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Natural DisastersVolcanoes: Types and Eruption Styles

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Magma Review

Magma Type Basalt Andesite Rhyolite

SiO2 45-55% 55-65% 65-75%

Viscosity Low Intermediate High

Gas content Low Intermediate High

Temperature 1000-1200°C 800-1000°C 650-800°C

Basalt Andesite

Rhyolite

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Types of Volcanoes

Basaltic VolcanoesShield volcanoes

Cinder cones

Andesitic Volcanoesstratovolcanoes

Rhyolitic VolcanoesLava domes

calderas

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Basaltic Volcanoes

Shield Volcanoes• Large, broad gentle slopes – resembles a “warrior’s shield”• Fluid basalt able to travel great distance• Low volatile (gas) content – generally non-explosive

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Shield Examples

Hawaiian Islands• “Big Island” – Mauna Loa, Kilauea, and Mauna Kea

Mauna Loa (largest volcano)

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Basaltic volcanoes

Cinder Cones• Small size, steep slope• Erupt when magma encounters groundwater• Single eruptive period (months to few years)• Built up by loose cinders around small crater

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Cinder Cone Examples

Amboy Crater

Cima Volcanic Field

Red Hill

California Cinder Cones

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Andesitic Volcanoes

Stratovolcanoes• Alternating layers of lava, debris, and ash• Moderate volume and size• Moderate to high volatile content• Violently explosive• Erupts repeatedly

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Stratovolcano Examples

Cascade Ranges

Mt. Hood

Mt. Rainier Mt. Lassen

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Rhyolitic Volcanoes

Lava Domes• Rough dome with many spines• Very high viscosity – magma does not travel far• Low volatile (gas) content

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Lava Domes Examples

Central California

Wilson ButtePanum Crater

Mono Craters

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Rhyolitic Volcanoes

Giant Continental Calderas• Circular, enclosed depressions• Typically filled with water to form lakes• Result from collapse volcanic structure

– Due to underlying emptying of magma

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Continental Caldera Example

Yellowstone National Park

Crater Lake, Oregon

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Explosive Eruption Styles

Hawaiian

Phreatomagmatic

Strombolian

Vulcanian

Pelean

Plinian

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Hawaiian

• Low-viscosity, fluid basalt• Non-explosive

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Phreatic & Phreatomagmatic

• Hot magma comes in contact with shallow groundwater• Water heats up and erupts steam, rock fragments, and

magma• Violently explosive• Phreatic = no new magma on surface

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Strombolian

• Distinct blasts of magma• Produce incandescent bombs• Small tephra cone results• Mildly explosive

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Vulcanian

• Sustained eruption of rock or viscous magma to several km

• Collapse to produce pyroclastic flow• Widespread tephra fall• Very explosive

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Pelean

• Collapse of lava dome

• Produce glowing avalanches (nuée ardentes)

• Violently explosive

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Plinian

• Sustained eruption columns up to 45 km

• Produce widespread ash deposits

• Collapse of column to pyroclastic flow

• Violently explosive

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Eruption Warning: Volcanic Precursors

• Active Volcano– Has shown activity in recorded history

• Extinct Volcano– Has not shown any historic activity

• Dormant Volcano– Has not shown activity in recorded history but

shows evidence of activity in geologic past

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Long-Term Forecasting

• Ancient studies help scientist assess hazards and risk posed by future volcanoes

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Short-Term Forecasting

• Seismic waves

• Magnetic field changes

• Electrical Resistivity

• Ground Deformation

• Changes in groundwater

• heat flow

• Gas composition