Ch 07 Volcano Case Histories: Killer Events · Volcanic Processes and Killer Events: Tsunami Volcano-generated tsunami have been responsible for 21% of volcano-caused deaths • 1883

7-1

Natural DisastersTenth Edition

Chapter 7

Volcano Case Histories:

Killer Events

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-2

Volcanism at Spreading Centers: Iceland (1 of 3)

• 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



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



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 7.1


Figure 7.2 Lava ignites buildings, Vestmannaeyjar, Iceland


Figure 7.3 Volcanism at Spreading Centers


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 (1 of 2)

Cascade Range, Pacific Coast United States and Canada

• Subduction of Juan de Fuca plate under Pacific Northwest is responsible for Cascade Range volcanoes


Volcanism at Subduction Zones: Cascade Range (2 of 2)

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 7.4


Figure 7.5


Volcanism at Subduction Zones: Eruption Frequency

• Eruption frequency: Mt. St. Helens

– Every century or so for last 2500 years

– 1975 study predicted eruption before 2000

– 1980 major eruption

– Photo left in 1974; Photo right in late 1980


Figure 7.6


Figure 7.7 Eruption Histories of Cascade Range Volcanoes


Volcanism at Subduction Zones: Mount St. Helens

Mount St. Helens, Washington, 1980

• Dozens of magnitude 3 earthquakes as magma pushed toward surface, expanding northern side outward

• Landslide occurred (recorded as 5.1 M earthquake)on 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 falls from eruption cloud


Figure 7.10 (a)


Figure 7.10 (b)


Figure 7.10 (c)


Figure 7.10 (d)


Figure 7.10 (e)


Figure 7.10 (f)


Figure 7.12


Figure 7.13

Mount St. Helens, Washington, 1980Building of lava dome continues today


Figure 7.11 Map of Materials After Mount Saint Helens Eruption


Volcanism at Subduction Zones: Lassen Peak

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 7.16


Figure 7.17 Mount Shasta, California


Volcanism at Subduction Zones: Mount Shasta

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 7.18


Figure 7.21 Eruption Volumes of Some Events


Figure 7.22 Volcanic Processes and Killer Events


Figure 7.23

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

a dozen volcanic processes


Table 7.1 Volcanic Causes of Deaths

275,000Deaths

530 VolcanicEvents

Pyroclastic flow 29% 15%

Tsunami 21% 5%

Lahar 15% 17%

Indirect (famine) 23% 5%

Gas 1% 4%

Lava flow <1% 4%

Pyroclastic fall (bombs)

2% 21%

Debris avalanche 2% 3%

Flood 1% 2%

Earthquake <1% 2%

Lightning <1% 1%

Unknown 7% 20%

Data Source: Simkin,T.,Siebert,L.,and Blong,R.,”Volcano Fatalities” in Science 291:255,2001


Volcanic Processes and Killer Events: Pyroclastic Flows

• In pyroclastic flows, the superhot, high speed turbulent cloud of ash, gas and air is deadly – they result in about 29% of volcanic deaths


Figure 7.24


Volcanic Processes and Killer Events: Eruption Column Collapse

Pyroclastic Flow 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 7.27


Volcanic Processes and Killer Events: Pyroclastic Surges

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

– Overran nine villages, killed 2,000 people

• Plinian column up to 20 km high, two more pyroclastic surges

• Eruption changed (cooled) global climate


Figure 7.28


Volcanic Processes and Killer Events: Dome Collapse

Dome Collapse: 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 famed volcano photographers, Maurice and Katya Krafft


Volcanic Processes and Killer Events: Mount Pelee (1 of 2)

Mount Pelee, Martinique, 1902

• Many pyroclastic flows occur from hot ash and gas overspilling crater rim

• Few deadliest pyroclastic flows generated by directed blasts


Volcanic Processes and Killer Events: Mount Pelee (2 of 2)

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 pyroclastic flow enveloped town, killing 29,000 people

• Area is fully settled again now


Figure 7.25


Figure 7.26


Volcanic Processes and Killer Events: Pyroclastic Flows Over Water

Krakatau, Indonesia, 1883

• Blast on August 27 generated pyroclastic flow that traveled across 40 km (25 mi) of sea surface in Sunda Straits to Katimbang on Sumatra – and was still hot enough to kill more than 2,000 people on Sumatra


Volcanic Processes and Killer Events: Tsunami

Volcano-generated tsunami have been responsible for 21% of volcano-caused deaths

• 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


Volcanic Processes and Killer Events: Lahars (1 of 2)

• Lahars are volcanic mudflows and volcanic debris flows that are fluid when moving, but then solidify

