IV. Volcanoes

A. Basics
  1. general
    1. explosiveness depends on amount of gas and amount of silicon
    2. explosive volcanoes are felsic or intermediate and contain lots of silicon
    3. gentle, lava flow volcanoes are mafic or intermediate and contain little silicon
  2. felsic and intermediate explosive volcanoes
    1. high in silicon makes the magma very viscous (thick and gooey)
    2. light in color
    3. explosive, composite or strato-volcanoes
      • ash,
      • pyroclastic flows, nuee ardente,
      • tuff
      • lahars
  3. mafic and intermediate volcanoes
    1. low in silicon (<52%) makes the magma have low viscosity (thin and runny)
    2. dark in color due to high iron and magnesium content
    3. gentle lava flows make shield volcanoes
      • pahoehoe
      • aa
    4. more gas-rich lavas form cinder cones

B.  Subduction related - usually felsic to intermediate

  1. general
    1. Cascade Range
    2. generally explosive strato-volcanoes
    3. produce light colored, silicon rich, ash, tuff, pyroclastic rocks
  2. Mt. Shasta
    1. second and third highest Cascade peaks
    2. constructional edifice, largely unaffected by glaciation
  3. Mt. Lassen
    1. ancestral Mt. Tehama
    2. erupted in 1914-1915
    3. Bumpass Hell geothermal area
    4. southern end of subduction zone
    5. region is also influenced by extension (cinder cones and lava flows)
C. Extension related
  1. general
    1. Basin and Range (last 28 my, mostly last 10my) extension
    2. stretches and thins crust (lithosphere)
  2. mafic volcanoes
    1. Medicine Lake Highlands
      • shield volcano with minor cinder cones and spatter cones
      • 700,000 years
      • minor young silica-rich domes and flows
    2. Lava Flow National Monument (related to Medicine Lake)
      • lava flows from vents on the north flank of the shield volcano
      • extensive lava tubes
    3. Mojave desert
      • young from north (10 my) to south
      • mostly cinder cones and lava flows
      • related to northward extension of rift zone?
  3. felsic and bimodal volcanoes
    1. Long Valley Caldera
      • 3.6 my mafic flows
      • 2 my felsic eruptions
      • 760,000 major eruption
        • Bishop Tuff
        • caldera formation (2-3km deep, 17x32 km)
        • 200,000 year recurrence interval?
      • currently magma at 5-15km
    2. Mono-Inyo
      • rhyolite and obsidian domes and flows
      • Panum crater (650 years)
      • Inyo craters (530 years)
    3. Coso volcanics
      • 6 million years
      • bimodal - mafic flows and felsic domes
      • time/volume pattern

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