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Metamorphic Rock Classification

Metamorphic rocks form when pre-existing rocks protoliths are transformed by heat, pressure, or chemically active fluids without melting. Understanding metamorphism is key to interpreting tectonic history. Types of Metamorphism Regional Metamorphism - Large-scale, associated with mountain building - High temperature AND pressure - Produces foliated rocks - Found in orogenic belts Contact Metamorphism - Local, around igneous intrusions - High temperature, low pressure - Produces non-foliated rocks - Creates "metamorphic aureoles" Dynamic Metamorphism - Along fault zones - High pressure, lower temperature - Creates mylonites and fault breccias Foliated Metamorphic Rocks Foliation = parallel alignment of platy minerals or compositional banding | Rock | Grade | Protolith | Description | |------|-------|-----------|-------------| | Slate | Low | Shale | Dull, excellent cleavage, microscopic grains | | Phyllite | Low-Med | Shale | Silky sheen, wrinkled surfaces | | Schist | Medium | Shale/Basalt | Visible mica flakes, wavy foliation | | Gneiss | High | Various | Alternating light/dark bands | | Migmatite | Very High | Various | Partial melting, mixed igneous/metamorphic | Metamorphic Grade Sequence Shale → Slate → Phyllite → Schist → Gneiss → Migmatite Increasing temperature and pressure → Non-Foliated Metamorphic Rocks | Rock | Protolith | Key Features | |------|-----------|--------------| | Marble | Limestone | Crystalline calcite, fizzes in HCl | | Quartzite | Sandstone | Interlocking quartz, very hard | | Hornfels | Various | Fine-grained, massive, dark | | Serpentinite | Peridotite | Green, waxy, associated with ophiolites | | Soapstone | Various | Soft, talc-rich, soapy feel | Index Minerals Certain minerals only form at specific temperature-pressure conditions: - Chlorite, Muscovite : Low grade - Biotite, Garnet : Medium grade - Staurolite, Kyanite : Medium-high grade - Sillimanite : High grade These "index minerals" help determine metamorphic grade and P-T history.

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • How rocks are classified — U.S. National Park Service. Rock-group classification using composition, texture, and geologic processes.
  • Collecting rocks — U.S. Geological Survey. Field context, specimen observation, and responsible rock-collecting guidance.

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