Metamorphic rocks are transformed from existing rocks (either igneous, sedimentary, or even other metamorphic rocks) through a process called metamorphism.
Understanding Metamorphism
Metamorphism occurs when a rock is subjected to significant changes in its environment, specifically:
- Extreme Pressure: Deep burial under layers of sediment or tectonic forces can generate immense pressure.
- High Temperature: Increased temperature, often due to proximity to magma or geothermal gradients within the Earth, provides the energy needed for chemical reactions.
- Chemically Active Fluids: The presence of hot, chemically reactive fluids can alter a rock's mineral composition.
These conditions cause the original rock's minerals to recrystallize or change into new minerals that are more stable under the new conditions. The rock doesn't melt; it remains a solid throughout the process.
The Transformation Process
The exact transformation pathway depends on the original rock type (called the protolith) and the specific metamorphic conditions:
-
Increased Pressure: Pressure compacts the rock, reducing pore space and aligning minerals in a preferred orientation, creating a foliated texture (e.g., slate, schist, gneiss).
-
Increased Temperature: Heat provides the energy for chemical reactions to occur. New, more stable minerals form. The higher the temperature, the more dramatic the change.
-
Chemical Changes: Hot fluids can transport ions into and out of the rock, changing its chemical composition. This is common around magma intrusions or along fault lines.
What Happens After Metamorphism?
The metamorphic rock can undergo further changes:
- Continued Metamorphism: If the metamorphic rock is subjected to even higher pressures and temperatures, it can transform into a different type of metamorphic rock.
- Melting: If the temperature gets high enough, the metamorphic rock will melt, forming magma. Upon cooling and solidification, this magma becomes an igneous rock.
- Weathering and Erosion: Metamorphic rocks exposed at the Earth's surface are subject to weathering and erosion, eventually breaking down into sediments that can form sedimentary rocks.
Examples of Metamorphic Transformations
| Protolith (Original Rock) | Metamorphic Rock | Metamorphic Conditions |
|---|---|---|
| Shale | Slate | Low-grade metamorphism |
| Granite | Gneiss | High-grade metamorphism |
| Limestone | Marble | Variable, often moderate |
| Sandstone | Quartzite | Variable, often moderate |
