1. Differential Stress: This means that the pressure applied to the rock is not equal in all directions. Imagine squeezing a sponge – it gets thinner in the direction you're squeezing but thicker in the other directions. This unequal pressure causes the minerals within the rock to align themselves perpendicular to the direction of the greatest stress.
2. Heat: Heat is needed to make the minerals in the rock mobile enough to be able to re-crystallize and align themselves. The higher the temperature, the easier it is for the minerals to move and change their orientation.
So, it's the combination of differential stress and heat that allows for the formation of foliation in metamorphic rocks.
