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Volcanic Activity. Plumbing System of a Volcano. Fig. 5.1. Cross Section of the East Pacific Rise. Fig. 5.29. Volcanic Materials. Lavas – appearance depends upon viscosity Low viscosity – low SiO 2 lavas; high T; low volatile content - smooth lava

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Volcanic materials l.jpg
Volcanic Materials

  • Lavas – appearance depends upon viscosity

    • Low viscosity – low SiO2 lavas; high T; low volatile content - smooth lava

    • High viscosity – high SiO2 lavas; low T; high volatile content – rough lava

  • Pyroclastic material – fragmented material due to explosive volcanic activity

    • Associated with magmas with high volatile gas content

  • Volcanic gases – water, carbon dioxide, sulfur, sulfur dioxide, hydrogen sulfide

  • Basaltic lavas l.jpg
    Basaltic Lavas

    • Pahoehoe – rope-like, glassy

    • Aa – fragmented, dull surfaces



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    Basaltic Lavas

    • Pillow lavas – underwater “pillow” or “tube” shaped lava flow

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    Pyroclastic Materials

    • Pyroclastic name based upon fragment size

      ash – sand size

      lapilli – walnut size

      bombs (> 1 inch, cools in flight)

      blocks (> 1 inch, solid before ejection)

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    Volatile Gases Associated with Volcanism

    • Steam (H2O)

    • Carbon dioxide (CO2 )

    • Hydrogen sulfide (H2S)

    • Sulfur vapor

    • Many other constituents

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    Sulfur-encrusted fumerole:

    Galapagos Islands

    Sulfur-encrusted fumerole:Galapagos Islands

    Fig. 5.26

    Christian Grzimek/Photo Researchers

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    Volcanic Landforms

    • Shield volcanoes

    • Cinder cones

    • Sratovolcanoes or composite volcanoes

    • Domes

    • Calderas

    • Volcanic Necks and Pipes

    • Fissure Eruptions

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    Shield Volcanoes

    • Very large

    • Composed of low viscosity basalt

    • Volcanoes have a gentle, shield shape

    • Example: Mauna Loa, Hawaii

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    Shield Volcano

    Fig. 5.10

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    Cinder Cone

    • A relatively small volcanic cone composed of unconsolidated pyroclastic material

    • Cinder cones may be composed of basalt, andesite, or rhyolite fragments

    • Usually active for a short period of time

    • Example: Paŕicutin, Mexico

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    Cinder Cone

    Fig. 5.12

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    Paŕicutin, Mexico

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    Paŕicutin, Mexico

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    Cerro Negro Cinder Cone, near Managua, Nicaragua in 1968

    Fig. 5.13

    Mark Hurd Aerial Surveys

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    Stratovolcanoes or Composite Volcano

    • Steep, symmetrical volcanic cones

    • Composed of alternating layers of lava and volcanic ash

    • Usually composed of andesitic material

    • Examples: Fujiyama, Japan; Mt. Saint Helens, Wa

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    Mount Saint Helens – Before May 1980

    Mount Saint Helens, Before May, 1980

    Emil Muench/Photo Researchers

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    After May, 1980

    Mount Saint Helens – After May, 1980

    David Weintraub/Photo Researchers

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    Volcanic Domes

    • Small, dome or “inverted” cup-shaped landforms

    • Usually composed o rhyolie

    • Example: Lava Dome in the Mount Saint Helens crater

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    Mount St. Helens lava dome





    Lyn Topinka/USGS

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    • A large, circular depression produced by the collapse of a volcano following the emptying of a subterranean magma chamber

    • Example: Crater lake, Oregon

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    Crater Lake, Oregon

    Fig. 5.17

    Greg Vaughn/Tom Stack

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    Volcanic Necks and Pipes

    • Remnants of volcanic vents and conduit systems

    • Exposed after erosion has removed the surrounding, soft volcanic rubble and country rock

    • Examples: Shiprock, N.M.

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    Fissure Eruptions

    • Very extensive, sheet-like lava flows that originate from long cracks (fissures) rather than central vents or volcanoes.

    • Flood lavas or basalts are associated with this type of activity

    • Example: Columbia river basalts, Washington and Oregon

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    Columbia Plateau Flow Basalts

    Fig. 5.2

    Martin G. Miller

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    Relationship of Volcanism and Plate Tectonics

    • Volcanoes of the Earth are associated with:

      • Plate Boundary Volcanism

        Divergent Plates

        Convergent Plate

    • Interplate Volcanism

      Hot spot or plume volcanoes

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    Volcanism Associated with Plate Tectonics

    Fig. 5.30

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    Types of Volcanic Hazards

    • Lava Flows: e.g. Hawaii, 1998

    • Gas: e.g. Lake Nyos (Cameroon), 1984

      • 1700 people killed

    • Ash fall: e.g. Mt. Pinatubo, 1991

    • Pyroclastic flows: e.g. Mt. Pelee, 1902

      • 28,000 killed

    • Lahars (mudflows): e.g. Nevado del Ruiz, 1985

      • 23,000 killed

    • Tsunami: e.g. Krakatoa, 1883

      • 36,417 killed

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    Volcanic Hazards

    • Basaltic lavas (flows) may cover homes and roads, but flows move so slow that there is little loss of life

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    Pyroclastic Activity

    • Usually rapid and explosive, accompanied by large volumes of poisonous gases. Can produce large losses of life and property.

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    Vesuvian Type Pyroclastic Eruption

    • Mount Vesuvius, Italy (79 A.D.)

    • Three day rain of volcanic ash buried the cites of Pompeii and Herculaneum

    • The cities along with >2000 people were buried intact by large ash falls.

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    Escaping a Pyroclastic Flow at Mount Unzen, Japan, 1991A type of Nuee Ardente (“glowing ash flow” eruption)

    Fig. 5.9

    AP/Wide World Photos

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    Phreatic explosion

    • Produced by groundwater seepage into the interior of a volcano

    • Water is converted into superheated steam with subsequent explosive release (flash vaporization)

    • Example – Krakatoa, Indonesia (1883) >100 megaton explosion; 36,000 people killed by tsunami or “tidal wave” produced by the explosion

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    Fig. 5.18

    Maritime Safety Agency, Japan

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    Volcanic Mudflows - Lahars

    • Hot volcanic gases and phyroclastic eruptions can melt glacial ice on the flanks of volcanoes, producing a fast moving wall of mud that can travel tens of miles

    • Example Nevado del Ruíz, Colombian Andes (1985) 20,000 deaths

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    Lahar from Nevado del Ruíz (1985)

    Barbara and Robert Decker