How is foliation related to the dominant regional stresses?
1 Answer
Any mineral in a rock subjected to a stress field will start to rotate so that the long axis of the mineral is perpendicular to the stress field.
Explanation:
This tendency of elongated minerals to rotate into this position gives rise to foliation in metamorphic minerals (like in gneisses, schists, slates). This happens in compressional stress fields.
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Quick quiz for 10 points - what direction was the stress field in the following gneiss? Parallel to the hammer or perpendicular?
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Answer: the compressional stress field must have been perpendicular to the hammer ie. top to bottom squeeze!
