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

# Texture of igneous rocks

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2081 Kartik (new course), 3 marks; 2066 Bhadra (old course), 3 marks; 2064 Jestha (old course), 4 marks; 2063 Baisakh (old course), 3 marks (asked 4 times)
- Video: [Igneous Textures](https://www.youtube.com/watch?v=sYL8xXRg3q8) — Crust to Coast
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/texture-of-igneous-rocks
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Define rock texture. Describe the texture of igneous rocks.

## Short answer

Texture refers to grain size, shape, and crystallinity. Igneous textures include holocrystalline (all crystals), glassy, and hypocrystalline; phaneritic (coarse, slow cooling), aphanitic (fine, rapid), porphyritic (two-stage); and vesicular with gas holes. Coarse interlocking textures are strong; glassy and vesicular ones are weaker.

## Answer

**Texture** of a rock is the size, shape, arrangement and degree of crystallinity of its mineral grains, that is, the overall appearance of the rock on a small scale. In igneous rocks it depends mainly on the rate of cooling of the magma.

#### Textures of igneous rocks
1. **Based on degree of crystallinity**
   - **Holocrystalline**: wholly crystalline (granite).
   - **Hypocrystalline (hemicrystalline)**: partly crystals, partly glass.
   - **Holohyaline (glassy)**: wholly glass, from very rapid cooling (obsidian).
2. **Based on grain size**
   - **Phaneritic**: crystals visible to the naked eye, from slow cooling (granite, gabbro). Coarse, medium and fine grained.
   - **Aphanitic**: crystals too small to see, from rapid cooling (basalt, rhyolite).
   - **Porphyritic**: large crystals (phenocrysts) set in a fine-grained or glassy groundmass; two-stage cooling (porphyry).
   - **Pegmatitic**: very coarse crystals (pegmatite).
3. **Special textures**
   - **Vesicular**: gas bubble holes (pumice, scoria).
   - **Amygdaloidal**: vesicles filled with secondary minerals (zeolite, calcite).
   - **Ophitic**: laths of plagioclase enclosed in pyroxene.
   - **Graphic**: intergrowth of quartz and feldspar.
4. **Based on grain shape**: euhedral (perfect faces), subhedral, anhedral.

#### Significance
Coarse, interlocking crystalline rocks are strong; glassy and vesicular rocks are weaker and more porous.

## Check yourself

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

1. What mainly controls the texture of igneous rocks?
   A. Amount of weathering
   B. Direction of folding
   C. Degree of cementation
   D. Rate of cooling of the magma
   Answer: D. Cooling rate decides crystal size and crystallinity. (Key point: Controlling factor)

2. Which pairing is correct?
   A. Phaneritic: slow cooling, visible crystals; aphanitic: rapid cooling, tiny crystals
   B. Phaneritic: rapid cooling; aphanitic: slow cooling
   C. Both form only by very slow cooling
   D. Both are wholly glassy
   Answer: A. Slow cooling gives coarse visible crystals (granite); rapid cooling gives fine grains (basalt). (Key point: Phaneritic vs aphanitic)

3. Porphyritic texture consists of
   A. only equal-sized interlocking crystals
   B. large phenocrysts in a fine-grained or glassy groundmass, from two-stage cooling
   C. gas-bubble holes in glass
   D. an intergrowth of quartz and feldspar
   Answer: B. Two-stage cooling gives big early crystals set in a finer groundmass. (Key point: Porphyritic texture)

4. How does amygdaloidal texture differ from vesicular?
   A. It is wholly glassy
   B. It has no gas cavities at all
   C. Its vesicles are filled with secondary minerals such as calcite or zeolite
   D. It has plagioclase laths enclosed in pyroxene
   Answer: C. Amygdales are gas holes later filled with secondary minerals. (Key point: Amygdaloidal)

## Related in Engineering Geology I

- [Metamorphic rock formation; igneous rock texture](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ametamorphic-rock-formation-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
- [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](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atexture-of-sedimentary-rocks) — Engineering Geology I, chapter 4, asked 2×
- [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