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,
is unstable at high temperatures and will decompose into lime (CaO) and
carbon dioxide (CO2).
These are the primary metamorphic rocks: