Ultrahigh-pressure (UHP) metamorphism occurs when rocks are subjected to pressures greater than 2.5 GPa, often during continental collision. UHP metamorphic rocks, such as coesite-bearing eclogite, are significant in geology for providing evidence of deep subduction and the dynamics of crustal processes.
Reference: Chopin, C. (2003). “Ultrahigh-Pressure Metamorphism: Tracing Continental Crust into the Mantle.” Earth and Planetary Science Letters, 212(1-2), 1-14.