1. Starting Point: Pre-Existing Rocks
* Metamorphic rocks are not formed from magma or lava. They begin their journey as existing rocks:
* Igneous Rocks: Formed from cooled magma or lava (e.g., granite, basalt)
* Sedimentary Rocks: Formed from compressed and cemented sediments (e.g., sandstone, limestone)
* Other Metamorphic Rocks: Metamorphic rocks can even be transformed into other metamorphic rocks!
2. The Agents of Change
* Heat: The most common agent, provided by:
* Magma intrusions: Molten rock pushing into existing rock layers
* Deep burial: Pressure from overlying rocks generates heat
* Regional metamorphism: Heat associated with tectonic plate collisions
* Pressure: The weight of overlying rocks or tectonic forces squeeze the original rock. This can be:
* Confining Pressure: Equal pressure from all sides
* Differential Pressure: Unequal pressure, often due to tectonic forces
* Chemically Active Fluids: These can be present in the original rock or introduced by magma intrusions. They interact with the existing minerals to create new ones.
3. Transformation and Recrystallization
* Mineralogical Changes: The original minerals in the rock are unstable under the new conditions and begin to break down.
* Recrystallization: New minerals form, often larger and more organized than the original minerals. This is where the rock takes on its metamorphic characteristics.
* Texture Changes: The rock's original texture, like layering in sedimentary rocks, can be changed by pressure and heat. New textures, such as foliation (banding), may develop.
4. The End Result: A Metamorphic Rock
* Examples: Slate (from shale), marble (from limestone), gneiss (from granite or other metamorphic rocks)
Key Concepts to Remember
* No melting: Metamorphic rocks don't melt; they transform in the solid state.
* Solid-state transformation: Changes occur through rearrangement of atoms and minerals, not melting and reforming.
* Variety: A wide variety of metamorphic rocks exist, reflecting the diverse starting materials and conditions of formation.
Let me know if you'd like more detail on any specific aspect of metamorphic rock formation!
