Powerpoint Presentation Physical Geology, 10e - PowerPoint PPT Presentation

1 / 47
About This Presentation

Powerpoint Presentation Physical Geology, 10e


Isolated thunderstorm rained and hailed, eroded soil around 200 ton boulder in ... acoustical energy propagates as internal waves, fluidizing rock debris (acoustic ... – PowerPoint PPT presentation

Number of Views:142
Avg rating:5.0/5.0
Slides: 48
Provided by: stevek93


Transcript and Presenter's Notes

Title: Powerpoint Presentation Physical Geology, 10e

Mass Movements Natural Disasters, 5th edition,
Chapter 9
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

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.

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

  • 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

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

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)

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

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

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

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)

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

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
  • Anything that floats in water will float even
    more easily in quicksand does not suck objects

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
  • 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

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

Adverse Geologic Structures
  • Weaknesses due to pre-existing geologic
  • Ancient slide surfaces sliding creates a smooth,
    slick layer of ground-up materials that can
    easily slide over and over again, especially when
  • 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

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

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

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

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

  • 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
  • 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

  • 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

  • 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

  • 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
  • Point Fermin, California, 1929
  • Sandstone block on clay layer slid seaward, with
    no resisting mass

  • Translational Slide Vaiont, Italy, 1963
  • Fractured rock layers dip toward valley on both
  • 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

  • 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
  • Heavy snowmelt overtopped dam and caused flood

  • Translational Slide Turnagain Heights,
    Anchorage, Alaska, 1964
  • Magnitude 9.2 earthquake triggered numerous mass
  • Rocks composed of glacially ground, clay-rich
  • 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

  • Mass movements that behave like fluids internal
    movements dominate, slip surfaces absent or
  • 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

  • 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
  • Unstable land used for farming until residential
    development built in 1950s

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

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

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
  • Shattered, flowed 3 km across valley, 130 m up
    opposite side
  • Buried southern end of town, killing about 70
  • 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

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

Movement of Highly Fluidized Rock Flows
  • 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
  • Elm sturzstrom was in contact with ground
  • Identical flow features on ocean floor, Moon,
    Mars (no atmosphere)

Movement of Highly Fluidized Rock Flows
  • 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)

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

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

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

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

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

  • 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
  • Removal of fluid volume and decrease in
    pore-fluid pressure compacts rock, lowering
    ground above

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

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

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

Long-Term Subsidence, Venice, Italy
  • Venice is built on soft sediments that compact
    under weight of city itself, as global sea level
  • 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
  • More sediment to raise ground level
  • Pump seawater or carbon dioxide into sand below
    city to pump up region

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

How To Create a Cave
  • Caves usually occur in limestone
  • Equilibrium equation to create or dissolve
  • 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
Write a Comment
User Comments (0)
About PowerShow.com