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Other volcanic features
• Flood basalts
– Largest volcanic events in terms of volume
– Volatiles, viscosity are low, so not very
explosive
– Cover large areas w/ thick layers of lava
• Lava domes
– Hardened dome/plug of magma cools quickly
– Can form in hours, continue for years, decades
Flood Basalts
Lava Domes
Lava
Dome,
Mt. St.
Helens
Volcanic Hazards
Outline
• Types of
volcanic
hazards
Volcano Classification
• Active: currently erupting or has erupted in
historical time
• Dormant: presently inactive but may erupt
again (examples: Cascades)
• Extinct: not active, not expected to be active
for long time in future
Cumulative Deaths Due to Volcanic
Hazards in past 2000 years
Causes of
Deaths
How bad can it be?
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1815
1822
1826
1883
1902
1902
1919
1951
1982
1985
Indonesia
Indonesia
Indonesia
Indonesia
Martinique
Guatemala
Indonesia
New Guinea
Mexico
Columbia
92,000 died*
5,500 killed
3,000 killed
36,417 killed
29,025 killed
6,000 killed
5,110 killed
2,942 killed
1,700 killed
23,000 killed
Pyroclastic Flow
• Most deadly of all hazards
• Superhot, fast moving cloud of volcanic
gas, steam, debris
• 200-700 degrees C, 80 km/hr (hurricane
speeds!)
Generation of
pyroclastic
flow
• Eruption
column
collapse
• Dome
collapse
• Directed
blasts
Pyroclastic flows
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Mayon, Philippines, 1984
Effects of Pyroclastic Flows
• Destroys nearly everything in path
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Structures
Burial of sites
Burn forests, crops, structures
Melt snow to form lahars
Building destruction
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Remnant of building after eruption of El Chichon, Mexico in 1982
Note reinforcement rods bent in direction of flow
Effects of pyroclastic flows
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Mt. St. Helens - forest destruction
Effects on people
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Physical impact
Burns
Inhaling superhot gases
Usually quick death
Examples
• Many examples of devastating pyroclastic
flows
– Unzen, Japan
– Chichon, Mexico
– Mt. Pelee, Caribbean*
Mt. Pelee
• 1902-1903, 1929-1932 significant pyroclastic
flows
• Many from directed blasts, deadliest in 1902
– Viscous magma filled crater
– May 1902, several pyroclastic flows, traveling ~190
km/hr
– Destroyed St. Pierre (~30,000 people) and other towns
in area
Mt. Pelee
Large area
affected by 1902
pyroclastic flows
Eruption Debris
• Includes cloud, bombs, ash fall
• Bombs kill small # of people every year
• Clouds
– Planes fly into cloud - dangerous
• Over 25 years, ~60 jets damaged
• Ash fall
– Bury cities
Ash fall hazards
Roof collapse
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Mid-Afternoon daylight gone
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Water systems can
clog
Kamchatka eruption,
seen by space shuttle
Cloud
Hazard
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Ash from Mt.
Spurr traveled
around for days
(satellite
image)
Example - Mt. Vesuvius 79 AD
• Cause of volcano: subduction
• Region near Pompeii also had earthquakes, but
city of ~20,000 people not deterred to move
• Significant volume of material erupted (4 km3) in
first wave
– Pompeii buried by ash/pumice 3 m deep
• Later pyroclastic flows continued to bury city
Vesuvius
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Remains of Pompeii after 79 AD
eruption
Tsunami
• Wave caused by undersea vertical
displacement (like earthquakes)
• Volcanic eruption can also cause tsunami
– Pyroclastic flow or lava dome avalanche
– Collapse of cone or crater
Example - Krakatau 1883
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Stratovolcano between Java and Sumatra
Subduction zone
Collapse led to tsunami ~35 m (115 ft)
Death toll ~36,000
New cone appeared in 1927
Collapse of
Krakatau
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Anak Krakatau
Lahars
• Mudflow of volcanic debris, produced when
pyroclastic debris or lava mix with rain,
snow, ice, lakes
• Can travel quickly (40 mph) and significant
distances (10s of miles)
Dark pathways - lahars
triggered by Mt. St.
Helens eruption, 1985
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“Cloud” upriver: lahar traveling
down riverbed in Guatemala
Home destroyed from lahar - Mt. Unzen, Japan lava dome
collapse and later rains, 1993
Example - Nevado del Ruiz 1985
• Colombian volcano, small glacier on top
• Small eruption caused melting, sending
mud down the mountain
• Covered most of town of Armero with ~26
ft thick mud
– Killed ~22,000
Nevado del Ruiz, 1985
River valley
with lahar
remains
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Town of Armero
Famine
• Indirect hazard
• Eruptions change climate
– Can harm plants, animals
Iceland eruption, 1783
• Fissure eruption of basalt
• Released gas rich in SiO2 and fluorine
– Slowed grass growth and added significant
amounts of fluorine to grass
– Killed significant population of livestock
• ~20% of Iceland population died due to
famine
Tambora, 1815
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• Indonesian volcano, most violent eruption
in last 200 years
• Significant Plinian eruption
• Next pyroclastic flows (12mi3 of material)
• Deaths in region ~90,000 (10% during
eruption, 90% later)
• 1816 - year without summer
– Affected global climate
Gas
• Gas can escape from basaltic magmas
– CO2 gas is common
• Depending on gas, can kill plants in region
• Can also be trapped in water
Example - Lake Nyos, Cameroon
• Area of failed rifts with some remaining
volcanic activity
• 1986 - gas burp from the lake
– Sent rivers of gas down into valleys ~25 km (16
mi)
• Effects - ~1700 deaths, no animals alive
– Plants ok though
Why?
• CO2 gas leaking out of basaltic magma
• Dissolved in bottom waters of lake (held
down by pressure)
• Some event caused overturn
– Eruption, earthquake, landslide, temp change
all possible triggers
• Gas release “river” ~50 m thick
Lake Nyos
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Efforts to pump water,
gas out of lake
Lake Nyos
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Continued hazard?
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YES!
Still some gas remaining
Continues to build up
Possible ways to pump gas out to reduce
risk
Lava Flows
• Not big hazards in terms of fatalities
• Moves too slow to kill people
• Can destroy towns, villages
Kilauea, HI
• Few lives lost
• Mostly
structural
losses
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1973 Flows in Iceland
• Lava flows threatened town, harbor
• Built diverters to control flow
• Sprayed water to harden flows, move to
flow in other direction away from harbor
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Man vs. volcano!
Actually gained
land and
improved
harbor
Next Time
• U.S. volcanoes and related hazards