In general the degree of alteration is a function of the difference in chemistry, and conditions of formation, of the existing rock as compared to the intrusive. Regional Metamorphism can cover large areas, often in association with batholiths or stocks, in roots of mountain chains. Shields found in every continent are such ancient roots. Progressive metamorphism is common with higher grades having coarser textures, better defined banding, and changed mineral associations. Most have a shale parent, though some are volcanic.
Resultant rocks range from coarse fault-breccias to fine fault gouges .
Weak platy minerals become smeared out and resistant blocky minerals pulverized. Sometimes recrystallization occurs forming mylonite .