Textural features of metamorphic rocks have been discussed in the previous lecture. Here, we concentrate on the development of foliation, one of the most common purely metamorphic textures, and on the processes involved in forming compositional layering commonly observed in metamorphic rocks. Most foliation is caused by the preferred orientation of phylosilicates, like clay minerals, micas, and chlorite. We here review the differences between hydrostatic and differential stress. Stress and Preferred Orientation Pressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth.
Pressure is defined as a force acting equally from all directions.
It is a type of stress , called hydrostatic stress or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress. Normally geologists talk about stress as compressional stress. Note that extensional stress would act along the direction of minimum principal stress. If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock.