Metamorphic rocks are formed when other rocks (generally sedimentary, but potentially igneous or even older metamorphic rocks) are subjected to heat and pressure. This process changes the character of the initial rocks and minerals in distinctive ways. Extreme pressures compact the original rocks, often flattening crystals into distorted shapes. In some cases, the pressures can change minerals into other minerals with the same or different chemistry. High temperatures can encourage chemical changes and diffusion. Heat and pressure can also force some minerals to decompose and/or separate from the host rock. For example, calcite is unstable at high temperatures and will decompose into lime (CaO) and carbon dioxide (CO2).
These are the primary metamorphic rocks: