Blueschist facies rocks form under metamorphic conditions characterized by which combination of temperature and pressure?

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

Answer: (D)

Blueschist facies form when rocks are subjected to high pressures while remaining relatively cool, a hallmark of subduction zone metamorphism. As oceanic lithosphere is pushed down into the mantle, the pressure increases rapidly, but the slab stays cold, so temperatures stay low compared with other metamorphic environments. This combination favors specific high-pressure, low-temperature mineral assemblages (such as glaucophane) that give blueschist its characteristic blue color.

So the defining condition is low temperature with high pressure. Scenarios with high temperature (even at high pressure) or low pressure lead to different metamorphic facies, not blueschist.