 H2CO3
• Ca is calcium ion
• HCO3 is bicarbonate ion
• CaCO3 is calcite limestone
• H2CO3 is carbonic acid
• Left to right limestone is precipitated
• Right to left limestone is dissolved
• Controlled by amount of carbonic acid, which is
  controlled by amount of carbon dioxide