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Where:
1. Almost all volcanoes are found @ Plate Boundaries.
Most of those volcanoes are found along the Pacific Rim – a subduction zone called “The Ring of Fire”
2. Hot spots
Volcanoes
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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.
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Melting Rock to Make Magma
1. Heat Melts RockHeat 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.
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Magma Composition and Characteristics
Granitic
Ocean Crust Melt (Mafic Magmas) Continental Crust
Melt(Felsic Magmas)
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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
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Structure of Volcanoes
Caldera : depression formed from collapsed volcanoes (usually extinct); usually fill up with water & b/cm lakes
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Eruption Types: Quiet = low silica/ high water/ high temp. / Low viscosity/ low gas content
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Eruption TypesExplosive Eruptions: high-silica / Low temps./ low water / high viscosity/ high gas content
“pyroclastic”
Mount St. Helens, WA
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Pre 1980 eruption
Eruption Types
Mount St. Helens, WA
Post 1980 eruption
Today
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Volcano Hazards1.Lava flows (burns/
fire)2.Ash (buries,
suffocates)3.Pyroclastic flows
(gas, ash, superheated rock fragments <bombs>)
4.Mudflows/Lahars5.Acidification of
water6.Climate
change/mass extinction
Least Hazard
Great Hazard
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Lahar: Volcanic landslide/mudslide
Eruptions and Eqs that accompany them trigger landslides;Lava melts snow at the top of the volcano MudslideVery Hazardous wipe out villages/ fatalities
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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
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Volcano Types2. Strato or Composite VolcanoBuilds 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. HelensMt. Pinatubo
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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
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Volcano Types4. Under Water Volcanoes
Seamount – an underwater mt. (volcano) that does not reach the surface
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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)
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Ring of Fire = Edge of the Pacific Plate. Most of the worlds volcanoes are found here due to SUBDUCTION of the pacific plate.
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3 Ways Volcanoes Form
1. Subduction Zones (Convergent Plate Boundaries)(Ring of Fire)
2. Rifting & Sea Floor Spreading (Divergent Plate Boundaries)(African volcanoes & Iceland’s volcanoes)
3. 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.
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Hot SpotsThe Hawaiian Islands were formed as seamounts grew over hot spots in the Pacific.
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Hot Spots Around the World
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Hot Spot Volcanoes
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Hot Spot Volcanoes
Hot spot stays in the same place while the plate moves above.
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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)
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Columbia Plateau in Idaho =
Lava Plateau
Basalt
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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.
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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)
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Types of Volcanoeshttp://vulcan.wr.usgs.gov/Glossary/
VolcanoTypes/volcano_types.htmlhttp://vulcan.wr.usgs.gov/Photo/Pictograms/
volcano_types.html
Volcano World – All things volcanohttp://volcano.und.edu/
Smithsonian’s Weekly Volcano Reporthttp://www.volcano.si.edu/reports/usgs/