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USGS CMG InfoBank: Textures and Conditions

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Comment: 06:48 - 07:53 (01:05)
Source: Annenberg/CPB Resources - Earth Revealed - 14. Intrusive Igneous Rocks
Keywords: "J. Lawford Anderson", crystal, phaneritic, granite, aphanitic, obsidian

Our transcription: Textures that result in the crystallization of those minerals tells us about the conditions under which that rock formed.
Some rocks have very large crystals that are easily visible to the naked eye, and we call that a "phaneritic texture".
Your everyday granite has a phaneritic texture.
That shows that rock had to crystallize at some depth several kilometers down in the Earth's crust.
That rock represented a different time.
It's exposed now at the surface only due to later tectonic activity.
Other igneous rocks crystallize near the surface.
Volcanic rocks, for example, record a very rapid crystallization of the same kind of magma, but as the crystallization becomes more rapid, the crystals are forced to be smaller and smaller becoming eventually "aphanitic" or invisible to the eye.
Or even faster the minerals don't have time to grow, and the result is volcanic glass which we call obsidian.
So we're learning to read textures from the rock, and we learn the conditions under which it forms.

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