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Chapter 5 Volcanoes and Volcanic Hazards
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The Nature of Volcanic Eruptions
All eruptions involve magma/lava The behavior of magma is determined by:
Temperature Composition Gases
These factors control the viscosity, which controls the nature of the eruption
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The Nature of Volcanic Eruptions
Viscosity - resistance to flow Dictates eruptive behavior High viscosity (cooler) – thick (like stew) Low viscosity (hotter) – very fluid (like soup)
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High ViscosityHigh silica content (Felsic)Andesite & Rhyolite (Granitic)Violent eruptionsContinental
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Low ViscosityLow silica content (mafic)BasaltMild eruptionsOceanic
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Dissolved Gases in magma/lavaDissolved water in magma reduces
viscosityGases expand within magma as it
ascends (decreasing pressure)The violence is related to how easily
the gases escape
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The Nature of Volcanic EruptionsQuiescent Hawaiian –Type Eruptions
Basaltic lavas (low viscosity)Weeks, months or years of lava flowing
Explosive EruptionsGranitic (high viscosity)Expels particles of fragmented lava and
gases
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Other Volcanic MaterialsPyroclastics – “fire fragments”
Ash/Dust - fine & glassyPumice - porous Cinders - pea-sizeLapilli - walnut-size Blocks – hardened or cooled lavaBombs – hot lava
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Development of Pyroclastic Flows
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Volcanic Features
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Crater Lake
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Volcanic LandformsCrater Lake and Wizard Island
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Volcanic LandformsFissure Eruptions & Basalt Plateaus
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Volcanic LandformsFissure Eruptions & Basalt Plateaus
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Shield Volcanoes
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Cinder Volcano
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Composite/Strato Volcano
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Caldera Volcano - Yellowstone
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Profiles of Volcanoes
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Mt. St. Helens Prior to the 1980 Eruption
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Mt. St. Helens after the 1980 eruption
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Mt. St. Helens
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Shiprock, New Mexico
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Plate Tectonics & Igneous Activity
Igneous activity is not randomMost volcanoes are in/near oceansBasaltic rocks = oceans & continentsGranitic rocks = continents
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Volcanoes of the Cascade Range
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World’s Major Volcanoes
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Plate Tectonics & Igneous Activity Intraplate volcanismOccurs within a tectonic plateHot Spot
HawaiiYellowstone Park
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Volcanoes & Climate
Explosive eruptions Emit gases & fine-grained debrisSolar radiation is reflected & filtered
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Volcanoes and Climate Mount Tambora, Indonesia – 1815
Krakatau, Indonesia – 1883
Mount St. Helens, Washington – 1980
Mount Pinatubo, Phillippines - 1991
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Mount Vesuvius 79 A.D.
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End of Chapter 5