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USGS CMG InfoBank: Shear Stress

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Comment: 05:51 - 06:52 (01:01)
Source: Annenberg/CPB Resources - Earth Revealed - 18. Metamorphic Rocks
Keywords: "shear stress", crystal, Chinese, "snowball garnet", geologist, metamorphism, mineral, inclusion, "multi-ringed model", plane

Our transcription: Directed shear stress, for example, helps explain the origin of a spectacular form of crystal growth.
These swirling images suggest several things: a cluster of spiral galaxies, the Chinese symbol for ying and yang.
They are, in fact, "snowball garnets", to the geologist, frozen slices of metamorphism in action.
The swirling pattern in a snowball garnet is formed by planes of tiny mineral inclusions that are swallowed up by the garnet as it grows.
Shear stress causes the garnet to rotate during growth distorting the planes into swirls.
The three dimensional form of the swirling pattern can be shown by means of a multi-ringed model.
Each ring represents the edge of a plane of minerals incorporated by the growing garnet.

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