Metamorphism is the change that takes place within a body of rock as a result of it being subjected to conditions that are different from those in which it formed. In most cases, but not all, this involves the rock being deeply buried beneath other rocks, where it is subjected to higher temperatures and pressures than those under which it formed. Metamorphic rocks typically have different mineral assemblages and different textures from their parent rocks, but they may have the same overall composition.
Thumbnail: Folded foliation in a metamorphic rock from near Geirangerfjord, Norway. Image used with permission (CC-SA-BY 3.0; Siim);