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Volcano Case Histories Killer Events Pat
Abbott, Natural Disasters, 5th edition, Chapter 8
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Volcanism at Spreading Centers

• Rifting at spreading centers worldwide creates 20
  km3 of new oceanic crust each year
• Iceland
• Volcanic plateau built of basaltic lava erupted
  from hot spot underlying mid-Atlantic Ocean
  spreading center
• 13 covered by glaciers
• 33 covered by active volcanoes
• Volcanic eruptions (mostly peaceful fissure
  eruptions) occur about every 5 years

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Volcanism at Spreading Centers

• Iceland Lava Flows of 1973
• Vestmannaeyjar fishing port, harbor built by
  volcanism
• Fissure opened 1 km from town and emitted 230
  million m3 lava and 26 million km3 pyroclastic
  material, increasing size of island by 20
• Destroyed 370 buildings and began to fill in
  harbor
• Icelanders bulldozed lava and ash into barriers
  to divert lava flow, and sprayed flows with
  seawater to cool and harden into walls that
  diverted further flows
• Used heat from flows to heat town

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Volcanism at Spreading Centers

• Iceland Jokulhlaup of 1996
• Fissures opened under glacier, melted 600 m of
  ice
• Meltwater accumulating in volcano crater flooded
  out suddenly as jokulhlaup flood (45,000 m3/sec)

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Volcanism at Subduction Zones

• Most of worlds famous volcanoes are subduction
  zone volcanoes
• Many regions around subduction zone volcanoes are
  heavily populated
• Erupt directly into atmosphere (not underwater)
  so have direct impact on worldwide climate

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Cascade Range, Pacific Coast United States and
Canada

• Subduction under Pacific Northwest is responsible
  for Cascade Range volcanoes
• Upper mantle (asthenosphere) is melted (aided by
  water released from sediments on top of
  subducting plate) and mixes with melted crustal
  rock into rising andesitic magma
• Some magma cools at depth into plutonic rocks
• Some magma erupts explosively at surface

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Cascade Range, Pacific Coast United States and
Canada

• Eruption frequency Mt. St. Helens
• Every century or so for last 2500 years
• 1975 study predicted eruption before 2000
• 1980 major eruption

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Mount St. Helens, Washington, 1980

• Dozens of magnitude 3 earthquakes as magma pushed
  to surface and expanded out northern side
• Magnitude 5.1 earthquake occurred and triggered
  landslide of north side of mountain, traveling
  great distance as mudflows (lahars)
• Landslide released pressure on magma and blast
  followed ? pyroclastic flows
• Blast opened throat of volcano for vertical
  column eruption (Plinian phase) lasting nine
  hours
• Continued pyroclastic flows and ash fall from
  eruption cloud
• Building of lava dome continues today

9
Lassen Peak, California, 1914-1917

• Lassen Peak not a volcano itself but lava dome
  (one of largest known) in remnants of enormous
  Mt. Tehama
• Formed from lava too viscous to flow away so
  solidified as plug
• 1914-1917 three years of eruptions in May
  (probably initiated by melting of snow) of ash
  clouds and collapse of overtopped lava dome,
  creating pyroclastic flows and lahars

10
Mount Shasta, California

• Active volcano erupted 11 times in last 3,400
  years, last in 1786
• Lower slopes are broad and smooth
• Pyroclastic flows spread widely as they move
  downhill
• Settled with three towns and one large reservoir
• Risk is low when comparing eruption recurrences
  to lifetime of person or building, but once
  established settlements persist for centuries

11
Killer Events and Processes

• Historic Record of Volcano Fatalities
• About 275,000 people killed during last 500 years
  by about 12 processes

12
Pyroclastic Flows

• Superhot, high speed turbulent cloud of ash, gas
  and air can kill thousands of people in one
  event
• Mount Mayon, Philippines, 1968
• Vulcanian eruptions sent ash clouds 10 km high,
  collapsed into pyroclastic flows down mountain
• Columns are more likely to collapse and form
  pyroclastic flows when eruptions are weaker and
  eruption column is cooler

13
Pyroclastic Flows

• El Chichon, Mexico, 1982
• Dormant for 550 years, then month of earthquakes
  led up to six hour long Plinian eruption
• Followed by five days of inactivity
• Pyroclastic surge flowing radially outward from
  volcano in all directions
• Overran nine villages, killed 2,000 people
• Plinian column up to 20 km high
• Two more pyroclastic surges and Plinian columns
• Change in global climate

14
Pyroclastic Flows

• Mount Unzen, Japan, 1991
• Unique in steady magma supply and topography ?
  chunks of lava dome frequently break off creating
  pyroclastic flows
• 7,000 pyroclastic flows between 1991 and 1995
• Cities and towns near the volcano, and farming
  villages on lower slopes, evacuated when threat
  grew
• 44 observers killed in 1991 by larger than usual
  flow, including volcano photographers Kraffts

