volcanic hazards in the u.s.. outline cascades –mt. st. helens –mt. rainier –crater lake...
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Volcanic Hazards in the U.S.
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Outline• Cascades
– Mt. St. Helens– Mt. Rainier– Crater Lake– Mt. Shasta– Lassen
• Other eruptions– Yellowstone– Long Valley– Valles Caldera
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Locations of U.S. Volcanoes
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Subduction Zone Volcanoes
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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
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Cascades
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Mt. St. Helens
• Very active over last 4000 years
• Eruptions ~ every century– Date past eruptions using old deposits
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St. Helens (pre-1980)
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Mt. St. Helens 1980
• March: small magnitude earthquakes– Indicates magma moving– Also northern side bulge
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Mt. St. Helens 1980
• April: 1.2 mi2 bulge rising 100 m
– Growing by 1.5 m/day
bulge
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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
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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
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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
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Mt. St. Helens FigsAsh removed in Idaho
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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
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Steam plume, Jan3 2005
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New dome growth indicates magma moving to surface, producing steam
Oct 2004
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Cascades
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Mt. Rainier• Classified as most
dangerous volcano in U.S.– Large glacier system
– Frequent earthquakes
– Active springs
– Just outside Seattle-Tacoma
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Why So Dangerous?
• Could have eruption/landslide like St. Helens– Springs act to weaken structure
• Significant lahar potential
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Previous Activity
• Mudflows date back 5,000+ years
• Most recent 500 years ago
• Significant population centers on lowlands (~100,000 people)
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Activity
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Cascades
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Crater Lake
• Today popular tourist spot
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Mt. Mazama 5677 B.C.
• Stratovolcano in Cascades• Major eruption of lava, pyroclastic debris
– Covered much of Pacific Northwest, Canada with thick layer
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Crater Formation
• Magma chamber empties, leaves void• Top collapses
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Crater Formation
• Magma chamber empties, leaves void• Top collapses
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Lake Formation
• Forms over time• Small volcanic cone in center
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Cascades
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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
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Mt. Shasta
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Lahar paths
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Pyroclastic Flow Danger
Note the increasing number of towns/developments in higher hazard zones
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Cascades
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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
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Lassen
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Outside Cascades
• Significant caldera events in – Yellowstone– Long Valley– Valles
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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
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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
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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