rhyolite

   This is the name of a type of extrusive igneous rock -- rock which when molten, erupts to the surface of the world, and then cools relatively quickly.  Consequently, it is fine-grained.
   When this same type of rock cools slowly, deep underground, it forms with larger crystals within, and is called granite, an intrusive igneous rock.
   Molten rhyolite is also very thick and viscous, and contains dissolved gases.   When the pressure above it is released, it erupts violently, causing explosive ash flows.
   For those of you technically disposed, rhyolite is approximately 2/3 orthoclase (K-rich feldspar), with minor amounts of quartz and other accessory minerals, and it is usually light in color.
   See also rhyodacite.

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