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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 Activity

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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 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

    • Pahoehoe – rope-like, glassy

    • Aa – fragmented, dull surfaces



    Basaltic Lavas

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

    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)

    Volcanic Bombs

    Volcanic Blocks

    Volcanic Tuff

    Volatile Gases Associated with Volcanism

    • Steam (H2O)

    • Carbon dioxide (CO2 )

    • Hydrogen sulfide (H2S)

    • Sulfur vapor

    • Many other constituents

    Sulfur-encrusted fumerole:

    Galapagos Islands

    Sulfur-encrusted fumerole:Galapagos Islands

    Fig. 5.26

    Christian Grzimek/Photo Researchers

    Volcanic Landforms

    • Shield volcanoes

    • Cinder cones

    • Sratovolcanoes or composite volcanoes

    • Domes

    • Calderas

    • Volcanic Necks and Pipes

    • Fissure Eruptions

    Shield Volcanoes

    • Very large

    • Composed of low viscosity basalt

    • Volcanoes have a gentle, shield shape

    • Example: Mauna Loa, Hawaii

    Mauna Loa, Hawaii

    Shield Volcano

    Fig. 5.10

    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

    Cinder Cone

    Fig. 5.12

    Paŕicutin, Mexico

    Paŕicutin, Mexico

    Cerro Negro Cinder Cone, near Managua, Nicaragua in 1968

    Fig. 5.13

    Mark Hurd Aerial Surveys

    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


    Fujiyama, Japan

    Mount Saint Helens – Before May 1980

    Mount Saint Helens, Before May, 1980

    Emil Muench/Photo Researchers

    After May, 1980

    Mount Saint Helens – After May, 1980

    David Weintraub/Photo Researchers

    Volcanic Domes

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

    • Usually composed o rhyolie

    • Example: Lava Dome in the Mount Saint Helens crater

    Fig. 5.11

    Mount St. Helens lava dome





    Lyn Topinka/USGS

    Inyo Obsidian Domes-California

    P. L. Kresan


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

    • Example: Crater lake, Oregon

    Formation of a Caldera

    Fig. 5.16

    Crater Lake, Oregon

    Fig. 5.17

    Greg Vaughn/Tom Stack

    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.

    Shiprock, N.M.

    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

    1971 Fissure Eruption, Kilauea, Hawaii

    Fissure Eruptions Form Lava Plateaus

    Fig. 5.20

    Extent of Columbia River Basalts

    Fig. 5.22

    Columbia Plateau Flow Basalts

    Fig. 5.2

    Martin G. Miller

    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

    The World’s Active Volcanoes

    Fig. 5.28

    Volcanism Associated with Plate Tectonics

    Fig. 5.30

    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

    Volcanic Hazards

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

    May 1990 Eruption of Kilauea, Hawaii

    James Cachero/Sygma

    January 17, 2002 Nyiragongo Volcano D.R. Congo

    January 17, 2002 Nyiragongo Volcano D.R. Congo

    January 17, 2002 Nyiragongo Volcano D.R. Congo

    January 17, 2002 Nyiragongo Volcano D.R. Congo

    January 17, 2002 Nyiragongo Volcano D.R. Congo

    Pyroclastic Activity

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

    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.

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

    Fig. 5.9

    AP/Wide World Photos

    Mt. Pelee, 1902

    Pyroclastic Flow from the 1998 Eruption on Montserrat

    R.S.J. Sparks

    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

    Fig. 5.18

    Maritime Safety Agency, Japan

    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

    Lahar from Nevado del Ruíz (1985)

    Barbara and Robert Decker

    U.S. Active Volcanoes

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