---
title: "Classification of igneous rocks"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:classification-of-igneous-rocks"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/classification-of-igneous-rocks
site: BCT Engineering
---

# Classification of igneous rocks

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2080 Bhadra, 3 marks
- Video: [Igneous Rocks & Their Classification | B.Sc. 2nd Year Geology | Complete & Easy Explanation](https://www.youtube.com/watch?v=7dahR917fAE) — GeoParth
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/classification-of-igneous-rocks
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Give a brief account of the classification of igneous rocks.

## Short answer

Igneous rocks are classified by mode of occurrence (plutonic, hypabyssal, volcanic), silica content (acidic >65% SiO₂, intermediate, basic,, ultrabasic),, texture (phaneritic, aphanitic, etc.), and mineral content (felsic vs mafic). Examples: granite is acidic plutonic, basalt is basic volcanic.

## Answer

Igneous rocks are formed by cooling of magma or lava. They are classified in several ways.

#### 1. By mode of occurrence (place of cooling)
- **Plutonic (intrusive)**: deep, slow cooling, coarse grains (granite, gabbro, diorite).
- **Hypabyssal**: shallow dykes and sills, medium grains (dolerite, porphyry).
- **Volcanic (extrusive)**: surface, fast cooling, fine or glassy (basalt, rhyolite, andesite, obsidian).

#### 2. By silica content (chemical)
| Class | SiO₂ | Colour | Examples |
|---|---|---|---|
| Acidic | > 65% | Light | Granite, rhyolite |
| Intermediate | 55-65% | Medium | Diorite, andesite, syenite |
| Basic | 45-55% | Dark | Gabbro, basalt, dolerite |
| Ultrabasic | < 45% | Very dark | Peridotite, dunite |

#### 3. By texture
Phaneritic (coarse), aphanitic (fine), porphyritic, glassy and vesicular.

#### 4. By mineral content
Felsic (quartz, feldspar rich) and mafic (iron-magnesium rich).

## Check yourself

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

1. Which classification by place of cooling is correct?
   A. Plutonic (deep), hypabyssal (shallow dykes and sills), volcanic (surface)
   B. Acidic, basic, ultrabasic
   C. Clastic, chemical, organic
   D. Phaneritic, foliated, bedded
   Answer: A. Place of cooling gives these three groups. (Key point: Mode of occurrence)

2. Which class has silica above 65%?
   A. Basic, as in gabbro
   B. Acidic, as in granite and rhyolite
   C. Ultrabasic, as in dunite
   D. Intermediate, as in diorite
   Answer: B. Acidic rocks are light-coloured with over 65% SiO2. (Key point: Silica content)

3. Basic rocks have
   A. Over 65% silica and light colour
   B. Under 45% silica
   C. 45-55% silica, dark colour, e.g. gabbro and basalt
   D. 55-65% silica
   Answer: C. Basic is 45-55%. (Key point: Basic rocks)

## Related in Engineering Geology I

- [Texture of igneous rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atexture-of-igneous-rocks) — Engineering Geology I, chapter 4, asked 4×
- [Intrusive versus extrusive igneous rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aintrusive-versus-extrusive-igneous-rocks) — Engineering Geology I, chapter 4, 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
- [Classification of sedimentary rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aclassification-of-sedimentary-rocks) — Engineering Geology I, chapter 4, asked 4×
- [Concordant versus discordant igneous bodies](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aconcordant-versus-discordant-igneous-bodies) — Engineering Geology I, chapter 4, asked 4×
- [Petrogenesis, igneous rock identification, rock significance](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Apetrogenesis-igneous-rock-identification-rock) — Engineering Geology I, chapter 4, asked 2×

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