What is happening when a metamorphic rock is forming?

Question

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Answer 1

The rock, being an igneous, sedimentary or even a metamorphic rock is being subjected to extreme heat and /or pressure without melting. This is by definition, a metamorphic rock. What actually happens is when an old rock is texturally or mineralogically (or the crystallography) transformed to a new rock and that new rock is distinct from the old

Answer 2

When Regional Metamorphic rocks form pressures from colliding plates force the movement of atoms, resulting in new assemblages of minerals. When igneous or sedimentary rocks form they normally contain "flaws" such as vacancies (or voids) in the minerals. Differential stresses force these vacancies to the edge of the mineral where they disappear (it's not magic, they were nothing to begin with). This process hardens the rock. Similarly, volatiles (water, carbon dioxide) are forced out of minerals, leading to new minerals that don't contain these volatiles. Then there are the entire realms of Contact Metamorphism and Hydrothermal Metasomatism, but that is enough for now.

Answer 3

Metamorphic rock requires high pressure and/or temperature to form

Answer 4

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