Blueschist facies is associated with which P-T combination?

• A. high pressure and low temperature
• B. Low pressure high temperature
• C. Moderate pressure moderate temperature
• D. Very high pressure very high temperature

Answer: (A)

Blueschist facies form under high-pressure but relatively low-temperature conditions. This combination occurs in subduction zones, where cold oceanic crust is pushed to great depths while temperatures stay comparatively low. The minerals that stabilize under these conditions, such as glaucophane (blue) and lawsonite, give rocks a distinctive blue-schist character.

If the environment were high pressure and high temperature, you’d expect eclogite facies rather than blueschist. If the setting were low pressure and high temperature, metamorphism would be more like contact metamorphism (hornfels). Moderate pressure and temperature align with amphibolite facies, not blueschist.