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USGS CMG InfoBank: Crystals and Cooling

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Comment: 05:46 - 06:48 (01:02)
Source: Annenberg/CPB Resources - Earth Revealed - 14. Intrusive Igneous Rocks
Keywords: "James Sadd", dike, andesite, magma, gneiss, heat, crystal, granite, rhyolite, phenocryst

Our transcription: This dike formed when a hot andesitic magma welled up into a fracture in this gneiss.
As the magma cooled, it transferred heat into the surrounding gneiss.
Heat transfer was, of course, more rapid here at the contact resulting in the gneiss being cooked by the magma heat.
Because crystal size in igneous rocks is a good indicator of relative cooling rate, we can also see that the magma itself cooled more quickly at the contact.
The smaller crystals here at the contact tell us that the magma cooled more quickly there than here at the center where the crystals are somewhat larger.
Based upon texture, geologists infer that granites crystallize from magma cooling slowly deep underground.
In contrast, rhyolites form from magma that cools slowly at first allowing a small amount of the larger crystals called phenocrysts to develop.
Then it cools quickly as the result of an eruption stopping crystal growth altogether.

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