Model Answer
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Rock cleavage refers to the tendency of a rock to split along closely spaced parallel planes. It’s a fundamental structural feature developed due to directed pressure during metamorphic events, or sometimes during deformation of sedimentary rocks. While the *cause* of cleavage is important, morphological classification focuses on *how* it manifests – the geometry and characteristics of the cleavage planes themselves. Understanding these morphological variations provides insights into the stress history and deformation mechanisms experienced by the rock. This note will detail the primary morphological classifications of rock cleavage, outlining their defining features.
Morphological Classification of Rock Cleavage
Cleavage is categorized based on several morphological characteristics. These include the orientation of cleavage planes, their spacing, their persistence, and their relationship to other rock features.
1. Parallel Cleavage
This is the most common type of cleavage. Cleavage planes are uniformly oriented throughout the rock mass. It’s often observed in fine-grained metamorphic rocks like slates. The planes are generally closely spaced, resulting in a fissile texture.
- Characteristics: Uniform orientation, closely spaced planes, slatey texture.
- Formation: Typically develops under conditions of regional metamorphism with directed pressure.
- Example: Phyllite exhibits a sheen due to the alignment of mica minerals along cleavage planes.
2. Crenulation Cleavage
Crenulation cleavage is characterized by small, wave-like or curved cleavage planes. These planes are often associated with micro-folding within the rock.
- Characteristics: Curved or wavy cleavage planes, micro-folds, often seen in rocks that have undergone multiple deformation events.
- Formation: Develops during deformation where the rock is subjected to changing stress orientations.
- Example: Observed in moderately metamorphosed sedimentary rocks where earlier cleavage has been refolded.
3. Spaced Cleavage
In spaced cleavage, cleavage planes are discontinuous and separated by areas of rock with no cleavage. This creates a patchy appearance.
- Characteristics: Discontinuous cleavage planes, areas of unaffected rock, often associated with localized deformation.
- Formation: Develops in rocks with pre-existing compositional variations or inhomogeneities.
- Example: Common in shales and siltstones where clay mineral alignment is not uniform.
4. Closely Spaced Cleavage
This type is similar to parallel cleavage but with even closer spacing between planes. The rock may appear almost laminated.
- Characteristics: Very closely spaced, parallel planes, laminated appearance, high degree of foliation.
- Formation: Indicates intense deformation and recrystallization.
- Example: Some schists exhibit closely spaced cleavage due to the alignment of platy minerals.
5. Intersection Cleavage (Composite Cleavage)
This occurs when two or more cleavage sets intersect within the rock. The resulting pattern is more complex than simple parallel cleavage.
- Characteristics: Two or more intersecting cleavage sets, creating a polygonal or rhombic pattern.
- Formation: Develops in rocks subjected to multiple deformation events with different stress orientations.
- Example: Common in rocks from complexly deformed orogenic belts.
| Cleavage Type | Characteristics | Formation | Example |
|---|---|---|---|
| Parallel | Uniformly oriented, closely spaced | Regional metamorphism | Slate |
| Crenulation | Curved, wavy, micro-folds | Multiple deformation events | Refolded sedimentary rocks |
| Spaced | Discontinuous, patchy | Localized deformation | Shale |
| Intersection | Intersecting sets | Multiple stress orientations | Complexly deformed rocks |
Conclusion
The morphological classification of rock cleavage provides a valuable tool for understanding the deformation history of rocks. By carefully observing the orientation, spacing, and persistence of cleavage planes, geologists can infer the stress conditions and deformation mechanisms that acted upon the rock. Recognizing these different cleavage types is crucial for interpreting regional geological structures and understanding the evolution of mountain belts and other deformed terrains. Further research into the microstructural features associated with each cleavage type can provide even more detailed insights into the processes involved.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.