S(h)immering greenstone or pounamu! NZ volcanoes

Gas content
Mt. Victoria (extinct)
  •   Fire fountain of lava
  •   Low cone, steep sides
  •   E.g. Mt Victoria; Mt Eden
  •   Produces scoria
  • Mt. Ruapehu (dormant)
  •   Big explosion ash released
  •   Large volcano strato
  •   E.g. Mt Ruapehu; Mt Taranaki/Egmont
  •   Produces andesite, ignimbrite, rhyolite, pumice
  • NON-
    Rangitoto (extinct)
  •   No explosion
  •   Wide cone, not high
  •   E.g. Rangitoto Island
  •   Produces basalt
  • Table Mountain (extinct)
  •   No explosion ash released if magma reaches surface
  •   No cone causes land to bulge
  •   E.g. Table Mountain in Coromandel; Warkworth Dome
  •   Produces granite (plutonic)

  • Silica content

    Rule of thumb
    The higher the silica content, the stickier (more viscous) the magma/lava (flows fast if runny and slowly if sticky);
    the higher the gas content, the more explosive the eruption (gas escapes due to pressure change at vent causing material to be erupted explosively).

    Pictures courtesy of Google Images; based on idea from Rhys Hodson (TGS)

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