1. The Starting Material: Foliated metamorphic rocks begin as either sedimentary or igneous rocks.
2. Pressure and Heat: These rocks are buried deep within the Earth's crust or are involved in mountain building processes. This causes them to experience high temperatures and immense pressure.
3. Mineral Alignment: The directed pressure forces the minerals within the rock to align themselves perpendicular to the direction of pressure. This alignment creates a distinct layering or banding called foliation.
4. Types of Foliation: The type of foliation depends on the intensity of the pressure and the minerals present. Some common types include:
* Schistosity: A distinct layered appearance with visible mineral grains.
* Gneissic banding: Alternating light and dark bands of different minerals.
* Slatey cleavage: A flat, layered appearance that allows the rock to break easily into thin sheets.
* Phyllitic texture: A silky, wavy appearance caused by fine-grained minerals.
Examples of foliated metamorphic rocks:
* Slate: Formed from shale under low-grade metamorphism.
* Schist: Formed from shale or other rocks under medium-grade metamorphism.
* Gneiss: Formed from igneous or sedimentary rocks under high-grade metamorphism.
* Phyllite: Formed from shale or other fine-grained rocks under moderate metamorphism.
Key takeaway: Foliated metamorphic rocks form when existing rocks are subjected to intense pressure and heat, causing the minerals to align in a specific direction, creating a layered appearance called foliation.
