Chapter 5 Volcanoes and Volcanic Hazards
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
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)
High ViscosityHigh silica content (Felsic)Andesite & Rhyolite (Granitic)Violent eruptionsContinental
Low ViscosityLow silica content (mafic)BasaltMild eruptionsOceanic
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
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
Other Volcanic MaterialsPyroclastics – “fire fragments”
Ash/Dust - fine & glassyPumice - porous Cinders - pea-sizeLapilli - walnut-size Blocks – hardened or cooled lavaBombs – hot lava
Development of Pyroclastic Flows
Volcanic Features
Crater Lake
Volcanic LandformsCrater Lake and Wizard Island
Volcanic LandformsFissure Eruptions & Basalt Plateaus
Volcanic LandformsFissure Eruptions & Basalt Plateaus
Shield Volcanoes
Cinder Volcano
Composite/Strato Volcano
Caldera Volcano - Yellowstone
Profiles of Volcanoes
Mt. St. Helens Prior to the 1980 Eruption
Mt. St. Helens after the 1980 eruption
Mt. St. Helens
Shiprock, New Mexico
Plate Tectonics & Igneous Activity
Igneous activity is not randomMost volcanoes are in/near oceansBasaltic rocks = oceans & continentsGranitic rocks = continents
Volcanoes of the Cascade Range
World’s Major Volcanoes
Plate Tectonics & Igneous Activity Intraplate volcanismOccurs within a tectonic plateHot Spot
HawaiiYellowstone Park
Volcanoes & Climate
Explosive eruptions Emit gases & fine-grained debrisSolar radiation is reflected & filtered
Volcanoes and Climate Mount Tambora, Indonesia – 1815
Krakatau, Indonesia – 1883
Mount St. Helens, Washington – 1980
Mount Pinatubo, Phillippines - 1991
Mount Vesuvius 79 A.D.
End of Chapter 5