This is another 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.
   Molten rhyodacite is also very thick and viscous, and contains dissolved gases.  When the pressure above it is released, it erupts violently, causing explosive ash flows.
   When this same type of rock cools slowly, deep underground, it forms with larger crystals within, and is called granodiorite, an intrusive igneous rock.
   For those of you technically disposed, rhyodacite is approximately 2/3 plagioclase (Ca-rich feldspar), 1/3 orthoclase (K-rich feldspar), with minor amounts of quartz and other accessory minerals.  It occurs with rhyolite in the Superstition Mountains, and is usually light in color inside.


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