AS

Volcanoes III: Volcano Distribution and Characteristics

Eruption Styles

  • two main styles:

    1. effusive: outpouring of molten lava from vent
    2. explosive: gas-driven violent eruption producing pyroclastic deposits

Effusive Eruptions

  • low gas content and low viscosity magma = effusive volcanism
    • “tame” volcano with lots of lava flows
    • fewer hazards

Explosive Eruptions

  • high gas content and vicious magma = explosive volcanism
    • explosive volcano with lots of pyroclastic (“fire fragments”) material
    • several associated hazards

Global Distribution of Volcanoes

  • volcanoes occur at:
    • plate boundaries
    • hotspots

Plate Boundaries: 3 Main Types

  • divergent, convergent, transform
    • at transform there isn’t any volcanoes
  • crust composition and melt origin will strongly influence the type of volcanism

Oceanic Divergent Boundary

  • two oceanic plates spread apart across a rift zone
    • rift zone: a long, linear volcano
  • new oceanic crust is created as older crust is pulled away
  • volcanism is extensive (lots of lava produced), but not explosive
  • when the two plates have spread far enough apart that an ocean has formed between two continents, the divergent boundary is called a mid-ocean ridge
    • most voluminous volcanism on Earth
    • not exposed on Earth’s surface (so we don’t see the eruptions), except in Iceland because there is a hotspot there
  • initial source of melting: partial melting of the mantle (due to decompression melting)
  • secondary source of magma: oceanic crust
  • type of magma formed: mafic (hot, low viscosity/SiO2, low gas)
  • hazard: low viscosity lava, low gas content, effusive eruptions, and underwater = no hazard

Continental Divergent Boundary

  • two continental plates spread apart to form a rift valley
    • ex: east african rift valley
  • initial source of melting: partial melting of the mantle
  • secondary source of magma: continental crust
  • type of magma formed: mostly intermediate (cooler, high viscosity/SiO2, lots of gas)
  • hazard: vicious lava with lots of trapped gas = high hazard (lower hazard in places with more mafic lava)

Subduction Zones

  • oceanic plate sub-ducts under another oceanic plate or continental plate
    • releases water
    • water causes partial melting of the mantle (ultramafic = more extreme mafic composition than mafic; super hot, super low silica)
    • magma composition becomes more silica rich as it rises and incorporates oceanic (mafic)/continental (felsic) crust
  • at the surface:
    • volcanic arcs: arc-shaped lines of volcanoes
    • 75% of active volcanoes happen in the Pacific Ring of Fire

Oceanic Oceanic Convergent Boundary

  • initial source of melting: partial melting of the mantle (due to water released from sub-ducting plate)
  • secondary source of magma: (modified) oceanic crust- mafic to intermediate
  • type of magma formed: mostly mafic (hot, low viscosity/SiO2, low gas)
  • hazard: generally low

A “Local” Continental Volcano

  • produced by the Cascadia Subduction Zone
    • subduction of the Juan de Fuca plate (oceanic) under North American plate (continental)
  • active volcanoes (last 2 million years)

Oceanic-Continental Convergent Boundary

  • initial source of melting: partial melting of the mantle (due to H2O released from sub-ducting plate)
  • secondary source of magma: continental crust- felsic (lots of cooler, high viscosity/SiO2 magma with lots of trapped gas)
  • hazard: YES