From the Latin word for
"fire", igneous refers to rocks derived from the solidification of molten rock
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
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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