---
title: "Metamorphic rock formation; igneous rock texture"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:metamorphic-rock-formation-igneous-rock"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/metamorphic-rock-formation-igneous-rock
site: BCT Engineering
---

# Metamorphic rock formation; igneous rock texture

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2069 Chaitra, 5 marks
- Video: [Origin of Metamorphic Rocks](https://www.youtube.com/watch?v=0QTkumoJnuU) — 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/metamorphic-rock-formation-igneous-rock
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Write down the formation process of metamorphic rock. Describe the texture of igneous rock.

## Short answer

Metamorphic rocks form from protoliths altered by heat, pressure, and fluids without melting, producing foliated or non-foliated textures. Igneous rock textures depend on cooling rate: phaneritic (slow, coarse), aphanitic (fast, fine), porphyritic, glassy, and vesicular.

## Answer

#### Formation of metamorphic rock
**Metamorphism** is the change of an existing rock (parent rock, or protolith) into a new rock in the solid state, by heat, pressure and chemically active fluids, without melting.

1. A parent rock is buried deep, comes near hot magma, or is squeezed during mountain building.
2. **Heat** causes recrystallisation and growth of new, stable minerals.
3. **Pressure** (confining and directed) compacts the rock and aligns platy minerals, forming foliation, slaty cleavage and schistosity.
4. **Fluids** (water, CO₂) circulate and promote reactions.

Types of metamorphism:
- **Contact (thermal)**: near intrusions, mainly heat; non-foliated rocks (marble, hornfels, quartzite).
- **Regional**: widespread, with heat and directed pressure during mountain building; foliated rocks (slate, phyllite, schist, gneiss).
- **Dynamic (cataclastic)**: along faults; crushed rock.

Example: shale to slate to phyllite to schist to gneiss with increasing grade.

#### Texture of igneous rocks
Texture depends on the cooling rate and the gas content of the magma.
- **Phaneritic** (coarse): slow cooling at depth, visible crystals (granite).
- **Aphanitic** (fine): quick cooling (basalt).
- **Porphyritic**: large phenocrysts in a fine matrix.
- **Glassy**: very rapid cooling (obsidian).
- **Vesicular**: gas holes (pumice).
- **Pegmatitic**: very coarse crystals.

By degree of crystallinity, textures are also holocrystalline (all crystals), hypocrystalline (crystals and glass) and holohyaline (all glass).

## Check yourself

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

1. What does directed pressure do in metamorphism?
   A. Melts minerals
   B. Deposits cement
   C. Creates vesicles
   D. Aligns platy minerals, forming foliation, slaty cleavage and schistosity
   Answer: D. Confining pressure compacts; directed stress aligns. (Key point: Role of pressure)

2. Contact metamorphism is mainly due to
   A. heat near intrusions, producing non-foliated rocks like marble
   B. directed pressure in mountain belts
   C. movement along faults
   D. weathering
   Answer: A. Regional gives foliated rocks. (Key point: Contact type)

3. Which igneous texture comes from quick cooling?
   A. Phaneritic, e.g. granite
   B. Aphanitic, e.g. basalt
   C. Pegmatitic
   D. Holocrystalline coarse
   Answer: B. Slow cooling gives phaneritic. (Key point: Cooling rate)

4. What does glassy texture show?
   A. Slow cooling
   B. Gas holes
   C. Very rapid cooling, as in obsidian
   D. Large phenocrysts
   Answer: C. Obsidian is glassy. (Key point: Glassy)

## Related in Engineering Geology I

- [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 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
- [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
- [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×
- [Identify metamorphic rock; properties of sandstone and granite](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-metamorphic-rock-properties-of) — Engineering Geology I, chapter 4