Natural Disasters
Volcanoes: Types and Eruption Styles
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Magma Review
Basalt
Andesite
Rhyolite
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
Types of Volcanoes
Basaltic Volcanoes
Shield volcanoes
Cinder cones
Andesitic Volcanoes
stratovolcanoes
Rhyolitic Volcanoes
Lava domes
calderas
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
Shield Examples
Mauna Loa (largest volcano)
Hawaiian Islands
• “Big Island” – Mauna Loa, Kilauea, and Mauna Kea
Basaltic volcanoes
Cinder Cones
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Small size, steep slope
Erupt when magma encounters groundwater
Single eruptive period (months to few years)
Built up by loose cinders around small crater
Cinder Cone Examples
Cima Volcanic Field
Amboy Crater
California Cinder Cones
Red Hill
Andesitic Volcanoes
Stratovolcanoes
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Alternating layers of lava, debris, and ash
Moderate volume and size
Moderate to high volatile content
Violently explosive
Erupts repeatedly
Stratovolcano Examples
Mt. Hood
Mt. Rainier
Cascade Ranges
Mt. Lassen
Rhyolitic Volcanoes
Lava Domes
• Rough dome with many spines
• Very high viscosity – magma does not travel far
• Low volatile (gas) content
Lava Domes Examples
Mono Craters
Panum Crater
Central California
Wilson Butte
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
Continental Caldera Example
Crater Lake, Oregon
Yellowstone National Park
Explosive Eruption Styles
Hawaiian
Phreatomagmatic
Strombolian
Vulcanian
Pelean
Plinian
Hawaiian
• Low-viscosity, fluid basalt
• Non-explosive
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
Strombolian
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Distinct blasts of magma
Produce incandescent bombs
Small tephra cone results
Mildly explosive
Vulcanian
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Sustained eruption of rock or viscous magma to several km
Collapse to produce pyroclastic flow
Widespread tephra fall
Very explosive
Pelean
• Collapse of lava dome
• Produce glowing
avalanches
(nuée ardentes)
• Violently explosive
Plinian
• Sustained eruption columns
up to 45 km
• Produce widespread ash
deposits
• Collapse of column to
pyroclastic flow
• Violently explosive
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
Long-Term Forecasting
• Ancient studies help
scientist assess hazards
and risk posed by
future volcanoes
Short-Term Forecasting
• Seismic waves
• Magnetic field changes
• Electrical Resistivity
• Ground Deformation
• Changes in groundwater
• heat flow
• Gas composition