Lecture 12: Metamorphic Rocks

Metamorphic Rocks

• Definition: Metamorphic rocks form from parent rocks (protoliths) due to heat and/or pressure.

Metamorphism

• Temperature Range: 200°C to 850°C.

  • Influenced by geothermal gradient and contact with magma.

  • Types:

  • Contact Metamorphism: heat with minimal or no pressure.

  • Burial Metamorphism: both heat and pressure due to depth of burial.

  • Regional Metamorphism: heat/pressure due to tectonic processes, primarily in convergent zones.

Grades of Metamorphism

• Low Grade: low temperature and/or pressure (e.g., Slate from Shale).

• Intermediate Grade: moderate temperature and/or pressure (e.g., Phyllite).

• High Grade: high temperature and/or pressure (e.g., Gneiss from Granite).

Classification of Metamorphic Rocks

• Changes in texture and mineralogy due to increasing temperature and pressure.

• Common Protoliths:

  • Granite, Shale, Quartz Sandstone, Limestone/Dolostone, Mafic Igneous Rocks (e.g., Gabbro).

Metamorphic Processes

• Recrystallization: minerals change size/shape without melting.

• Foliation: alignment of minerals due to differential stress.

• Mineral Growth: new minerals form and existing minerals may separate.

Common Metamorphic Rocks

• Low Grade:

  • Slate (from Shale).

  • Phyllite (recrystallized clay minerals into mica).

• Medium Grade:

  • Mica Schist (large foliated micas).

• High Grade:

  • Gneiss (banded, felsic and mafic minerals).

Non-Foliated Metamorphic Rocks

• Marble (from Limestone).

• Quartzite (from Sandstone).

Key Points

• Metamorphic minerals form based on specific temperature and pressure conditions.

• Unique geological processes like subduction impact metamorphism significantly.

• Geological events such as continental collisions lead to further metamorphism of various rock types.