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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
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Outline • Cascades • Mt. St. Helens • Mt. Rainier • Crater Lake • Mt. Shasta • Lassen • Other eruptions • Yellowstone • Long Valley • Valles Caldera
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
Mt. St. Helens • Very active over last 4000 years • Eruptions ~ every century • Date past eruptions using old deposits
Mt. St. Helens 1980 • March: small magnitude earthquakes • Indicates magma moving • Also northern side bulge
Mt. St. Helens 1980 bulge • April: 1.2 mi2 bulge rising 100 m • Growing by 1.5 m/day
Mt. St. Helens 1980 • May 18th: • Magnitude 5.1 earthquake • Triggered landside on north side
Mt. St. Helens 1980 • May 18th: • Lahars forming after landslide • Snow, ice near top of mountain Bridge destroyed by lahar
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
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
Effects of 1980 eruption • 62 deaths • Significant stream/valley modification • Clogged Columbia River, shipping channels • Forest destruction
Ash removed in Idaho Mt. St. Helens Figs
Current Activity, Mt. St. Helens • Sept. 23, 2004 - volcano started grumbling again • Earthquakes, uplifted crater floor, some steam eruptions 9 days later
Current Activity, Mt. St. Helens • Growth of new lava dome
Current Activity, Mt. St. Helens New dome growth indicates magma moving to surface, producing steam Oct 2004 Steam plume, Jan3 2005
Mt. Rainier • Classified as most dangerous volcano in U.S. • Large glacier system • Frequent earthquakes • Active springs • Just outside Seattle-Tacoma
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)
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
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
Pyroclastic Flow Danger Note the increasing number of towns/developments in higher hazard zones
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
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
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
Valles Caldera • Associated with Rio Grande Rift • Last eruption ~1 million years ago
Next Time • Volcanic eruption prediction efforts