Which metamorphic facies is typically formed under moderate pressure and low temperature?

• A. Amphibolite
• B. Sanidinite
• C. Blueschist
• D. Greenschist

Answer: (D)

Metamorphic facies are defined by the mineral assemblages stable at specific P–T conditions. When rocks experience relatively low temperatures and low-to-moderate pressures, hydrous, clay- to mica-rich minerals become stable and recrystallize into greenschist facies. The typical mineral suite includes chlorite, epidote, albite, quartz, and often muscovite or serpentine, which gives the rocks a characteristic green tint. The textures are generally fine-grained and may show some foliation or cleavage from deformation.

This is the metamorphic zone you get at gentle metamorphism, before temperatures rise enough to shift into higher-grade facies. The other options correspond to different P–T realms: blueschist forms under high pressure and relatively low temperature, amphibolite forms at higher temperatures, and sanidinite occurs at very high temperatures.