---
title: "Gabbro versus basalt"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:gabbro-versus-basalt"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/gabbro-versus-basalt
site: BCT Engineering
---

# Gabbro versus basalt

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2062 Baisakh (old course), 2 marks
- Video: [Geology: Basalt vs Gabbro](https://www.youtube.com/watch?v=usDTebhFlJk) — Glen Warren
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/gabbro-versus-basalt
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Differentiate between gabbro and basalt.

## Short answer

Gabbro is a coarse-grained plutonic mafic rock (plagioclase, pyroxene, olivine) with visible grains; basalt is a fine-grained volcanic mafic rock with the same composition but grains invisible. Gabbro cools slowly deep, basalt rapidly at surface.

## Answer

| Point | Gabbro | Basalt |
|---|---|---|
| Class | Plutonic (intrusive) igneous | Volcanic (extrusive) igneous |
| Cooling | Slow, deep in crust | Rapid, at the surface |
| Texture | Coarse grained, phaneritic | Fine grained, aphanitic (sometimes vesicular) |
| Composition | Basic (mafic): plagioclase, pyroxene, olivine | The same basic (mafic) composition |
| Colour | Dark, grains visible | Dark, grains not visible |

## Check yourself

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

1. Gabbro is which type?
   A. Volcanic igneous
   B. Plutonic igneous
   C. Sedimentary
   D. Metamorphic
   Answer: B. It cools slowly deep in the crust. (Key point: Class)

2. Why is basalt fine grained?
   A. Slow cooling at depth
   B. Metamorphism
   C. Rapid cooling at the surface
   D. Cementation
   Answer: C. Fast cooling gives no time for large crystals. (Key point: Texture difference)

3. How do their compositions compare?
   A. Gabbro acidic, basalt basic
   B. Gabbro basic, basalt acidic
   C. Both acidic
   D. Same mafic composition
   Answer: D. Both have plagioclase, pyroxene and olivine. (Key point: Composition)

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