Chapter 5 Volcanoes and Other Igneous Activity

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