• Water flowing downhill mixes with old pyroclastic debris from previous eruptions to form huge mudflows

• Lahars can flow at velocities of 65 km/hr, tens of kilometers from volcano

• Lahars are responsible for about 15% of deaths by volcano

• May be primary events during volcanic eruptions or occur later due to addition of water such as from heavy rain or melting glacial ice


Volcanic Processes and Killer Events: Lahars (2 of 2)

Nevada Del Ruiz, Colombia, 1985:

• Very high volcano in the Andes Mountains has a large ice cap

• Initial Plinian eruption sent pyroclastic debris falling onto ice cap, melting ice, creating lahars flowing 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


Volcanic Processes and Killer Events: Mount Rainier

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


Figure 7.32


Volcanic Processes and Killer Events: Debris Avalanches

• Stratovolcanoes are riddled with fractures

• Hot water and gases chemically decompose volcanic rock over time

• Sector collapses flow downslope as debris avalanches, as at Mount Shasta in California

• Debris avalanches are responsible for 2% of volcano-caused deaths


Volcanic Processes and Killer Events: 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 human population died of famine


Volcanic Processes and Killer Events: Tambora

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 2,650m

– 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”


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


Figure 7.33 Volcanic ash

Tiny pieces of sharp glass that are easy to inhale but do great damage to respiratory systems, including causing death


Volcanic Processes and Killer Events: Gas (1 of 3)

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 in 1986

– Residents lost consciousness – only four awoke

– 1,700 people killed, 3,000 cattle died, all local wildlife died

• Gas was carbon dioxide

– Abundant 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




• 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


Figure 7.35


Figure 7.36


Figure 7.37

Degassing pipe allows high-pressure, gas-rich water to escape before building to a catastrophic volume.


Volcanic Processes and Killer Events: 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

– Lava flows are responsible for less than 1% of volcano deaths. Although they are frequent, the lava usually flows quite slowly.


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 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 involcano hazard zones


TABLE 7.2 VEIs of Notable Volcanic Disasters (Volcanic Explosivity Index)

VEI Volcano

8 Yellowstone, 600,000 years ago; Toba, 74,000 years ago

7 Tambora, 1815

6 Vesuvius, 79; Krakatau,1883; Pinatubo, 1991

5 St. Helens, 1980

4 Pelée, 1902

3 Nevado del Ruiz, 1985

2 -

1 -

0 Lake Nyos, 1986

Toba eruption almost drove humans into extinction


Volcano Monitoring and Warning: Long Valley (1 of 2)

Long Valley, California, 1982

• Abundant crustal melting (no hot spot), including colossal eruption 760,000 years ago, created Long Valley caldera and erupted pyroclastic debris (Bishop Tuff) 100’s of meters thick

• Giant continental caldera

– Rare giant eruptions

– Frequent small eruptions

▪ Long Valley 600 years ago

▪ Mono Lake ~200 years ago


Volcano Monitoring and Warning: Long Valley (2 of 2)

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.G.S. issued Notice of Potential Volcanic Hazard, lowest level alert

– House prices dropped 40%, tourism slowed

– Residents erupted – volcano did not erupt


Figure 7.38


Figure 7.40


Volcano Monitoring and Warning: Mount Pinatubo (1 of 3)

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

• Intense monitoring program by U.S. and Philippine scientists began




• June 7

– Degassed magma reached surface, formed lava dome

• June 12 (Philippine Independence Day)

– Large explosive eruptions began

– Evacuation removed almost everyone; military base closed

• June 15

– Cataclysmic eruption

– More than 5 km3 magma and rock, blasted up to 35 km height in atmosphere

– Pyroclastic flows 200 m deep

– Typhoon (hurricane) arrived and washed volcanic debris downslope as lahars




• Assessment:

– 300 people killed but a million 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 (1 of 3)

1. 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



2. 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 moved



3. Gas Measurements

• Magma approaching surface loses gas as pressure drops

• Mammoth Mountain, California

– CO2 from magma kills 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


Figure 7.42 LIDAR

LIDAR image shows ground deformation as magma moves below ground at Three Sisters volcanoes in Oregon


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, California, and Yellowstone

– Each with own website to report current activity


TABLE 7.3 Volcanic-Alert Levels, US Geological Survey

Normal Typical background activity of a volcano

Advisory Elevated unrest above known background activity

Watch Heightened/escalating unrest with increased potential for eruptive activity or minor eruption underway

Warning Highly hazardous eruption underway or imminent

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Ch 07 Volcano Case Histories: Killer Events · Volcanic Processes and Killer Events: Tsunami Volcano-generated tsunami have been responsible for 21% of volcano-caused deaths • 1883