Blueschist metamorphism is characterized by which combination of temperature and pressure?

• A. High temperature and high pressure
• B. Low temperature and low pressure
• C. Low temperature and high pressure
• D. High temperature and low pressure

Answer: (C)

Blueschist metamorphism forms where rocks are buried to great depths under high pressures but stay relatively cool in temperature. This happens in subduction zones, where the oceanic plate sinks rapidly and the surrounding rock is subjected to high pressure while the geothermal gradient remains low, so temperatures aren’t high. Under these conditions minerals such as glaucophane (the blue amphibole) and lawsonite stabilize, giving the characteristic blue color and defining blueschist facies. So the key T-P combo is low temperature with high pressure. If temperature were high at the same pressure, you’d get a different metamorphic grade (like eclogite); if both were low, greenschist facies would form.