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Chapter 7 Fires Within: Igneous Activity The Nature of Volcanic Eruptions Characteristics of a magma determine the “violence” or explosiveness of an eruption Composition Temperature Dissolved gases The above three factors actually control the viscosity of a magma

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Chapter 7 fires within igneous activity l.jpg

Chapter 7 Fires Within: Igneous Activity


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The Nature of Volcanic Eruptions

  • Characteristics of a magma determine the “violence” or explosiveness of an eruption

    • Composition

    • Temperature

    • Dissolved gases

  • The above three factors actually control the viscosity of a magma


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    The Nature of Volcanic Eruptions

    • Viscosity is a measure of a material’s resistance to flow

    • Factors affecting viscosity

      • Temperature—Hotter magmas are less viscous

      • Composition—Silica (SiO2) content

        • Higher silica content = higher viscosity

        • Lower silica content = lower viscosity


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    The Nature of Volcanic Eruptions

    • Dissolved gases

      • Gases expand within a magma as it nears Earth’s surface due to decreasing pressure

      • The violence of an eruption is related to how easily gases escape

  • In summary

    • Basaltic lavas = mild eruptions

    • Rhyolitic or andesitic lavas = explosive eruptions


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    Materials Extruded from a Volcano

    • Lava flows

      • Basaltic lavas exhibit fluidbehavior

      • Types of basaltic flows

        • Pahoehoe lava (resembles a twisted or ropey texture)

        • Aa lava (rough, jagged blocky texture)

  • Dissolved gases

    • 1%–6% by weight

    • Mainly H2O and CO2


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    Aa Lava Flow

    Figure 7.5 A


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    Materials Extruded from a Volcano

    • Pyroclastic materials—“Fire fragments”

      • Types of pyroclastic debris

        • Ash and dust—Fine, glassy fragments

        • Pumice—Porous rock from “frothy” lava

        • Cinders—Pea-sized material

        • Lapilli—Walnut-sized material

        • Particles larger than lapilli

          • Blocks—Hardened or cooled lava

          • Bombs—Ejected as hot lava


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    A Volcanic Bomb

    Figure 7.6 (top)


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    Volcanic Structures

    • General features

      • Opening at the summit of a volcano

        • Crater—Summit depression <1 km diameter

        • Caldera—Summit depression >1 km diameter produced by collapse following a massive eruption

      • Vent—Surface opening connected to the magma chamber

      • Fumarole—Emit only gases and smoke


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    Volcanic Structures

    • Types of volcanoes

      • Shield volcano

        • Broad, slightly domed shape

        • Generally cover large areas

        • Produced by mild eruptions of large volumes of basaltic lava

        • Example = Mauna Loa on Hawaii


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    Mauna Loa, a Shield Volcano on Hawaii

    Figure 7.9


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    Volcanic Structures

    • Cinder cone

      • Built from ejected lava (mainly cinder-sized) fragments

      • Steep slope angle

      • Small size

      • Frequently occur in groups


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    Cinder Cone Volcano

    Figure 7.12


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    Volcanic Structures

    • Composite cone (stratovolcano)

      • Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mount St. Helens)

      • Large, classic-shaped volcano (1000s of ft. high and several miles wide at base)

      • Composed of interbedded lava flows and pyroclastic debris

      • Most violent type of activity (e.g., Mt. Vesuvius)


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    Mount St. Helens—Prior to the 1980 Eruption


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    Mount St. Helens after the 1980 Eruption



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    Volcanic Structures

    • Nuée ardente

      • Nuée ardente—A deadly pyroclastic flow

        • Fiery pyroclastic flow made of hot gases infused with ash and other debris

        • Also known as glowing avalanches

        • Move down the slopes of a volcano at speeds up to 200 km per hour


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    A Nueé Ardente on Mount St. Helens

    Figure 7.15


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    Volcanic Structures

    • Lahar—Volcanic mudflow

      • Mixture of volcanic debris and water

      • Move down stream valleys and volcanic slopes, often with destructive results


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    Other Volcanic Landforms

    • Caldera

      • Steep-walled depressions at the summit

      • Generally >1 km in diameter

      • Produced by collapse

  • Pyroclastic flow

    • Felsic and intermediate magmas

    • Consists of ash, pumice, and other debris

    • Example = Yellowstone Plateau


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    Formation of Crater Lake, Oregon

    Figure 7.18


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    Other Volcanic Landforms

    • Fissure eruptionsandlava plateaus

      • Fluid basaltic lava extruded from crustal fractures called fissures

      • Example = Columbia River Plateau

  • Lava domes

    • Bulbous mass of congealed lava

    • Associated with explosive eruptions of gas-rich magma


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    Other Volcanic Landforms

    • Volcanic pipes and necks

      • Pipes—Short conduits that connect a magma chamber to the surface

      • Volcanic necks (e.g., Shiprock, New Mexico)—Resistant vents left standing after erosion has removed the volcanic cone


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    Intrusive Igneous Activity

    • Most magma is emplaced at depth in the Earth

      • Once cooled and solidified, is called a pluton

  • Nature of plutons

    • Shape—Tabular (sheetlike) vs. massive

    • Orientation with respect to the host (surrounding) rock

      • Concordant vs. discordant


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    Intrusive Igneous Activity

    • Types of intrusive igneous features

      • Dike—A tabular, discordant pluton

      • Sill—A tabular, concordant pluton (e.g., Palisades Sill in New York)

      • Laccolith

        • Similar to a sill

        • Lens or mushroom-shaped mass

        • Arches overlying strata upward


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    Igneous Structures

    Figure 7.22 B



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    Intrusive Igneous Activity

    • Intrusive igneous features continued

      • Batholith

        • Largest intrusive body

        • Surface exposure >100+ km2 (smaller bodies are termed stocks)

        • Frequently form the cores of mountains


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    Plate Tectonics and Igneous Activity

    • Global distribution of igneous activity is not random

      • Most volcanoes are located within or near ocean basins

      • Basaltic rocks = oceanic and continental settings

      • Granitic rocks = continental settings


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    Distribution of Some of the World’s Major Volcanoes

    Figure 7.26


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    Plate Tectonics and Igneous Activity

    • Igneous activity at plate margins

      • Spreading centers

        • Greatest volume of volcanic rock is produced along the oceanic ridge system

        • Mechanism of spreading

          • Decompression melting occurs as the lithosphere is pulled apart

          • Large quantities of basaltic magma are produced


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    Plate Tectonics and Igneous Activity

    • Subduction zones

      • Occur in conjunction with deep oceanic trenches

        • An island arc if in the ocean

        • A volcanic arc if on a continental margin

      • Associated with the Pacific Ocean Basin

        • Region around the margin is known as the “Ring of Fire”

        • Majority of world’s explosive volcanoes


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    Plate Tectonics and Igneous Activity

    • Intraplate volcanism

      • Occurs within a tectonic plate

      • Localized volcanic region in the overriding plate is called a hot spot

        • Produces basaltic magma sources in oceanic crust (e.g., Hawaii and Iceland)

        • Produces granitic magma sources in continental crust (e.g., Yellowstone Park)



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