1. Original Rock:
- Starts with a pre-existing rock (igneous, sedimentary, or even another metamorphic rock). This rock may have no layering or only a weak original layering.
2. Directed Pressure:
- The rock is subjected to intense pressure from one direction. This pressure is often uneven, causing the rock to deform and flatten.
3. Mineral Alignment:
- As the pressure increases, the minerals within the rock start to realign themselves perpendicular to the direction of the pressure. This alignment is driven by the tendency of minerals to minimize their surface energy under stress.
4. Foliation Formation:
- The aligned minerals create distinct planar structures within the rock, known as foliation. This can manifest in several forms:
- Schistosity: Distinct, parallel, and often wavy planes of platy minerals like mica and chlorite.
- Gneissic banding: Alternating bands of light and dark minerals.
- Slatey cleavage: Fine-grained, closely spaced planes that allow the rock to split into thin sheets.
- Phyllitic texture: Shiny, wavy, and wrinkled planes with a silky sheen.
5. Metamorphic Grade and Foliation:
- The intensity of the pressure and the temperature during metamorphism determine the degree of foliation.
- Low-grade metamorphism: Results in fine-grained, weakly developed foliation (like slatey cleavage).
- High-grade metamorphism: Leads to stronger, more pronounced foliation (like gneissic banding).
Key Points:
* Foliation is a key characteristic of metamorphic rocks.
* It develops in response to directed pressure, which causes mineral alignment.
* Foliation type and intensity reflect the conditions of metamorphism.
Examples:
* Slate: A fine-grained, low-grade metamorphic rock with slaty cleavage.
* Schist: A medium-grade metamorphic rock with schistosity.
* Gneiss: A high-grade metamorphic rock with gneissic banding.
