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USGS CMG InfoBank: Foliation

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Comment: 07:50 - 08:54 (01:04)
Source: Annenberg/CPB Resources - Earth Revealed - 18. Metamorphic Rocks
Keywords: "J. Lawford Anderson", "directed stress", compression, metamorphism, structure, temperature, pressure, mineral, layering, geologist, foliation, "shear stress", "metamorphic rock", "mica schist", subduction, recrystallization

Our transcription: Directed stress involving compression helps explain the origin of a very common metamorphic structure.
As temperature and pressure increase, minerals recombine to make new, more stable minerals.
The minerals grow in the directions of lowest pressure perpendicular to the directed stress.
This results in a layering which geologists call "foliation."
Shear stress, too, can cause foliation.
I'll hold a piece of a metamorphic rock we call a "schist," a "mica schist," and we can see it's very layered.
That layering is a preservation of a stress field generated within a subduction zone environment.
As the rock is recrystallized under great pressure and great temperature, it is also recording in it the intensity of the stress field.
We see stresses that had to be in some sort of direction to planerize the micas forming the mica schist.

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