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Pyroclastic Flows

• Mount Pelee, Martinique, 1902-1903, 1929-1932
• Most pyroclastic flows from hot ash and gas
  overspilling crater
• Few deadliest pyroclastic flows generated by
  directed blasts
• 1902 Small pyroclastic flow killed 40 people and
  raised tension, but mayor of St. Pierre (town of
  25,000) used militia to prevent people from
  leaving before election
• Enormous nuee ardente (glowing cloud) enveloped
  town and killed all but two residents
• Area is fully settled again now

16
Tsunami

• Krakatau eruption and caldera collapse in 1883
  killed more than 36,000 people
• Less than 10 killed directly by volcanic
  eruptions
• More than 90 killed by volcano-caused tsunami
• Mt. Unzen, Japan, 1792 earthquake triggered
  collapse of lava dome, sending avalanche/pyroclast
  ic flow to ocean, creating tsunami that killed
  15,000 people

17
Lahars

• Kelut, Indonesia, 1586, 1919
• Pyroclastic material on slopes of volcano quickly
  decomposes to fertile soil in tropical climate,
  bringing people to live on slopes of volcano
• Water in summit crater lake is frequently forced
  out and downhill by intruding magma
• Water flowing downhill mixes with old pyroclastic
  debris from previous eruptions to form huge
  mudflows ? lahar (Indonesian term)
• Lahars can flow at velocities of 65 km/hr, tens
  of kilometers from volcano

18
Lahars

• Nevado del Ruiz, Colombia, 1985
• Very high volcano is topped by large ice cap
• Initial plinian column eruption sent pyroclastic
  debris falling down on to ice cap, melting ice
  and creating lahars flowing down mountain and
  killing 1,800
• Later eruptions melted more ice, creating bigger
  and bigger lahars, finally reaching town of
  Armero (27,000 residents), killing 22,000 people
• Lahars were repeat of 1845 events, when 1,000
  people were killed (fewer residents)

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Lahars

• Mount Rainier, Washington
• On Alert
• Considered very dangerous by volcanologists
  because of
• Great height
• Extensive glacial cap
• Frequent earthquakes
• Active hot-water spring systems
• Mountain may fail in massive avalanche and/or
  melted ice may cause floods or lahars, even
  without eruption
• Osceola mudflow, 5,600 years ago, spread more
  than 120 km from mountain and over enormous area,
  now densely populated

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Indirect Famine

• Laki, Iceland Fissure Eruption of 1783
• Greatest lava eruption of historic times
• Fissure eruption with lava flow of 5,000 m3/sec
  (1/3 volume of Mississippi River) over 50 days
• Accompanied by enormous volume of gases
• Haze of SO2 and fluorine killed Iceland livestock
  ? about 20 of population died of famine

21
Indirect Famine

• Tambora, Indonesia, 1815
• Most violent and explosive eruption of last 200
  years
• Two extremely violent Plinian eruptions tore open
  the volcano so that 50 km3 of magma erupted in
  pyroclastic flows over one week
• Reduced elevation of mountain from 4,000 m to
  2,650 m
• Created 6 km wide, 1 km deep caldera
• Eruption caused 117,000 deaths
• 10 by eruption
• 90 from famine or disease, after pyroclastic
  fallout damaged crops

22
Death at Ashfall, Nebraska

• 10 million years ago waterholes in grassland
  savanna, migrating wildlife
• Eruption at Yellowstone 1,300 km away blanketed
  Nebraska with 0.3 m thick layer of volcanic ash,
  reworked by wind and dumped in waterholes
• Volcanic ash is tiny, sharp pieces of glass and
  rock, dangerous to inhale ? deprived animals of
  adequate oxygen over long term
• Animals very well preserved in ash after death
  fossils excavated at Ashfall Fossil Beds
  Historical Park in Nebraska

23
Gas

• Killer Lakes of Cameroon, Africa
• East African Rift Valley failed rift with
  string of crater lakes
• Lake Nyos is young, high crater formed by
  explosion few hundred years ago, filled with rain
  water
• 1986 gigantic volume of gas burst out of Lake
  Nyos and swept down valleys, 50 m thick, up to 45
  mph
• Four villages overwhelmed by cloud of gas
• Residents lost consciousness only four awoke
• 1,700 people killed, 3,000 cattle died, all local
  wildlife died
• Gas was carbon dioxide
• Dose determines the poison
• Too much carbon dioxide killed fauna but did not
  affect flora
• Carbon dioxide had leaked up from basaltic magma
  underlying lake, part of Cameroon volcanic line
  of East African failed rift

