Lecture Notes on Metamorphic Rocks

Introduction to Rock Types

• Three major groups of rocks:
  1. Igneous Rocks: Form when magma solidifies.
  2. Sedimentary Rocks: Formed by weathering and erosion of rocks and minerals near Earth's surface.
  3. Metamorphic Rocks: Focus of today's lesson.

Overview of Metamorphic Rocks

• Metamorphic rocks often host economic mineral deposits.
• Form bedrock under much of North America.
• Formation process: Physical or chemical changes of a rock type into another.
• Examples:
  • Limestone -> Marble
  • Granite -> Alignment of minerals
• Everyday analogy: Making toast is akin to metamorphism.

Learning Objectives

1. Understand how metamorphism occurs.
2. Identify examples of typical metamorphic rocks.

Process of Metamorphism

• Occurs in solid state after initial rock formation.
• Triggered by changing conditions:
  • Temperature
  • Pressure
  • Presence of hot fluids
• Everyday analogies:
  • Cooking food (e.g., baking pizza, toasting marshmallow).
• Temperature Window:
  • Between 200°C to 800°C.
  • Below Earth's surface - depths of several kilometers.
  • Near heat sources (e.g., magma chambers).
• Geothermal Gradient:
  • Necessary temperatures at depths of 7 km or more.

Types of Metamorphism

Contact Metamorphism

• Occurs when rocks are in contact with magma bodies.
• Very high temperatures (magma over 1200°C).

Regional Metamorphism

• Associated with plate boundaries (convergent and divergent).
• Convergent boundaries:
  • High temperature & pressure in over-riding plate.
  • Low temperature, high pressure in descending oceanic crust.
• Oceanic ridges:
  • High temperatures & circulating hot fluids.
• Results in distinct assemblage of metamorphic rocks.

Changes in Rocks During Metamorphism

• Shape change.
• Mineral growth or rotation.
• Formation of new minerals.
• Foliation:
  • Alignment of minerals perpendicular to pressure/stress.
  • Example: Balloon/marshmallow expansion analogy.

Identifying Metamorphic Rocks

• Foliated Rocks:
  • Form under regional metamorphism.
  • Examples: Granite transforming with mineral realignment.
• Non-Foliated Rocks Examples:
  • Shale progression: Shale -> Slate -> Phyllite -> Schist -> Gneiss.
  • Grain size increases with higher metamorphic grade.
  • Quartzite: Metamorphosed sandstone.
  • Marble: Metamorphosed limestone.

Conclusion

• Metamorphism results in compositional and textural changes in the solid state.
• Driven by temperature and pressure.
• Results in:
  • Foliated Rocks: Slate, Phyllite, Schist, Gneiss.
  • Non-Foliated Rocks: Marble, Quartzite.

Reflection

• Assess understanding of learning objectives:
  1. How metamorphism occurs.
  2. Identifying typical metamorphic rocks.