ERTH 101: Exploring Planet Earth - Week 7: Metamorphic Rocks

Metamorphic Rocks: Form when a preexisting rock (protolith) undergoes a solid-state change in response to temperature, pressure, and chemical environment at depth in the Earth.
Metamorphism Definition: The process of metamorphosis, which includes physical and chemical changes in the rock.

Lithification Temperature Range: Typically occurs between roughly 150-300 degrees Celsius.
- Reasoning for this Range:
- Below 150 °C, reactions necessary for cement formation are too slow.
- Above 300 °C, significant changes lead to melting, forming igneous rocks.
Geothermal Gradient: Approximately 25-30 °C/km.
- Calculation for Depth to Reach 300 °C:
- ext{Depth} ext{ (km)} ext{ ≈ } rac{300 °C}{30 °C/km} ≈ 10 ext{ km}
Metamorphism Influences: Driven by high temperature and high pressure, ensuring rocks do not reach melting temperatures.

Preview of Metamorphic Origins: Various environments lead to different metamorphic processes.
- High Pressure vs. Low Pressure, High Temperature vs. Low Temperature change the rock characteristics depending on the conditions.

Main Types of Metamorphism:
- Hydrothermal, Subduction zone metamorphism, Contact metamorphism, and Regional metamorphism.
Facies Types:
- Zeolite Facies: Low T, low P.
- Blueschist Facies: Low T, high P.
- Eclogite Facies: High T, high P.
- Granulite Facies: High T, high P.

Key Processes: Based on mineral changes and textural alterations.
- Recrystallization: Same mineral, different shape/size (e.g., limestone to marble).
- Phase Changes: Transformation of one mineral into another with the same chemical composition but different structures.
- Neocrystallization: New minerals grow from the old ones leading to significant textural and compositional shifts.
- Pressure Solution: Dissolving and reprecipitation happening under high stress conditions.
- Plastic Deformation: Change in shape when subjected to high temperature and pressure.

Protolith Definition: The precursor to a metamorphic rock.
Foliation: Parallel alignment of platy minerals resulting in a planar structure (e.g., schist, gneiss).
- Types of Foliated Rocks:
- Slate, Phyllite, Schist, Gneiss: Each representing increasing metamorphic grade and mineral alignment.

Influencing Factors:
- Mineral stability depends on temperature, pressure, and composition.
- Metasomatism: Alteration through hydrothermal fluids which provide ions and molecules changing the rock's composition.

General Classification of Metamorphic Grade:
- Low Grade: 250-400 °C, Intermediate Grade: 400-600 °C, High Grade: 600 °C +.
Example of Prograde Metamorphism of a Pelitic Rock:
- Low Grade: Shale to slate (recrystallization of clays).
- Intermediate Grade: Slate to schist (growth of micas).
- High Grade: Schist to gneiss (neocrystallization).

Use of Index Minerals: Minerals indicating the metamorphic conditions of temperature and pressure experienced by the rock (e.g., kyanite, garnet).
Geothermobarometry: Using minerals to estimate the metamorphic grade based on pressure-temperature conditions.

Types of Environments for Metamorphism:
- Contact Metamorphism: Proximity to intrusions (high T, low P).
- Regional Metamorphism: Involves broad regional changes (low T, high P).
- Dynamic Metamorphism: Shear stress along faults.
- Hydrothermal Metamorphism: Interaction with heated fluids.
- Shock Metamorphism: Caused by sudden high-pressure events (e.g., meteorite impacts).

Metamorphic rocks arise from the transformation of existing rocks under varied temperature and pressure conditions, leading to different textures and compositions depending on the protolith and the metamorphic process involved.