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Volcanoes
A Hot Topic
What is a volcano?
• A mountain formed by lava and/or
pyroclastic material
Nature of Volcanoes
Explosive vs. Effusive
(Erupts violently)
(Quiet eruptions)
Nature of Eruption
Controlled by Viscosity
• A material’s resistance to
flow
• Factors controlling
viscosity:
– Composition
– Temperature
– Dissolved gases
Factors Controlling Viscosity
• Composition
– ↑ SiO2; ↑ Viscosity
– ↓ SiO2; ↓ Viscosity
• Temperature
–↑ temperature; ↓ Viscosity
• Dissolved Gases
– ↓ pressure; Gases come
out of solution
– How easily gases escape
determine eruptive style
Factors Controlling Viscosity
Andesite
Basalt
Magma Type
Rhyolite
Viscosity
Basalt
50%
Low
Andesite
60%
Intermediate
Rhyolite
70%
High
Gas content
Low
Intermediate
High
SiO2
Explosiveness Nonexistent Intermediate
Explosive
What is Extruded?
• Lava
• Dissolved Gases
• Pyroclastic Material
Types of Lava
• Aa
– Rough, jagged surface
– Higher viscosity
– Lower temperature
• Pahoehoe
– Smooth, ropy surface
– Lower viscosity
– Higher temperature
Dissolved Gases - Volatiles
• H2O
– steam
• CO2
– Greenhouse gas
• SO2
– “Rotten egg” smell
• HCl
– Acid rain
Pyroclastic Material
• Ground-hugging avalanche
that rushes down the side of a
volcano
• Material consists of:
– Hot ash
– Pumice
– Rock fragments
– Dust
Types of Volcanoes
• Shield Volcanoes
– Largest, quiet eruption
• Composite Cone (Stratavolcano)
– Most explosive
• Cinder Cones
– Small, classic shape
Shield Volcanoes
• Broad, gently sloping - “Warrior’s Shield”
• Covers large area – lava tubes carry lava great distance
• Erupts mainly basalt
Shield Volcano
Hawaiian Islands
Mauna Loa, Hawaii
Composite (Stratovolcano)
• Alternating layers of lava
and pyroclastic material
• Large, classic-shaped
• Associated with
subduction zones
• Multiple eruptions
Composite Volcano
Cascade Ranges
Mount Lassen, California
Cinder Cones
• Cone built by cinders
– lava fragments
• Explosive, but small
• One eruptive event
Cinder Cones
California
Amboy Crater
Cima Volcanic Field
Deadly Volcanoes
Lateral Blasts
• An explosion of
rock, ash, and gas
released at side of
volcano
• May exceed speed
of sound
St. Pierre, 1902; 29,000 killed
Deadly Volcanoes
Nuée Ardentes
• Pyroclastic flow of
hot ash and gas
• Also known as “glowing
avalanche”
• Move at speeds up to
200 km/hr
Montserrat, 1996
Deadly Volcanoes
Lahars
• Volcanic mudflow
– Consist of water,
rock, ash, and mud
– May travel >80 km
distance at 35-60
km/hr
Nevado del Ruiz, Colombia
• Causes:
– Heat melts snow and ice
– Rainfall eroding loose volcanic debris
– Landslides of water-saturated debris
In 1985, 23,000 people died
Mount St. Helens,
Washington
• Lateral Blast
before
During
After
Mount St. Helens,
Washington
• Lateral Blast
• Pyroclastic flow
• Lahar
Predicting Eruptions
• Seismicity
– Ground shaking
• Geomorphology
– Changes in ground surface shape
• Geophysical properties
– Electrical conductivity, gravity anomaly
• Gas content
– ↑SO2
Mount Pinatubo, Philippines
Successful Prediction
• Erupted in 1991
• Prediction saved
1,000’s of lives
• Cloud circled entire
equator
• Global temperatures
decreased