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

• April: 1.2 mi2 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

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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

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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 FigsAsh 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

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Steam plume, Jan3 2005

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New dome growth indicates magma moving to surface, producing steam

Oct 2004

Cascades

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

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