Lecture 2.5 Metamorphic rocks

Introduction to Metamorphic Rocks

• Definition: Metamorphic rocks are one of the three categories of rocks in the rock cycle, characterized by their formation through the alteration of existing rocks via temperature, pressure, and fluids.

• Etymology: The term comes from "meta" (change) and "amorphous" (form).

• Formation: Both igneous and sedimentary rocks can undergo metamorphism and transform into metamorphic rocks.

The Rock Cycle

• Metamorphism is part of the rock cycle, indicating how different rock types can evolve into one another.

• Metamorphic rocks can also undergo further changes, leading to remetamorphosis.

• Significance: Metamorphic rocks serve as a record of geological processes and past tectonic motions that shaped the Earth.

Understanding Metamorphism

• Definition: Metamorphism is the process wherein solid rock changes in composition or texture without melting.

• Parent Rock (Protolith): The original rock that undergoes metamorphism.

• Location: Most metamorphic activity occurs deep within the Earth's crust.

Agents of Metamorphism

• Temperature: Generally begins around 150°C and increases with depth.

• Pressure Types:

  • Confining Pressure: Equal pressure from all directions, often due to overlying sediment.

  • Directed Stress (Differential Stress): Unequal pressure, typically found at convergent plate boundaries. This pressure changes the rock structure more effectively than confining pressure.

• Chemically Reactive Fluids: Play a significant role by altering mineral chemistry. Commonly comprised of water and carbon dioxide.

Metamorphic Rock Characteristics

• Texture Change: Metamorphic rocks can exhibit significant changes in mineral arrangement and texture due to heat and pressure.

• Chemical Composition: Typically remains stable, despite changes in mineralogy.

• Examples of Textures:

  • Foliated: Mineral grains are aligned due to directed stress (e.g., gneiss).

  • Non-foliated: Minerals lack preferred orientation (e.g., quartzite, marble).

Mechanisms of Metamorphism

• Grain Orientation: During metamorphism, grains in the rock can realign, reflecting the stress conditions:

  • Original random orientation to an aligned structure.

• Slip within Mineral Crystals: Atoms slide past one another within a mineral due to stress.

• Pressure Solution: Some minerals dissolve under pressure and redeposit elsewhere, altering shape.

Case Studies of Metamorphism

• Example: Metaconglomerate formed under confining pressure versus differential stress.

• Indicators of stress conditions can be observed in the alignment of mineral grains.

Hydrothermal Metamorphism

• Definition: Involves the alteration of minerals due to the interaction with hot, chemically reactive fluids, often from magma.

• Mechanisms: Can include the removal of materials from the parent rock during seawater circulation.

• Black Smokers: Hydrothermal vents that release mineral-rich water into the ocean.

Foliation and Lineation

• Definitions:

  • Foliation: Aligned minerals creating planar structures.

  • Lineation: Minerals aligned in a linear manner.

• Examples of minerals exhibiting these characteristics include micas for foliation and hornblende for lineation.

• Interaction: Rocks may display both foliation and lineation under complex stress conditions.

Metamorphic Grades

• Definition: Describes the extent of metamorphic change a rock has undergone,

  • Low Grade: Begins with rocks just above sedimentary conditions (e.g., shale to slate).

  • High Grade: Experienced significant changes (e.g., gneiss).

• Index Minerals: Specific minerals that form at known temperature and pressure conditions provide insight into metamorphic history.

Metamorphic Facies and Processes

• Facies: Groups of minerals stable at specific temperature and pressure ranges.

• Contact Metamorphism: Occurs when high-temperature magma intrudes rocks, leading to low pressure conditions.

• Regional Metamorphism: Results from large-scale tectonic processes, typically at convergent boundaries where rocks can be buried.

Summary

• The study of metamorphic rocks reveals significant geological history.

• Understanding the variables of metamorphism, texture, and processes helps in the identification of metamorphic environments and rock types.