1) Heat: Metamorphic rocks are formed when pre-existing rocks are subjected to high temperatures. This heat can come from a variety of sources, such as the intrusion of magma, the collision of tectonic plates, or the geothermal gradient.

2) Pressure: Metamorphic rocks are also formed when pre-existing rocks are subjected to high pressures. This pressure can come from the weight of overlying rocks, the compression of tectonic plates, or the growth of new minerals.

3) Chemical Reactions: Metamorphic rocks can also be formed when pre-existing rocks undergo chemical reactions. These reactions can be caused by the presence of water, carbon dioxide, or other fluids.

4) Time: Metamorphic rocks take a long time to form. The process can take millions or even billions of years.