Metamorphic rock is formed underground through a process that changes a rock's molecular structure due to pressure, heat and chemical reactions. A metamorphic rock forms from a parent rock called a protolith. Depending on conditions, a protolith can transform into any metamorphic rock. Because protoliths are capable of undergoing vast changes, identifying them is sometimes difficult for geologists.
Metamorphic rocks can only be formed when a rock's physical and chemical composition is altered without the parent rock melting. The conditions the rocks are exposed to determine the exact chemical and mineral composition of the resulting metamorphic rock.
Under extreme pressures, such as between two colliding tectonic plates, the minerals of a metamorphic rock group together and align to form foliation, which appears as stripes in the rock. One example of a heavily foliated rock is gneiss.
Alternatively, highly heated areas, such as near magma chambers, produce vastly different metamorphic rocks. One example is hornfels.
Another area for metamorphism is at a subduction zone where oceanic plates collide with and bend under continental plates. Because these high-pressure areas are near the ocean, they are cooler and produce different metamorphic results. One example of this is the creation of a blue mineral called glaucophane. This mineral in the rock foliates from high pressure and creates blueschist, a blue-tinted version of schist.
A protolith may change a number of times before reaching its final metamorphic stage. For example, gneiss may begin as shale that turns into slate, phyllite, schist and finally gneiss.