The alignment of minerals into bands is caused by massive force of pressure and high temperature which results from mountain building processes caused by plate collisions. The heat and pressure combine to realign the existing minerals from a random orientation, which then creates the visible lines found in the metamorphic rocks. <span />
The main reason behind the rock changes metamorphism is that the minerals should be stable under the new temperature and weight conditions.
The requirement for stability may make the structure of minerals modify and form new minerals. Particles may move between minerals to make minerals of various concoction structure
