Question: Is it possible for Gneiss, through regional metamorphism, to change pack to schistose or Phyllite or Slate? how long could this take?

Katy Mee answered on 17 Jun 2010:

No it’s not possible to turn gneiss back to phyllite, slate or schistose through metamorphism. Think of it like baking a cake – you start with the raw ingredients, mix them together and apply heat and you get a cake. It is now impossible to go back to the raw ingredients because the application of heat (and pressure in the case of metamorphism) has changed the physical and chemical state of the rock. The next stage would be migmatite.

Theoretically you could get gneiss from any of those rocks you mention via the rock cycle – so for example if the gneiss is subducted and melted, then brought back up to the surface via a plume creating an igneous rock. This may then be denuded and the denuated materials form a sedimentary rock that then goes through the metamorphic processes to become schistose, phyllite or slate again!

So in short, no it can’t be turned back into gneiis dreictly via metamorphism but it can be recycled.