1. Our Geologic Environment
Volcanoes

2. Volcano Anatomy Key Terms: Magma / Lava Magma Chamber Neck / Vent
Crater / Caldera
Intrusive (Igneous Rocks)
Extrusive
Lava
Cinder
Ash

3. Type I: Shield Volcano Shield Shaped Flat (low-sloping sides)
Tall (10,000 m)
Caldera and Flank Eruptions
Wide (10-100s of km)
Passive eruptions, flowing lava
Mauna Kea, Hawaii

4. Shield Volcano Eruptions
pahoehoe!
Iceland
a) Lava is hot
b) Lava is basaltic
c) Eruptions are not (very) explosive.

5. Type II: Cinder Cones 1. Cone Shaped 2. Short (100s of meters)
3. Narrow (1-2 km)
4. Steep (up to 30-40o)
5. Built of Rhyolitic Cinder Blocks
6. Highly explosive, sticky lava
Black Butte, California

6. Type III: Composite Volcanoes
Mt. Mayon, Philippines
A mix of shield and cinder cone characteristics

7. Composites– The Danger
Mt. Fuji, Japan
Mt. Pinatubo, Philippines
Long spells of inactivity
Some eruptions are passive
Lava: andesitic, lower temperature, more gas
Pressure builds beneath surface, released in explosions

8. Mount St. Helens, Washington
Before
After
May 18, 1980
Mt St Helen’s Eruption 1980

9. Magma Composition is Key
Basaltic
Andesitic
Rhyolitic
1. More SiO2 (silica) = More viscous magma
a) Sticky… won’t flow
b) Traps gas… buoyant
2. The more viscous the magma, the more explosive the eruption

10. Volcanoes & the 3 Plate Boundary Types
2. Convergent: andesite volcanoes
1. Divergent: basalt flows
3. Transform: little volcanic activity

11. #1 Most Common Location: Mid-Ocean Ridge #2 Subduction Zones (The “Ring of Fire”)
… where seafloor is being separated… volcanoes!
Volcanoes
Hawaii
… and where it is being subducted too.

12. And #3: Hotspots … about two dozen spots
Plates move over spots making multiple volcanoes

13. Volcanic hazards Incineration Burial Gases Ash (tephra)
Lava
Pyroclastics… nuee ardente
Lahars
Gases
Ash (tephra)
Tsunami (Krakatoa 1884 – 120 ft)

14. The story of Heimaey Iceland is atop a spreading center and a hotspot
Heimaey Island

15. Haemey, Iceland, 1973
January 23
USGS

16. Heimaey Harbor
Making lava dams
New basalt flow
USGS

17. Vesuvius Quiet for 100’s of years until…... infamous eruption 79 A.D.
In Pompeii
Hot poisonous gases
Buried by 6 m of ash
uncovered in 1700’s
~3000 lives lost
Herculaneum
buried by 60 m of mudflows

18. People and Volcanoes, ca. 79CE
Casts of animal and people killed by gases, buried by ash.
Preserving a snap-shot of Roman life.

19. Cascade Volcanoes
Mt. St. Helens, 1980

20. Lahars can be deadly
Armero, Columbia, Volcanic mudflow from Nevado de Ruiz buried >23,000 people

21. Mt. Rainier Lahar Map Tacoma Mt. Rainier from Tacoma Small lahars
Moderate lahars
Large lahars
Lava/ash flows

22. Volcanic gas hazards
CO2 gas built up in lake in volcano crater
Gases escaped, heavier than air
Suffocated sleeping people
Lake Nyos, Cameroon, people and 3000 cattle died.

23. Boiling Springs
Temps 180oF and up.
Color from
Algae
minerals

24. Volcanic Ash Ash cloud, Ephrata WA May 18, 1980
Ephrata WA, 145 miles from Mt.
Saint Helens

25. St Helens Ash Map

26. Volcanic SO2 and ash circles Earth
SO2 gas forms sulfuric acid droplets.
Ash and droplets in atmosphere reflect sunlight

27. Volcano-induced climate change
Temperature change ( ºC)
Mt. Pinatubo eruption
Global temperature dropped by ~0.2º C for 2 years after eruption

28. Hazards reviewed Basaltic volcanoes: lava Andesitic volcanoes:
Relatively predictable, don’t cover large area, easy to get away from
Andesitic volcanoes:
Explosive, nuee ardentes move rapidly, cover large area quickly
Mudflows
Ash (tephra)

29. How to reduce hazard? Volcanic hazards maps based on geologic data
regulate human activity in hazard areas
best thing to do is not to live so close to volcanoes
Predict eruptions
Earthquake swarms
Unusual ground tilting, “inflation”
Gas emissions

30. US volcanic hazards No volcanoes here! Based on past activity.
Brown: high; green, blue, beige – decreasing hazard
Based on past activity.
Active volcano: eruption within past 500 years
Dormant: eruption > 500 but < 10,000 years ago.
Extinct: last eruption > 10,000 years ago