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Volcanic Hazards in the U.S.
Outline
• Cascades
–
–
–
–
–
Mt. St. Helens
Mt. Rainier
Crater Lake
Mt. Shasta
Lassen
• Other eruptions
– Yellowstone
– Long Valley
– Valles Caldera
Locations of U.S. Volcanoes
Subduction Zone Volcanoes
Cascades
• Subduction of Juan de Fuca plate beneath
North America
• Water released from slab aids melting above
• Magma travels toward surface, some cools,
other erupts
• 6-7 of these volcanoes have erupted in last
200 years
Cascades
Mt. St. Helens
• Very active over last 4000 years
• Eruptions ~ every century
– Date past eruptions using old deposits
St. Helens (pre-1980)
Mt. St. Helens 1980
• March: small magnitude earthquakes
– Indicates magma moving
– Also northern side bulge
Mt. St. Helens 1980
mi2
• April: 1.2
bulge rising 100
m
– Growing by
1.5 m/day
bulge
Mt. St. Helens 1980
• May 18th:
– Magnitude 5.1
earthquake
– Triggered
landside on
north side
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Mt. St. Helens 1980
• May 18th:
– Lahars forming after landslide
• Snow, ice near top of mountain
Bridge destroyed by
lahar
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Mt. St. Helens 1980
• May 18th:
– Lateral blast - pyroclastic
flow
• Occurred after landslide
removed pressure on side of
mountain
• Destroyed area ~550 km2
• Timescale: minutes from
earthquake
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Mt. St. Helens 1980
• May 18th:
– Vertical eruption
• Plinian eruption,
reached 12 mi (20
km) high
• 1 km2 ash
Mt. St. Helens, post eruption
• Rebuilding
– Different shape
– Lava dome builds
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Effects of 1980 eruption
•
•
•
•
62 deaths
Significant stream/valley modification
Clogged Columbia River, shipping channels
Forest destruction
Mt. St. Helens Figs
Ash removed in Idaho
Current Activity, Mt. St. Helens
• Sept. 23, 2004 - volcano started grumbling
again
– Earthquakes, uplifted crater floor, some steam
eruptions 9 days later
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Current Activity, Mt. St. Helens
• Growth of new lava dome
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Current Activity, Mt. St. Helens
New dome growth indicates magma
moving to surface, producing steam
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Oct 2004
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Steam plume, Jan3 2005
Cascades
Mt. Rainier
• Classified as most
dangerous volcano in
U.S.
–
–
–
–
Large glacier system
Frequent earthquakes
Active springs
Just outside SeattleTacoma
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Why So Dangerous?
• Could have eruption/landslide like St.
Helens
– Springs act to weaken structure
• Significant lahar potential
Previous Activity
• Mudflows date back 5,000+ years
• Most recent 500 years ago
• Significant population centers on lowlands
(~100,000 people)
Activity
Cascades
Crater Lake
• Today
popular
tourist spot
Mt. Mazama
5677 B.C.
• Stratovolcano in Cascades
• Major eruption of lava, pyroclastic debris
– Covered much of Pacific Northwest, Canada with thick
layer
Crater Formation
• Magma chamber empties, leaves void
• Top collapses
Crater Formation
• Magma chamber empties, leaves void
• Top collapses
Lake Formation
• Forms over time
• Small volcanic cone in center
Cascades
Mt. Shasta and Shastina
• 2nd and 3rd largest volcanoes in Cascades
– Last eruption in 1786, at least 3x in last 750
years
– Development ongoing in region
Mt. Shasta
Lahar paths
Pyroclastic Flow Danger
Note the increasing
number of
towns/developments
in higher hazard
zones
Cascades
Lassen Peak
• Actually lava dome forming in region of
past large volcano
• Significant eruptions in 1914-1917
– Lava flows, pyroclastic flows, ash cloud
– Eruptions may have been helped by melting
snow adding to groundwater system
Lassen
Outside Cascades
• Significant caldera events in
– Yellowstone
– Long Valley
– Valles
Yellowstone, WY
• Hotspot volcanism, makes tracks
across U.S.
• Youngest volcanics at
Yellowstone
– Eruptions at 2 million, 1.3 million,
600,000 years ago
– Catastrophic eruptions! (600,000
yr event - VEI 8)
– Active system, geysers, geothermal
activity today
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Yellowstone event 600,000 years
ago
• Erupted ~1,000
km3 of magma
– Mt. St. Helens
1km3 of
magma
• Created caldera
75 km long, 45
km wide
Long Valley Caldera
• Not traditional hotspot, but active magma
system
• Large eruption ~760,000 years ago
– Covered 1,500 km3 with pyroclastic flow
• “Activity” in 1980, more next time
Long Valley
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Valles Caldera
• Associated with Rio Grande Rift
• Last eruption ~1 million years ago
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Next Time
• Volcanic eruption prediction efforts