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. If not enough heat and temperature were applied another metamorphic rock could have been formed called phyllite, which is not as hard as slate. However, if the shale was in an area that was exposed to higher pressures and moderate temperatures, it might have been transformed into schist. The clay in the shale could have been converted to mica, which gives schist its shiny look.Granite is a light-colored rock made of quartz, feldspars, mica, and small amounts of hornblende. The crystals of all these minerals are randomly arranged. Granite can be metamorphosed into a rock called gneiss (pronounced like "nice"). Gneiss has about the same mineral composition as granite, but the pressure of metamorphism causes the minerals to line up, giving gneiss a distinct banded appearance. Schist may also be converted into gneiss, if increased pressure and temperature is added. Metamorphic rocks are a mixed up group that have been under a lot of stress!
The metamorphic system can also react differently if fluids are part of the system. Serpentinite, a mottled green rock, is usually formed with high pressure and low temperature. The original rock could contain a large amount of olivine (i.e., basalt). The olivine (Mg2SiO4) reacts with water (H2O) to form the mineral serpentine (Mg3Si2O5(OH)4)plus magnesium oxide (MgO). Serpentinite is found in areas where faulting occurs. Along the San Andreas fault zone in California, serpentinite is so abundant it is recognized as the California state rock. The pressure of shearing seems to be ideal for the serpentinite to form a fibrous pattern. This form of serpentinite is a variety of asbestos, which is used as a fire retardant