Volcanoes A Hot Topic. What is a volcano? A mountain formed by lava and/or pyroclastic material.
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Transcript of Volcanoes A Hot Topic. What is a volcano? A mountain formed by lava and/or pyroclastic material.
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:
– Temperature
– Composition
– 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
Magma Type Basalt Andesite Rhyolite
SiO2 50% 60% 70%
Viscosity Low Intermediate High
Gas content Low Intermediate High
Explosiveness Nonexistent Intermediate Explosive
Basalt Andesite Rhyolite
What is Extruded?
• Lava
• Dissolved Gases
• Pyroclastic Material
Types of Lava
• Aa
• Pahoehoe– Smooth, ropy surface
– Rough, jagged surface
– Higher viscosity
– Lower temperature
– Lower viscosity
– Higher temperature
Dissolved Gases - Volatiles
• H2O
• CO2
• SO2
• HCl
– steam
– Greenhouse gas
– “Rotten egg” smell
– 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
– Largest, quiet eruption
– Most explosive
– Small, classic shape
• Shield Volcanoes
• Composite Cone (Stratavolcano)
• Cinder Cones
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)
• Associated with subduction zones
• Large, classic-shaped
• Alternating layers of lava and pyroclastic material
• Multiple eruptions
Composite VolcanoCascade Ranges
Mount Lassen, California
Cinder Cones
• Cone built by cinders – lava fragments
• Explosive, but small
• One eruptive event
Cinder Cones California
Cima Volcanic FieldAmboy Crater
Deadly VolcanoesLateral 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 VolcanoesNuée Ardentes
• Pyroclastic flow of hot ash and gas
• Also known as “glowing avalanche”
• Move at speeds up to 200 km/hr
Montserrat, 1996
Deadly VolcanoesLahars
• Causes:
• Volcanic mudflow
– Heat melts snow and ice
– Rainfall eroding loose volcanic debris
– Landslides of water-saturated debris
– Consist of water, rock, ash, and mud
– May travel >80 km distance at 35-60 km/hr
Nevado del Ruiz, Colombia
In 1985, 23,000 people died
Mount St. Helens, Washington
• Lateral Blast
beforeDuringAfter
Mount St. Helens, Washington
• Lateral Blast
• Pyroclastic flow
• Lahar
Predicting Eruptions
• Seismicity
• Geomorphology
• Gas content
• Geophysical properties
– Ground shaking
– Changes in ground surface shape
– Electrical conductivity, gravity anomaly
– ↑SO2
Mount Pinatubo, PhilippinesSuccessful Prediction
• Erupted in 1991
• Prediction saved 1,000’s of lives
• Cloud circled entire equator
• Global temperatures decreased