Key Points on Metamorphic Rocks

Metamorphic Rocks

• Formed from pre-existing rocks (protoliths) via changes in temperature and pressure.

• Types of protoliths: Igneous, Sedimentary, Older Metamorphic.

• Comprise 27% of Earth's crust, 12% of land area, 20% of North America.

• Serve as records of Earth's history and sources of minerals/gems.

Metamorphic Environments

• Most metamorphism occurs at convergent plate margins.

• Types of metamorphism:

  • Contact Metamorphism: Near igneous intrusions, limited pressure changes.

  • Regional Metamorphism: In subduction zones; large scale, high pressure and temperature.

Processes of Metamorphism

• Solid-state transformations without melting; influenced primarily by heat.

• Reaction examples involve minerals reacting under temperature and pressure changes.

Temperature and Pressure

• Geothermal Gradient: Varies between 10-60 °C/km in crust.

• Pressure increases with depth; lithostatic pressure typically applies.

• Differential stress from plate collisions causes folding and mineral adjustments (foliation).

Foliation

• Foliation: Preferred mineral alignment under directed pressure.

• Types: Unfoliated vs. Foliated rocks; Foliated rocks have minerals aligned, impacting strength and stability.

Classification of Metamorphic Rocks

• Based on texture, environment, and composition:

  • Texture: Foliated (e.g. Gneiss, Schist) vs. Non-foliated (e.g. Quartzite, Marble).

  • Environment: Regional vs. Contact Metamorphism.

  • Composition: Based on protolith (e.g. Sandstone to Quartzite).

Index Minerals & Zones

• Index minerals indicate metamorphic grade (e.g. Chlorite, Biotite, Garnet).

• Zones reflect highest-ranking index minerals in rocks, useful in geological mapping.

Metamorphic Facies

• Sets of minerals that form under consistent temperature and pressure; helpful for assessing metamorphic conditions.

Resources from Metamorphic Rocks

• Valuable for construction materials (Marble, Slate), metals (Au, Zn, Cu), and gemstones (Garnets, Corundum).