igneous

   From the Latin word for "fire", igneous refers to rocks derived from the solidification of molten rock or magma.
   The are two kinds: intrusive and extrusive.  Intrusive igneous rock never made it to the surface of the Earth (until erosion later exposed its cooled remains), cooled very slowly, and the minerals that make it up therefore had time to grow into large crystals.  Granite and granodiorite are intrusive igneous rocks.  They form much of the rock visible in the rocks of the Phoenix area.
   Extrusive igneous rocks came out to the surface, cooled quickly, and therefore are very fine-grained.  We commonly call them lava flows, or volcanics.  Rhyolite and dacite are names of two types, and they form much of the rock in the Superstition Mountains, for example.

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