What is the process by which metamorphic rock is formed?
High pressures and temperatures can cause hydrous minerals to change to non-hydrous minerals like garnet. Rounded mineral crystals can become flattened, and minerals that grow in the stressed rock can display a preferred orientation that is perpendicular to the direction of the stress and display a characteristic known as foliation, with distinct banding or platyness. In rocks such as quartzite and limestone, where the minerals are relatively stable along a broad range of temperatures and pressures, the mineral crystals simply become larger. In cases where the metamorphism occurs because of proximity to hydrothermal fluids, a chemical exchange will take place, changing the original composition of the host rock.
