Chapter 7
Volcano Case Histories:
Killer Events
Lecture PowerPoint
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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
Volcanism at Spreading Centers
Iceland – Lava Flows of 1973
• Vestmannaeyjar fishing port,
harbor built by volcanism
• Fissure opened 1 km from town,
emitted 230 million m3 lava and
26 million km3 pyroclastic
material, increased size of island
by 20%
• Destroyed 370 buildings and
began to fill in harbor
Figure 7.2
Volcanism at Spreading Centers
Iceland – Lava Flows of 1973
• To divert flows, Icelanders bulldozed lava and ash into barriers,
sprayed flows with seawater to harden into walls
• Used heat from flows to heat town
Figure 7.3
Figure 7.4
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)
• Destroyed Iceland’s longest bridge,
telephone lines and roads
Volcanism at Subduction Zones
• Most of world’s 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
Volcanism at Subduction Zones
Cascade Range, Pacific
Coast United States
and Canada
• Subduction of Juan de Fuca
plate under Pacific
Northwest is responsible for
Cascade Range volcanoes
Figure 7.5
Volcanism at Subduction Zones
Cascade Range, Pacific Coast United States and Canada
• Subduction
– 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
Figure 7.6
Volcanism at Subduction Zones
Cascade Range, Pacific Coast United States and Canada
• Eruption frequency: Mt. St. Helens
Figure 7.7
Volcanism at Subduction Zones
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
Figure 7.10
Volcanism at Subduction Zones
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)
Figure 7.11
Volcanism at Subduction Zones
Mount St. Helens,
Washington, 1980
• Landslide released pressure on
magma and blast followed 
pyroclastic flows
Figure 7.13
Figure 7.11
Volcanism at Subduction Zones
Mount St. Helens, Washington, 1980
• Blast opened throat of volcano for vertical column eruption
(Plinian phase) lasting nine hours
• Continued pyroclastic flows and ash falls from eruption cloud
Figure 7.11
Volcanism at Subduction Zones
Mount St. Helens, Washington,
1980
• Building of lava dome continues
today
Figure 7.11
Figure 7.14
Volcanism at Subduction Zones
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: eruptions
each May (probably
initiated by melting of
snow) of ash clouds
and collapse of
overtopped lava dome
 pyroclastic flows
and lahars
Figure 7.17
Volcanism at Subduction Zones
Mount Shasta, California
• Active volcano – 11 eruptions in 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
settlements persist for centuries
Figure 7.19
Killer Events and Processes
Figure 7.22
Killer Events and Processes
Historic Record of Volcano Fatalities
• About 275,000 people killed during last 500 years
by about a dozen volcanic processes
Figure 7.23
Killer Events and Processes
Pyroclastic Flows
• Superhot, high speed
turbulent cloud of ash, gas
and air – can kill thousands
of people in one event
Figure 7.24
Killer Events and Processes
Mount Mayon,
Philippines, 1984
• 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
Figure 7.25
Killer Events and Processes
El Chichon, Mexico, 1982
• Dormant for 550 years, then month of earthquakes led up to sixhour-long Plinian eruption, followed by five days of inactivity
• Pyroclastic surge flowing radially outward from volcano
– Overran nine villages, killed 2,000 people
• Plinian column up to 20 km high, two more pyroclastic surges
• Change in global climate
Figure 7.26
Killer Events and Processes
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 1994
• 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, Maurice and Katya
Krafft
Killer Events and Processes
Mount Pelee, Martinique,
1902
• Most pyroclastic flows from
hot ash and gas overspilling
crater
• Few deadliest pyroclastic
flows generated by directed
blasts
Figure 7.27
Killer Events and Processes
Mount Pelee, Martinique, 1902
• 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
Figure 7.28
Killer Events and Processes
Krakatau, Indonesia, 1883
• Blast on August 27 generated pyroclastic flow across sea
surface of Sunda Straits to Katimbang on Sumatra
• Killed more than 2,000 people on Sumatra
Killer Events and Processes
Tsunami
• 1883 Krakatau caldera collapse killed more than 36,000
people
– Less than 10% directly killed by eruption
– More than 90% killed by volcano-caused tsunami
Mount Unzen, Japan, 1792
• Earthquake triggered collapse of lava dome, sending
avalanche/pyroclastic flow to ocean, creating tsunami
that killed 15,000 people
Killer Events and Processes
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
Killer Events and Processes
Nevada Del Ruiz, Colombia, 1985:
• Very high volcano is topped by large ice cap
• Initial Plinian eruption sent pyroclastic debris falling down onto ice
cap, melting ice, creating lahars down mountain, killing 1,800
• Later eruptions melted more ice, creating 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)
Figure 7.29
Killer Events and Processes
Mount Rainier, Washington –
On Alert:
• Considered very dangerous by
volcanologists because of:
Figure 7.32
– 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
Killer Events and Processes
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
Killer Events and Processes
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% directly by eruption
– 90% from famine or disease, after pyroclastic fallout damaged
crops
• Affected global climate so that 1816 was known as “the
year without a summer”
Side Note:
Death at Ashfall, Nebraska
• 10 million years ago: savanna waterholes, migrating wildlife
• Eruption at Yellowstone 1,300 km away blanketed Nebraska with
0.3 m thick layer of volcanic ash, dumped by wind 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
Figure 7.34
Killer Events and Processes
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
Figure 7.35
Killer Events and Processes
Gas
Killer Lakes of Cameroon, Africa
• 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
• Lake water is stratified, with densest water at bottom absorbing
leaked carbon dioxide and trapping it there
Killer Events and Processes
Killer Lakes of Cameroon, Africa
• 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, more is continually being added
– 20 years for lake water to become oversaturated in CO2 again
– Degassing pipes installed to allow high-pressure gas to escape
before building up to explosive levels
Figure 7.36
Killer Events and Processes
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
VEIs of Some Killer Eruptions
• Does energy of eruption correlate to number of
fatalities?
– VEI is semi-quantitative estimate of magnitude of
volcanic eruption, using volume erupted and eruptioncolumn 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
VEIs of Some Killer Eruptions
Insert Table 7.2
Toba
-May have nearly driven human race to extinction
due to climate change
-Eruption equivalent to 560 Mount Pinatubo
eruptions
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 ~200 years ago
Figure 7.38
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
Figure 7.40
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”
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 by U.S. and Philippine
scientists began
Volcano Monitoring and Warning
Mount Pinatubo, Philippines, 1991
• June 7
– Degassed magma reached surface, formed lava dome
• June 12 (Philippine Independence Day)
– 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) arrived and washed volcanic debris
downslope as lahars
Volcano Monitoring and Warning
Mount Pinatubo, Philippines, 1991
• Assessment:
– 300 people killed but millions moved out of harm’s way
(20,000 estimated deaths without evacuation)
– $500 million property saved (include military aircraft)
Figure 7.41
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
Volcano Monitoring and Warning
Ground Deformation
• Ground surface rises and falls in response to magma movement
• Measured with tilt meters, strain meters, 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 killed trees
– 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
Volcano Monitoring and Warning
Volcano Observatories
• In 20th century, U.S. had powerful eruptions in Alaska, California,
Hawaii and Washington
• At least 65 active or potentially active volcanoes in U.S.
• U.S. Geological Survey established Volcano Hazards Program with
five volcano observatories:
– Alaska, Cascades, Hawaiian, Long Valley and Yellowstone
– Each with own websites to report current activity
Insert Table 7.3
End of Chapter 7