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
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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