Metamorphic Rock Lesson: Formation, Types, and Characteristics
Lesson Overview
- Metamorphism: The Process of Change
- Types of Metamorphism
- Metamorphic Rock Classification
- Metamorphic Grade and Index Minerals
- Metamorphism and Plate Tectonics
- Economic Importance of Metamorphic Rocks
- Significance of Metamorphic Rocks in Earth's History
Metamorphic rocks are one of the three major rock groups, formed through the transformation of pre-existing rocks under heat, pressure, and chemically active fluids. Unlike igneous rocks, which form from magma, and sedimentary rocks, which form from deposited particles, metamorphic rocks do not melt but undergo recrystallization in the solid state.
This lesson explores metamorphic rock formation, classification, textures, common examples, and their relationship with tectonic processes.
Metamorphism: The Process of Change
What Causes Metamorphism?
Metamorphism occurs due to three main factors:
- Heat: Drives chemical reactions, increases mineral size, and enables recrystallization.
- Pressure: Increases with depth, leading to deformation, alignment of minerals, and sometimes new mineral formation.
- Fluids (Water & Gases): Promote mineral growth and accelerate chemical reactions.
These three agents work together, altering a rock's mineralogy, texture, and structure.
Parent Rocks and Metamorphism
Any rock type-igneous, sedimentary, or even another metamorphic rock-can become a metamorphic rock if subjected to enough heat and pressure.
For example:
- Shale (sedimentary) → Slate (metamorphic)
- Limestone (sedimentary) → Marble (metamorphic)
- Granite (igneous) → Gneiss (metamorphic)
Metamorphic rocks retain similar overall chemistry to their parent rocks but develop new textures and minerals due to recrystallization.
Types of Metamorphism
Different geological conditions give rise to different types of metamorphism:
| Type of Metamorphism | Cause | Typical Rocks Formed |
|---|---|---|
| Contact Metamorphism | Heat from nearby magma | Marble, Quartzite, Hornfels |
| Regional Metamorphism | High pressure and temperature over large areas (mountain-building) | Slate, Schist, Gneiss |
| Dynamic Metamorphism | Intense pressure along fault zones | Mylonite |
| Burial Metamorphism | Deep burial under sediment | Low-grade slates and phyllites |
| Hydrothermal Metamorphism | Hot, mineral-rich fluids alter rocks | Serpentinite, Skarn |
| Shock Metamorphism | Sudden impact (e.g., meteorite) | Impactites |
Among these, regional metamorphism is the most widespread, occurring at convergent plate boundaries where large-scale tectonic collisions generate heat and pressure.
Metamorphic Rock Classification
Metamorphic rocks are classified into foliated and non-foliated types based on texture.
Foliated Metamorphic Rocks
These have a banded or layered appearance due to the alignment of minerals under directed pressure.
| Rock Type | Parent Rock | Texture | Formation Conditions |
|---|---|---|---|
| Slate | Shale | Fine-grained, slaty cleavage | Low-grade metamorphism |
| Phyllite | Slate | Silky sheen, wavy layers | Low to medium grade |
| Schist | Shale, Igneous | Visible mica flakes, foliation | Medium-grade |
| Gneiss | Schist, Granite | Alternating light/dark bands | High-grade |
These rocks form in mountain-building events where compression forces minerals into parallel layers.
Non-Foliated Metamorphic Rocks
Non-foliated rocks lack banding because they contain minerals that do not align under pressure.
| Rock Type | Parent Rock | Texture | Formation Conditions |
|---|---|---|---|
| Marble | Limestone | Crystalline, smooth | Contact/Regional |
| Quartzite | Sandstone | Hard, glassy | Contact/Regional |
| Hornfels | Shale, Basalt | Dense, fine-grained | Contact |
| Anthracite (Coal) | Bituminous Coal | Hard, shiny black | Burial/Regional |
Marble and quartzite are widely used in construction due to their durability and aesthetic appeal.
Metamorphic Grade and Index Minerals
Metamorphic grade refers to the intensity of metamorphism.
- Low-grade: Slate, phyllite (formed at 200–400°C)
- Medium-grade: Schist, garnet-bearing rocks (formed at 400–600°C)
- High-grade: Gneiss, sillimanite-bearing rocks (formed at 600–800°C)
Index Minerals
Certain minerals indicate the temperature and pressure at which a rock formed.
| Mineral | Metamorphic Grade | Typical Rock |
|---|---|---|
| Chlorite | Low | Slate |
| Biotite | Low to Medium | Phyllite, Schist |
| Garnet | Medium | Schist |
| Staurolite | Medium | Schist |
| Kyanite | Medium to High | Schist, Gneiss |
| Sillimanite | High | Gneiss |
For example, if a rock contains biotite, garnet, and staurolite, it is intermediate-grade.
Metamorphism and Plate Tectonics
Metamorphic rocks form in specific plate tectonic settings:
- Convergent Boundaries: Regional metamorphism (e.g., Himalayas)
- Subduction Zones: High-pressure metamorphism (e.g., Blueschist)
- Fault Zones: Dynamic metamorphism (e.g., Mylonite)
- Volcanic Arcs: Contact metamorphism near magma (e.g., Hornfels)
Metamorphic rocks are not common in ocean basins but are widespread in mountain belts and ancient continental shields.
Economic Importance of Metamorphic Rocks
Metamorphic processes concentrate valuable minerals, making them economically important.
| Resource | Associated Metamorphic Process |
|---|---|
| Tin, Tungsten, Copper | Contact metamorphism & hydrothermal fluids |
| Gold | Regional metamorphism |
| Gemstones (Garnet, Ruby, Sapphire) | Regional metamorphism |
| Marble, Slate, Quartzite | Construction materials |
For example, tin and tungsten deposits are often found in contact metamorphic zones.
Significance of Metamorphic Rocks in Earth's History
Metamorphic rocks preserve records of deep geological processes:
- Oldest Rocks on Earth: Some of the oldest known rocks (e.g., Acasta Gneiss, 4 billion years old) are metamorphic.
- Indicators of Tectonic Activity: The presence of schist, gneiss, or blueschist suggests past mountain-building or subduction.
- Uplift and Erosion: Once buried deep, these rocks reach the surface due to tectonic uplift and erosion.
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