---
title: "Petrology; engineering significance of rock classes"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:petrology-engineering-significance-of-rock"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/petrology-engineering-significance-of-rock
site: BCT Engineering
---

# Petrology; engineering significance of rock classes

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2080 Baisakh, 4 marks
- Video: [Petrology 101: From Microscopic to Macroscopic Rock Analysis](https://www.youtube.com/watch?v=5TSJRiI92Ak) — Professional Geology Club
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/petrology-engineering-significance-of-rock
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

What is petrology? Describe the engineering significance of the metamorphic and sedimentary rock classes.

## Short answer

Petrology studies rock origin, composition, and classification. Metamorphic massive rocks are strong, but foliated ones are weak along planes; sedimentary sandstone strength depends on cement, limestone may leak, shale is weak and swells, and bedding dip controls stability.

## Answer

**Petrology** is the branch of geology that deals with the origin, occurrence, composition, structure, texture and classification of rocks.

#### Engineering significance of metamorphic rocks
- **Massive types**: quartzite, marble and gneiss are strong and durable; they are used as aggregate and as building and decorative stones, and are good foundation rocks.
- **Foliated types**: slate, phyllite and schist split along foliation planes; strength is low along these planes, so slopes, tunnels and foundations can fail. Slate is used for roofing.
- Mica- and talc-rich rocks are slippery and weak.
- Weathering of schists produces thick, weak soils, which are common in the Nepal Lesser Himalaya.

#### Engineering significance of sedimentary rocks
- **Sandstone**: building stone, aggregate; silica-cemented sandstone is strong, clay-cemented sandstone is weak.
- **Limestone**: raw material for cement and lime, and aggregate, but solution cavities cause reservoir leakage and settlement.
- **Shale, mudstone**: weak; they swell and slake, cause slope failures and make poor foundations.
- **Conglomerate**: may be strong or weak depending on the cement.
- Bedding planes control stability: an unfavourable dip toward the cut face causes sliding.
- The Siwalik sedimentary rocks of Nepal are weak and erodible.

## Check yourself

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

1. Petrology deals with
   A. only fossils
   B. only minerals' crystal shape
   C. origin, occurrence, composition, structure, texture and classification of rocks
   D. only soils
   Answer: C. Branch of geology. (Key point: Petrology)

2. Why are slate, phyllite and schist hazardous?
   A. They dissolve
   B. They swell like clay
   C. They are glassy
   D. Strength is low across foliation planes
   Answer: D. Slopes, tunnels and foundations can fail. (Key point: Foliated metamorphic)

3. Shale is poor because it
   A. is weak, swells and slakes
   B. is very hard
   C. is a good aggregate
   D. is soluble
   Answer: A. It causes slope failure. (Key point: Shale)

4. Which sandstone is strong?
   A. Clay-cemented
   B. Silica-cemented
   C. Poorly cemented
   D. Mica-rich
   Answer: B. Cement controls strength. (Key point: Sandstone cement)

## Related in Engineering Geology I

- [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×
- [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
- [Engineering significance of rock-forming minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-significance-of-rockforming-minerals) — Engineering Geology I, chapter 3, asked 3×
- [Recognising sedimentary, metamorphic, igneous rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arecognising-sedimentary-metamorphic-igneous-rocks) — Engineering Geology I, chapter 4
- [Formation of sedimentary and metamorphic rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aformation-of-sedimentary-and-metamorphic) — Engineering Geology I, chapter 4
- [Identify metamorphic rock in the field](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-metamorphic-rock-in-the) — Engineering Geology I, chapter 4, asked 2×

## Related in other subjects

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- [Rock forming minerals and significance](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-forming-minerals) — Engineering Geology II, chapter 1, asked 6×
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