How is foliation related to the dominant regional stresses?

1 Answer
Mar 26, 2016

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.https://www.google.ca/search?q=stress+fields+metamorphism&biw=1531&bih=1042&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiq5YK5q9_LAhUD02MKHV52BesQ_AUIBigB#tbm=isch&q=compressional+stress+fields+metamorphism&imgrc=unUfdrTMarJ6fM%3A image source here

Quick quiz for 10 points - what direction was the stress field in the following gneiss? Parallel to the hammer or perpendicular?

https://www.google.ca/search?q=stress+fields+metamorphism&biw=1531&bih=1042&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiq5YK5q9_LAhUD02MKHV52BesQ_AUIBigB#tbm=isch&q=banded+gneiss&imgrc=xrf3Yo4QcNbOEM%3A

Answer: the compressional stress field must have been perpendicular to the hammer ie. top to bottom squeeze!