---
title: "Short note: Rock cleavage"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:short-note-rock-cleavage"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/short-note-rock-cleavage
site: BCT Engineering
---

# Short note: Rock cleavage

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2078 Bhadra, 4 marks; 2076 Chaitra, 2 marks; 2076 Asoj, 4 marks; 2069 Chaitra, 2 marks; 2065 Shrawan (old course), 2 marks (asked 5 times)
- Video: [Cleavage and Fracture](https://www.youtube.com/watch?v=dZ1u-zJxyVo) — TMartScience
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/short-note-rock-cleavage
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Write a short note on rock cleavage.

## Short answer

Rock cleavage is a secondary structure in metamorphic rocks formed by directional pressure, causing closely spaced parallel parting planes. Platy minerals align perpendicular to stress, producing slaty, fracture, or strain-slip types. It creates planes of weakness, affecting slope stability, excavation, and water seepage.

## Answer

**Rock cleavage** is the property of a rock to split easily along closely spaced, parallel planes, producing thin sheets or slabs. It is a secondary structure, developed in a rock after its formation, usually in fine-grained metamorphic rocks by strong directional pressure. It should not be confused with mineral cleavage, which depends on the atomic structure of a single mineral.

#### Formation
Under compression, platy minerals (mica, chlorite) and flattened grains reorient perpendicular to the direction of maximum stress, so the rock splits parallel to these planes, which are unrelated to the original bedding.

#### Types
- **Slaty cleavage**: very fine, perfectly parallel planes in slate; slate splits into thin slabs.
- **Fracture (shear) cleavage**: closely spaced parallel fractures cutting the rock into thin slices.
- **Strain-slip (crenulation) cleavage**: fine folding of an earlier cleavage.
- Schistosity (coarser, flaky) in schists and **gneissic banding** are related foliations.

```text
 Bedding   ====== ====== ======
 Cleavage   / / / / / / / /  (cuts across bedding)
```

#### Engineering significance
- Cleavage planes are planes of weakness along which the rock splits easily, so the rock's strength depends on the direction of loading.
- It decides stability of slopes, excavations and tunnel roofs (planar failure along cleavage).
- Useful for obtaining roofing slate and flagstone, but poor for aggregate and foundations.
- Allows water to seep, causing leakage and weathering.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Rock cleavage is best described as
   A. a secondary structure that splits the rock along closely spaced parallel planes
   B. the splitting of a single mineral along its atomic planes
   C. the layering formed during deposition of sediment
   D. a primary structure formed as magma cools
   Answer: A. Rock cleavage is a secondary structure formed after the rock, unlike mineral cleavage which depends on atomic structure. (Key point: Nature of rock cleavage)

2. How does cleavage develop under compression?
   A. Grains align parallel to the maximum stress
   B. Platy minerals and flattened grains reorient perpendicular to the maximum stress
   C. Minerals melt and recrystallise as glass
   D. Sediment is deposited in thin layers
   Answer: B. Reorientation perpendicular to maximum stress creates planes along which the rock splits. (Key point: Formation)

3. How are cleavage planes related to the original bedding?
   A. They always coincide with bedding
   B. They are always parallel to bedding
   C. They are unrelated and may cut across it
   D. They exist only in sedimentary rocks
   Answer: C. Cleavage is produced by pressure and is independent of bedding. (Key point: Relation to bedding)

4. What is the engineering significance of cleavage?
   A. It makes the rock ideal for aggregate and foundations
   B. It makes the rock impermeable
   C. It increases strength across the planes
   D. Weak planes reduce strength and control stability of slopes and tunnel roofs
   Answer: D. Cleavage planes are weak and permit planar failure and seepage, though they give roofing slate. (Key point: Engineering effect)

## Related in Engineering Geology I

- [Texture of sedimentary rocks and rock cleavage](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atexture-of-sedimentary-rocks-and) — Engineering Geology I, chapter 4
- [Metamorphism and rock cleavage significance](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ametamorphism-and-rock-cleavage-significance) — Engineering Geology I, chapter 4
- [Rock cleavage: types and significance](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-cleavage-types-and-significance) — Engineering Geology I, chapter 4
- [Metamorphic rock, cleavage; igneous rock significance](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ametamorphic-rock-cleavage-igneous-rock) — Engineering Geology I, chapter 4