---
title: "Metamorphism and rock cleavage significance"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:metamorphism-and-rock-cleavage-significance"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/metamorphism-and-rock-cleavage-significance
site: BCT Engineering
---

# Metamorphism and rock cleavage significance

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2062 Baisakh (old course), 1+1+3+3 marks
- Video: [GLG310 Lineation Foliation Cleavage Lecture](https://www.youtube.com/watch?v=lYkrb5vjvTA) — Ramon Arrowsmith
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/metamorphism-and-rock-cleavage-significance
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Define metamorphism and metamorphic rock. Describe the different types of rock cleavage found in metamorphic rocks. Give their engineering significance.

## Short answer

Metamorphism is solid-state change by heat, pressure, and fluids, producing metamorphic rock. Rock cleavage types include slaty, fracture, and flow (schistosity). Engineering concerns: weak planes cause slope failures when dipping out, tunnel overbreak, leakage pathways, but also ease of quarrying slate for roofing.

## Answer

#### Metamorphism and metamorphic rock
**Metamorphism** is the change in mineralogy, texture and structure of a pre-existing rock in the solid state, caused by heat, pressure and chemically active fluids. A **metamorphic rock** is the rock produced by this process, for example marble from limestone, slate from shale.

#### Types of rock cleavage
**Rock cleavage** is the property of a metamorphic rock to split along closely spaced parallel planes.
1. **Slaty cleavage** - very fine, perfectly parallel planes; rock splits into thin sheets (slate). Caused by parallel alignment of microscopic mica and chlorite perpendicular to the maximum stress.
2. **Fracture (strain-slip) cleavage** - closely spaced fractures along which small movement has taken place; the rock is crossed by parallel cracks.
3. **Flow cleavage** - arrangement of flattened grains and minerals produced by plastic flow in the rock (schist, phyllite).
4. **Schistosity** - coarse foliation of platy minerals in schist (sometimes treated as a separate type).

#### Engineering significance
- **Strength**: rock is weak along cleavage planes and strong across them, so strength is anisotropic.
- **Slopes**: cleavage dipping out of a cut slope gives sliding planes (landslides, rock slides).
- **Tunnels and excavation**: overbreak, roof falls, and squeezing along cleavage planes; tunnels should be aligned across the cleavage strike.
- **Seepage**: cleavage planes allow water to pass, so leakage under dams and reservoirs may take place.
- **Useful aspect**: slate cleavage gives roofing slabs and flooring stone that are easily quarried.

## Check yourself

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

1. Metamorphism is change in what?
   A. Only colour
   B. Rock into magma
   C. Sediment into rock
   D. Mineralogy, texture and structure of a pre-existing rock in solid state
   Answer: D. Heat, pressure and fluids act. (Key point: Metamorphism)

2. What causes slaty cleavage?
   A. Alignment of microscopic mica and chlorite perpendicular to maximum stress
   B. Alignment parallel to maximum stress
   C. Cementation
   D. Cooling
   Answer: A. Parallel alignment gives thin sheets. (Key point: Slaty cleavage cause)

3. When does cleavage destabilise a cut slope?
   A. When it dips into the hill
   B. When it dips out of the slope
   C. When horizontal only
   D. Never
   Answer: B. It provides a sliding plane. (Key point: Slope significance)

4. How is strength related to cleavage planes?
   A. Equal in all directions
   B. Strong along, weak across
   C. Weak along, strong across: anisotropic
   D. Zero everywhere
   Answer: C. Planes are weak. (Key point: Strength)

## Related in Engineering Geology I

- [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
- [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
- [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
- [Engineering significance of three rock types](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-significance-of-three-rock) — Engineering Geology I, chapter 4, asked 5×
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## Related in other subjects

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