Metamorphic Rock Grading Explained

Metamorphic rocks form when existing rocks change due to heat, pressure, or chemical reactions. Metamorphic grade indicates the intensity of these changes. Understanding this sequence is key for classification.

Rock Types and Metamorphic Grade Sequence

The rocks provided can be ordered based on their formation conditions, from low to high metamorphic grade:

• Shale (D): This is a sedimentary rock, the protolith (original rock) that undergoes metamorphism. It represents the lowest grade.
• Slate (A): Forms from shale under low-grade metamorphism. It's characterized by fine-grained texture and slaty cleavage.
• Phyllite (E): Represents an intermediate-grade metamorphism, evolving from slate. It has a characteristic silky sheen due to aligned mica crystals.
• Schist (C): Forms under medium- to high-grade conditions. Exhibits schistosity, where platy minerals (like micas) are visibly aligned.
• Gneiss (B): Represents high-grade metamorphism. Typically shows distinct compositional banding (gneissic banding).

Ordering Rocks: High to Low Grade

Arranging the given rock types from the highest metamorphic grade to the lowest:

1. Gneiss (B) - High Grade
2. Schist (C) - Medium to High Grade
3. Phyllite (E) - Medium Grade
4. Slate (A) - Low Grade
5. Shale (D) - Protolith (Lowest Grade)

Therefore, the correct sequence from high grade to low grade is B, C, E, A, D.