You notice that the grains in a metamorphic rock appear to be flattened and crushed together. How can you account for this?

1 Answer
Feb 16, 2016

This is usually a sign that the rock has been subjected to a stress field at some point.

Explanation:

Metamorphic rocks develop foliation when subjected to stress fields, such as being buried under kilometres of rock. Minerals tend to rotate perpendicular to the stress field, giving rise to foliation. For example, gneisses (1st pic below), or schists.

The other way is for a rock to be caught in a major fault zone which tends to also produce a foliation and the rock boy crushing the minerals and this is called a mylonite (2nd pic below).

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