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Shield Volcanoes Basaltic · PDF file Basaltic Volcanoes References: Encyclopedia of Volcanoes, pp. 283-289 Shield Volcanoes • Low angle volcano constructed principally of basalt

Jun 18, 2020

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  • Basaltic VolcanoesBasaltic Volcanoes

    References: Encyclopedia of Volcanoes, pp. 283-289

    Shield VolcanoesShield Volcanoes

    • Low angle volcano constructed principally of basalt lavas

    • Examples: Mauna Loa and Kilauea, Hawaii • Thin lava flows • Summit calderas • May have lateral rift zones • May have lava lakes in the crater

    SkaldbreidurSkaldbreidur Mauna KeaMauna Kea

    Mount Etna, ItalyMount Etna, Italy

    StratovolcanoesStratovolcanoes

    • A volcano constructed of alternating layers of lava and pyroclastic materials

    • Steep slope angles (~35°) • Examples: Fuji, Japan and Kliuchevskaya,

    Kamchatka • May exhibit persistent activity (Etna, Sicily) • Volumes of erupted lava proportional to

    repose time

  • Stratovolcano structure PopocatepetlPopocatepetl

    Cones in Arc SettingsCones in Arc Settings

    • Tholeiitic or calc-alkaline compositions • Merge to andesites • Continuous feeding from depth into a

    shallow chamber • May be associated with fissures and

    monogenetic scoria cones • Cones may be truncated by edifice collapse

    – Example: Augustine, Alaska

    MountMount AugustineAugustine

    Facies at a stratovolcano Monogenetic Volcano FieldsMonogenetic Volcano Fields

    • Composed of volcanoes that erupt only once

    • Common edifice is a scoria cone • Tuff rings and small shields are also

    common • Example: Pinacate, Mexico • Vent systems are dikes or pipes (diatremes)

  • Scoria ConesScoria Cones

    Tuff RingsTuff Rings

    Pillow LavasPillow Lavas

    Scoria ConesScoria Cones • Simplest and commonest volcanic form

    • Characterized by three parameters

    – Height, width, crater width

    • Standard initial slope of 30o

    • Conical shape

    • Occur in several environments

    Sunset Crater, ArizonaSunset Crater, Arizona

    Scoria Cone SectionScoria Cone Section

    DiatremesDiatremes

    • Breccia pipes

    • Kimberlite

    • Contains diamonds

    • Ultramafic magmas

    • Mixture of rocks

    • Driven by deep CO2

    Dia tre m eDia tre m e

    Tuff ConesTuff Cones

    • Massive deposits

    • Thickly bedded

    • Palagonitized

    • Bedding up to 30o

    • Wet surges Vulcano, ItalyVulcano, Italy

  • Tuff RingsTuff Rings

    • Thinly-bedded

    • Poorly-indurated

    • Beds less than 12o

    • Sandwave beds

    • Dry surges

    C e rro C o lor ad o, M e x ic oC e rro C o lor ad o, M e x ic o

    Basaltic Scoria Cone FieldsBasaltic Scoria Cone Fields

    • 10s to 1000s of cones • General elliptical shape • Aspect ratio of 2:1 to 5:1 • 10 to 70 km in length • Areas of extensional tectonics • Elongate perpendicular to σ3 • Widespread in western USA • Pinacate example

    San Francisco Volcanic Field

    San Francisco Volcanic Field

    Small FieldsSmall Fields

    • North rim of Grand Canyon

    • Scoria cones aligned along fault planes

    Flood Basalt FieldsFlood Basalt Fields

    • Extensive sheets of lava flows • Flood the landscape • Associate with crustal dilation • May coincide with mass bioextinctions • Examples: Deccan, India and Columbia

    River Plateau, USA

  • Flow UnitsFlow Units

    • Compound lava flows • Overlapping pahoehoe flows • Interconnected lava shields • Lava ponds in depressions • Inward directed dips of large fields • Cut by numerous dike swarms+

    Columnar JoinColumnar Join

    Central VolcanoesCentral Volcanoes

    • A volcano that erupts magmas of various compositions – Mafic to silicic

    • Many have a central caldera composed of silicic rocks and rhyolite domes

    • A shallow magma chamber is part of the eruptive system

    ShastaShasta

  • Volcano InteriorVolcano Interior

    • The eroded core of these volcanoes may expose a granite pluton

    • Feeding system may be a ring complex • These may be a host for economic mineral

    deposits

    Alteration ZonesAlteration Zones

    Albite, epidote, Chlorite

    Propylitic

    ChloriteChloritic

    Clays, no micaArgillic

    MicaSericitic

    AdulariaPotassic

    SilicaSilicic