---
title: "Intrusive versus extrusive igneous rocks"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:intrusive-versus-extrusive-igneous-rocks"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/intrusive-versus-extrusive-igneous-rocks
site: BCT Engineering
---

# Intrusive versus extrusive igneous rocks

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2081 Baisakh, 4 marks; 2066 Jestha (old course), 4 marks; 2063 Baisakh (old course), 2 marks (asked 3 times)
- Video: [Classification of Igneous Rocks: Intrusive vs. Extrusive](https://www.youtube.com/watch?v=TtvSyROolsk) — Professor Dave Explains
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/intrusive-versus-extrusive-igneous-rocks
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Differentiate between intrusive (plutonic) and extrusive (volcanic) igneous rocks with examples.

## Short answer

Intrusive rocks (granite, gabbro) cool slowly underground, are coarse-grained, and strong; extrusive rocks (basalt, rhyolite) cool rapidly on the surface, are fine-grained or glassy, and often vesicular. Intrusives make good foundations; extrusives vary in strength.

## Answer

| Basis | Intrusive (plutonic) | Extrusive (volcanic) |
|---|---|---|
| Place of cooling | Deep inside the Earth, below the surface | On or near the surface |
| Cooling rate | Very slow | Rapid |
| Texture | Coarse-grained, phaneritic | Fine-grained, glassy, vesicular |
| Crystal size | Large, visible | Small or none |
| Types | Plutonic (deep), hypabyssal (shallow) | Lava flows, pyroclastic |
| Structures | Batholith, stock, sill, dyke | Lava flow, volcanic cone, vesicles |
| Colour | Depends on composition: light (granite) to dark (gabbro) | Depends on composition: light (rhyolite) to dark (basalt) |
| Examples | Granite, gabbro, diorite, syenite | Basalt, rhyolite, andesite, obsidian |
| Engineering | Strong, good foundations | Variable; basalt strong but jointed, vesicular rocks weak |

## Check yourself

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

1. Why are intrusive rocks coarse-grained?
   A. They cool rapidly at the surface
   B. They contain gas bubbles
   C. They cool slowly deep inside the Earth, allowing large crystals to grow
   D. They are cemented sediments
   Answer: C. Slow cooling gives large visible crystals. (Key point: Cooling and texture)

2. Which list contains only extrusive rocks?
   A. Granite, gabbro, diorite, syenite
   B. Granite, basalt, diorite, obsidian
   C. Gabbro, rhyolite, syenite, andesite
   D. Basalt, rhyolite, andesite, obsidian
   Answer: D. Basalt, rhyolite, andesite and obsidian are volcanic; granite, gabbro, diorite and syenite are plutonic. (Key point: Examples)

3. Which structure belongs to intrusive rocks?
   A. Batholith, stock, sill and dyke
   B. Lava flow
   C. Volcanic cone
   D. Pyroclastic deposit
   Answer: A. Lava flows, cones and pyroclastics are extrusive. (Key point: Hypabyssal)

4. Why are extrusive rocks like basalt variable as foundations?
   A. All extrusive rocks are glassy and brittle
   B. Basalt is strong but jointed, and vesicular rocks are weak
   C. They are always foliated
   D. They are always soluble
   Answer: B. Extrusive rocks vary: jointed basalt leaks, vesicular rocks are weak. (Key point: Engineering)

## 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×
- [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 igneous rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aclassification-of-igneous-rocks) — Engineering Geology I, chapter 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×
- [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

## Related in other subjects

- [Sedimentary versus igneous stones](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Asedimentary-igneous-stones) — Civil Engineering Materials, chapter 2