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Comment: 20:08 - 21:02 (00:54)
Source: Annenberg/CPB Resources - Earth Revealed - 18. Metamorphic Rocks
Keywords: metamorphism, "metamorphic rock", temperature, pressure, fluid, "chemical reaction", ion, mineral, recrystallization, geologist, deformation, crust, "plate tectonics"

Our transcription: If metamorphic rocks form inside the Earth as temperatures and pressures rise, why aren't they un-metamorphosed as temperatures and pressures fall back down?
In part, this is because loss of fluids during metamorphism makes it impossible for certain chemical reactions to reverse themselves.
Also, as temperatures drop, ions cannot migrate easily through the rock, so minerals will not recrystallize.
So in most metamorphic rocks, geologists find a preserved record of the greatest temperatures and pressures occuring during crustal deformation.
With the development of plate tectonics theory, the temperature and pressure changes geologists have long seen in metamorphic rocks finally began to make sense.

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