1. Volcanoes
Where:
1. Almost all volcanoes are Plate Boundaries.
Most of those volcanoes are found along the Pacific Rim – a subduction zone called “The Ring of Fire”
2. Hot spots

2. Formation of Volcanoes
1. Magma forms: rock melts, forming liquid magma (melted rock + trapped gas)
2. Magma rises through the crust, erupting at the surface.
Magma rises b/c it is less dense than rock (it’s hotter & has gas in it).
3. Magma collects & melts more rock …Pressure builds as more gas is added.
4. Eruption: pressure gets too high.

3. Melting Rock to Make Magma
Heat Melts Rock
Heat from mantle; Heat from friction of grinding plates; heat from magma that intrudes into crust from other locations
2. Decrease Pressure on Rock
Decompression Melting: When pressure is reduced rock can melt at lower temps. Pressure depends on depth. As hot rock rises; it melts because there is less pressure.
* Rock deep in crust should melt; but is solid because of pressure)
3. Add Water: “Wet” rock melts at lower temps.
Subduction drives water out of the subducting rock (metamorphic change). “Dewatering the slab” lowers the temperature of rock above.

5. Magma Composition and Characteristics
Granitic
Ocean Crust Melt (Mafic Magmas)
Continental Crust Melt
(Felsic Magmas)

6. Structure of Volcanoes
 Magma collects in magma chamber before eruption
 Magma exits through a central vent or pipe to the crater at the summit.
 Each eruption adds a layer to the volcano
Eruption dates can be determined by isotopic dating the rock layers

7. Structure of Volcanoes
Caldera : depression formed from collapsed volcanoes (usually extinct); usually fill up with water & b/cm lakes

8. Eruption Types:
Quiet = low silica/ high water/ high temp. /
Low viscosity/ low gas content

9. Eruption Types
Explosive Eruptions: high-silica / Low temps./ low water / high viscosity/ high gas content
“pyroclastic”
Mount St. Helens, WA

10. Eruption Types
Mount St. Helens, WA
Pre 1980 eruption
Post 1980 eruption
Today

12. Volcano Hazards Lava flows (burns/ fire) Ash (buries, suffocates)
Pyroclastic flows (gas, ash, superheated rock fragments <bombs>)
Mudflows/Lahars
Acidification of water
Climate change/mass extinction
Volcano Hazards
Least Hazard
Great Hazard

13. Lahar: Volcanic landslide/mudslide
Eruptions and Eqs that accompany them trigger landslides;
Lava melts snow at the top of the volcano  Mudslide
Very Hazardous  wipe out villages/ fatalities

14. Volcano Types
1. Shield: Broad / Flat, Gently sloping cone, (b/c runny lava travels far before solidifying)
Quiet Eruptions: Liquid lava (low viscosity / high water / low silicates) fr. Single vent
Ex: Hawaiian Volcanoes

15. Volcano Types 2. Strato or Composite Volcano
Builds in layers of lava and ash & debris
Explosive “pyroclastic” eruptions (Hot gas, rock, and ash) High viscosity / high silicates /low water
Tall & Steep & Side Vents
Most Dangerous
Ex: Mt. St. Helens
Mt. Pinatubo

16. Volcano Types 3. Cinder Cones Simple, small, steep sided
Made from blobs of lava & ash ejected from a single vent that fall back to the surface.
No repeated eruptions
Ex: California Volcanoes – Lassen Peak

17. Volcano Types 4. Under Water Volcanoes
Seamount – an underwater mt. (volcano) that does not reach the surface

18. Volcanoes - Most are located near plate boundaries
“Ring of Fire” = Edge of Pacific Ocean stretching from Alaska to Japan to Indonesia, where most of the world’s volcanoes are located. (subduction zone)

19. Ring of Fire = Edge of the Pacific Plate
Ring of Fire = Edge of the Pacific Plate. Most of the worlds volcanoes are found here due to SUBDUCTION of the pacific plate.

20. 3 Ways Volcanoes Form
Subduction Zones (Convergent Plate Boundaries) (Ring of Fire)
Rifting & Sea Floor Spreading (Divergent Plate Boundaries) (African volcanoes & Iceland’s volcanoes)
Hot Spots – can be in the middle of a plate (Yellowstone, Hawaii)
– a region of active magma under a plate. This active magma forces its way through weaknesses in the crust to form volcanoes.

21. Hot Spots
The Hawaiian Islands were formed as seamounts grew over hot spots in the Pacific.

22. Hot Spots Around the World

23. Hot Spot Volcanoes

25. Hot Spot Volcanoes
 Hot spot stays in the same place while the plate moves above.

27. Other Extrusive Igneous Features (not volcanoes)
Lava Plateau: Large amounts of easily flowing lava erupting fr. cluster of long, thin cracks in crust. Lava spreads out over enormous area before solidifying.
Ex.: Columbia Platueau in Pacific NW (1 km thick / 200,000 square mi)

28. Columbia Plateau in Idaho = Lava Plateau
Basalt
Columbia Plateau in Idaho =
Lava Plateau

29. Intrusive Igneous Features
Batholith: Large type of intrusive igneous rock mass that can form the core of a mountain range (Sierra Nevada’s in Calif.)
Sill: Magma squeezes through cracks in rock layers and hardens / Paralles to rock layers.
Dike: Igneous rock cuts across rock layers.
Volcanic Neck: magma hardens in volcanic neck.

31. Half Dome at Yosemite National Park (Sierra Nevada Mts) = Forms when batholith that was originally under surface reaches surface (rock on top of erodes away or it is pushed upwards)

32. G. A. F. B. E. C. D.

36. Types of Volcanoes
Volcano World – All things volcano
Smithsonian’s Weekly Volcano Report