Metamorphic Rocks – Quick Review Notes

Characteristics & Formation of Metamorphic Rocks

• Form from pre-existing igneous, sedimentary, or metamorphic rocks altered by heat, pressure, or chemically active fluids (agents of metamorphism)
• Resulting rocks are harder, more compact, with recrystallized minerals and increased resistance to weathering

Metamorphism (Process)

• Definition: transformation of existing rock into a new rock with different minerals/textures
• Requirements: long-term exposure to elevated temperature, pressure, or both

Types of Metamorphism

• Regional Metamorphism
  • Occurs during deep burial and tectonic compression
  • Produces large, high-grade metamorphic zones over wide areas
• Contact Metamorphism
  • Occurs where hot magma/lava heats surrounding rocks at relatively low pressure
  • Produces narrow, baked zones around intrusions or flows

Metamorphic Grade (Regional Sequence)

• Increasing T & P sequence for shale protolith:
  • Slate → Phyllite → Schist → Gneiss → Migmatite (highest grade)
• Blueschist represents high-pressure, relatively low-temperature conditions (subduction zones)

Foliation

• Planar alignment of minerals or structures produced by directed pressure
• Classification:
  • Foliated rocks (layered/banded appearance)
  • Non-foliated rocks (massive, granular texture)

Foliated Rocks (in order of increasing metamorphism)

• Slate: very fine, slaty cleavage, splits in sheets
• Phyllite: fine, satin sheen, wrinkled cleavage
• Schist: medium-coarse, shiny muscovite/biotite, schistosity
• Gneiss: coarse, alternating light & dark bands (gneissic banding)

Non-Foliated Rocks

• Marble: recrystallized calcite; protolith = limestone; $ext{Hardness} \approx 3$; effervesces in dilute $\text{HCl}$
• Quartzite: interlocking quartz grains; protolith = sandstone; $ext{Hardness} \approx 7$
• Hornfels: fine-grained, various colors; protolith = shale/basalt
• Anthracite: glossy, low-density coal metamorph; protolith = bituminous coal

Examples Summary

• Foliated: Slate, Phyllite, Schist, Gneiss
• Non-Foliated: Marble, Quartzite, Hornfels, Anthracite

Rock Cycle Context

• Weathering → Transportation → Deposition → Lithification → Sedimentary rocks
• Burial/tectonics → Metamorphism → Metamorphic rocks
• Melting → Magma → Crystallization → Intrusive/Extrusive igneous rocks
• Uplift & exposure recycle materials through the cycle

Key Points for Quick Recall

• $2$ main metamorphism types: Regional (high $P$ & $T$) vs Contact (high $T$, low $P$)
• Metamorphic grade increases with both $T$ and $P$; foliation becomes coarser and minerals grow larger
• Non-foliated rocks usually form from uniform composition protoliths (e.g., limestone → marble, sandstone → quartzite)
• Foliation indicates differential stress; absence suggests uniform stress or homogeneous mineralogy