Metamorphism is difficult to understand because there are many combinations of temperature and pressure that can create rocks. For example, mud and clay quietly settle on the ocean floor. As more mud and clay settle on top of it, the weight of the sediments "squeezes" the water from the mud and clay on the bottom. It becomes cemented together by chemical interactions and it becomes a sedimentary rock called shale. The shale is put under moderate pressure and low temperature due to burial or plate movements. The new pressure and temperatures changed the chemical make up of the shale into the metamorphic rock called slate. 

Shale to slate metamorphism