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Gas

• Killer Lakes of Cameroon, Africa
• Lake water is stratified, with densest water at
  bottom absorbing leaked carbon dioxide and
  trapping it there
• Lake eventually became unstable (triggered by
  unknown disturbance, overturning of water
  layers), and CO2 burst out in huge bubble,
  flowing down mountainsides in dense cloud along
  ground
• About 1/3 of gas was left in lake and more is
  continually being added
• 20 years for lake water to become oversaturated
  in CO2 again
• Pump system could be installed to degas lake water

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Lava Flows

• Nyiragongo, Zaire, 2002
• Some stratovolcanoes like Nyiragongo of East
  African Rift Valley have lava lakes in summit
  crater
• In 2002, lava with exceptionally low viscosity
  flowed very fast down volcano slopes
• Killed 45 people living on the mountain
• Flowed through city of Goma (500,000 residents
  plus Rwanda civil war refugees), destroying
  buildings and forcing evacuation

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VEIs of Some Killer Eruptions

• Does energy of eruption correlate to number of
  fatalities?
• VEI (volcanic explosivity index) is
  semi-quantitative estimate of magnitude of
  volcanic eruption, using volume erupted and
  eruption-column height
• Some deadly events have low VEI
• Frequency of different VEI magnitudes is inverse
  correlation
• Bigger eruption, less frequent occurrence
• Smaller eruption, more frequent occurrence
• As population grows, more people live in volcano
  hazard zones

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Volcano Monitoring and Warning

• Long Valley, California, 1982
• Abundant crustal melting (no hot spot), including
  colossal eruption 760,000 years ago creating Long
  Valley caldera and erupting pyroclastic debris
  (Bishop Tuff) hundreds of meters thick
• Giant continental caldera
• Rare giant eruptions
• Frequent small eruptions
• Long Valley 600 years ago
• Mono Lake 150-250 years ago

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Volcano Monitoring and Warning

• Long Valley, California, 1982
• 1980 (few weeks after Mt. St. Helens eruption)
• Numerous earthquakes, including four magnitude 6
• 1982
• Resurgent dome rose 25 cm
• U.S. Geological Survey issued Notice of Potential
  Volcanic Hazard, lowest level alert
• House prices dropped 40, tourism diminished
• Residents extremely angry volcano did not erupt

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Volcano Monitoring and Warning

• Long Valley, California, early 1990s
• Trees began dying on Mammoth Mountain as CO2
  leaked from underlying magma into soil
• Small earthquakes resumed
• Ground surface began rising
• Volcanologists hesitant to release alert after
  false alarm of 1982
• Prepare for the worst, but hope for the best

30
Volcano Monitoring and Warning

• Mount Pinatubo, Philippines, 1991
• Volcano-warning success story
• Largest eruption in 20th century near populated
  area
• Nearly one million people (20,000 U.S. military)
  in danger zone
• After 500 years of quiet, magma moved toward
  surface
• Thousands of small earthquakes
• Three small steam-blast craters and SO2 gas
  emissions
• Intense monitoring program began

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Volcano Monitoring and Warning

• Mount Pinatubo, Philippines, 1991
• June 7
• Degassed magma reached surface, formed lava dome
• June 12
• Large explosive eruptions began
• Evacuation cleared everyone out and closed
  military base
• June 15
• Cataclysmic eruption
• More than 5 km3 magma and rock, up to 35 km in
  atmosphere
• Pyroclastic flows 200 m deep
• Typhoon (hurricane) blew in and washed volcanic
  debris downslope as lahars

32
Volcano Monitoring and Warning

• Mount Pinatubo, Philippines, 1991
• Assessment
• 300 people killed but millions moved out of
  harms way (20,000 estimated deaths without
  evacuation)
• 500 million property saved (include military
  aircraft)

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Volcano Monitoring and Warning

• Signs of Impending Eruption
• Several phenomena are being evaluated as signs of
  impending eruption
• Determine if reliable to justify evacuation
• Seismic Waves
• Magma rising toward surface causes rocks to
  break, sends off short-period seismic waves
• Magma rising through opened conduits sends off
  long-period seismic waves
• For two weeks before Mt. Pinatubo eruption, 400
  long-period events were recorded daily from 10 km
  deep magma moving into place for eruption

34
Volcano Monitoring and Warning

• Ground Deformation
• Ground surface rises and falls in response to
  magma movement
• Measured with tiltmeters, strainmeters,
  distance-meters, satellites
• Three Sisters volcanoes in Oregon
• Bulged upward 10 cm as about 21 million m3 magma
  rose
• Gas Measurements
• Magma approaching surface loses gas as pressure
  drops
• Mammoth Mountain, California
• CO2 from magma is killing trees, increasing worry
  about impending eruption
• Declining CO2 levels have relieved worry
• Galeras Volcano, Colombia
• Decrease in gas emissions relieved worry
• Volcano was plugged by sticky magma, gas pressure
  building
• Eruption killed seven volcanologists collecting
  data in crater