1. Foliation Development:
* Compressive Stress: When rocks are subjected to compressive stress, they tend to flatten and elongate in the direction perpendicular to the stress. This creates a preferred orientation of mineral grains, resulting in the formation of foliation.
* Types of Foliation: Different types of foliation develop depending on the intensity and duration of the stress:
* Slaty Cleavage: Fine-grained, closely spaced planes of weakness.
* Phyllitic Texture: Wavy, shiny surfaces due to the alignment of platy minerals like mica.
* Schistosity: Distinct layers of different mineral types.
* Gneissic Banding: Alternating layers of light and dark minerals.
* Shear Stress: Shear stress involves forces acting parallel to the rock's surface. This stress can also create foliation, but it tends to be more complex and irregular.
2. Mineral Alignment:
* Preferred Orientation: The stress applied during metamorphism forces minerals to align themselves in a specific orientation, maximizing their stability and minimizing their resistance to the stress.
* Platy Minerals: Minerals like mica and chlorite, which have a sheet-like structure, tend to align parallel to the direction of stress, leading to the development of foliation.
* Elongate Minerals: Minerals like hornblende and pyroxene, which are elongated, often align perpendicular to the direction of stress.
3. Mineral Growth and Recrystallization:
* Stress-Induced Recrystallization: Stress can encourage the recrystallization of minerals, leading to the formation of larger and more stable crystals. These crystals will often be oriented in a way that minimizes their resistance to the stress.
* Mineral Transformation: The stress can also lead to the transformation of some minerals into others, with the newly formed minerals having a preferred orientation.
4. Rock Structures:
* Folds: Compressive stress can cause rocks to bend and fold. The direction of the fold axis (the line along which the fold is most tightly curved) can provide information about the direction of the original stress.
* Faults: Shear stress can cause rocks to break and move along fractures called faults. The direction of fault displacement (the direction in which the rocks moved relative to each other) can also provide information about the direction of stress.
In Summary:
The direction of stress applied during metamorphism is a crucial factor in determining the texture and fabric of a metamorphic rock. By understanding the relationship between stress and mineral arrangement, geologists can interpret the history of deformation and the conditions under which metamorphic rocks were formed.
