1. Volcanoes
A Hot Topic

2. What is a volcano?
A mountain formed by lava and/or pyroclastic material

3. Nature of Volcanoes Explosive vs. Effusive (Erupts violently)
(Quiet eruptions)

4. Nature of Eruption Controlled by Viscosity
A material’s resistance to flow
Factors controlling viscosity:
Composition
Temperature
Dissolved gases

5. 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

6. Factors Controlling Viscosity
Basalt
Andesite
Rhyolite
Magma Type
Basalt
Andesite
Rhyolite
SiO2
50%
60%
70%
Viscosity
Low
Intermediate
High
Gas content
Explosiveness
Nonexistent
Explosive

7. What is Extruded?
Lava
Dissolved Gases
Pyroclastic Material

8. Types of Lava Aa Pahoehoe Rough, jagged surface Higher viscosity
Lower temperature
Pahoehoe
Smooth, ropy surface
Lower viscosity
Higher temperature

9. Dissolved Gases - Volatiles
H2O
steam
CO2
Greenhouse gas
SO2
“Rotten egg” smell
HCl
Acid rain

10. Pyroclastic Material
Ground-hugging avalanche that rushes down the side of a volcano
Material consists of:
Hot ash
Pumice
Rock fragments
Dust

11. Types of Volcanoes Shield Volcanoes Composite Cone (Stratavolcano)
Largest, quiet eruption
Composite Cone (Stratavolcano)
Most explosive
Cinder Cones
Small, classic shape

12. Shield Volcanoes Broad, gently sloping - “Warrior’s Shield”
Covers large area – lava tubes carry lava great distance
Erupts mainly basalt

13. Shield Volcano Hawaiian Islands
Mauna Loa, Hawaii

14. Composite (Stratovolcano)
Alternating layers of lava and pyroclastic material
Large, classic-shaped
Associated with subduction zones
Multiple eruptions

15. Composite Volcano Cascade Ranges
Mount Lassen, California

16. Cinder Cones Cone built by cinders – lava fragments
Explosive, but small
One eruptive event

17. Cinder Cones California
Cima Volcanic Field
Amboy Crater

18. 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

19. Deadly Volcanoes Nuée Ardentes
Pyroclastic flow of hot ash and gas
Also known as “glowing avalanche”
Mt. Pelee is in Martinique. A stratovolcano caused by the subduction of the North American Plate under the Carribean Plate
Move at speeds up to 200 km/hr
Montserrat, 1996

20. Deadly Volcanoes Lahars
Volcanic mudflow
Consist of water, rock, ash, and mud
May travel >80 km distance at km/hr
Nevado del Ruiz, Colombia
Causes:
In 1985, 23,000 people died
Heat melts snow and ice
Rainfall eroding loose volcanic debris
Landslides of water-saturated debris

21. Mount St. Helens, Washington
After
During
before
Lateral Blast

22. Mount St. Helens, Washington
Lateral Blast
Pyroclastic flow
Lahar

23. Predicting Eruptions Seismicity Geomorphology Geophysical properties
Ground shaking
Geomorphology
Changes in ground surface shape
Geophysical properties
Electrical conductivity, gravity anomaly
Gas content
↑SO2

24. Mount Pinatubo, Philippines Successful Prediction
Erupted in 1991
Prediction saved 1,000’s of lives
Cloud circled entire equator
Global temperatures decreased