---
title: "Concordant versus discordant igneous bodies"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:concordant-versus-discordant-igneous-bodies"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/concordant-versus-discordant-igneous-bodies
site: BCT Engineering
---

# Concordant versus discordant igneous bodies

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2079 Bhadra, 3 marks; 2075 Chaitra, 2 marks; 2078 Kartik, 2 marks; 2068 Baisakh, 3 marks (asked 4 times)
- Video: [FORMS OF IGNEOUS ROCKS | INTRUSIVE AND EXTRUSIVE FORMS | CONCORDANT AND DISCORDANT BODIES|](https://www.youtube.com/watch?v=6DScON584HU) — Geology Education
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/concordant-versus-discordant-igneous-bodies
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Distinguish between concordant and discordant bodies of igneous rocks (forms of igneous rock).

## Short answer

Concordant igneous bodies like sills and laccoliths are parallel to bedding; discordant bodies like dykes and batholiths cut across it. Concordant are sheet-like and conformable; discordant are cross-cutting and unconformable.

## Answer

Intrusive igneous bodies are classified by their relation to the surrounding country rock.

| Basis | Concordant bodies | Discordant bodies |
|---|---|---|
| Relation with country rock | Parallel to the bedding or foliation | Cut across the bedding or foliation |
| Magma path | Spreads between layers | Forces its way through layers |
| Shape | Sheet-like or lens-like, parallel to layers | Irregular or vertical, cross-cutting |
| Types | Sill, laccolith, lopolith, phacolith | Dyke, batholith, stock, volcanic neck |
| Contact | Conformable | Unconformable |
| Example | Sill, laccolith | Dyke, batholith |

```text
 Concordant (sill)      Discordant (dyke)
 ==================      ===|##|=====
 ##################      ===|##|=====
 ==================      ===|##|=====
```

- **Sill**: horizontal sheet between beds. **Laccolith**: lens with domed top. **Dyke**: vertical sheet cutting beds. **Batholith**: huge irregular mass, over 100 km², of granite. **Stock**: smaller version of batholith.

## Check yourself

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

1. How do concordant bodies relate to country rock?
   A. They cut across the bedding
   B. They are always larger than 100 km2
   C. They form only on the surface
   D. They lie parallel to the bedding or foliation
   Answer: D. Concordant bodies spread between layers; discordant ones cut across them. (Key point: Basic distinction)

2. Which pair lists only discordant bodies?
   A. Dyke and batholith
   B. Sill and laccolith
   C. Sill and dyke
   D. Laccolith and batholith
   Answer: A. Dykes, batholiths, stocks and volcanic necks are discordant; sills, laccoliths, lopoliths and phacoliths are concordant. (Key point: Examples)

3. What is the difference between a sill and a dyke?
   A. A sill cuts beds; a dyke lies between beds
   B. A sill is a horizontal sheet between beds; a dyke is a vertical sheet cutting beds
   C. Both lie between beds
   D. A sill is huge and irregular; a dyke is lens-shaped
   Answer: B. Sill is concordant, dyke is discordant. (Key point: Sill vs dyke)

## Related in Engineering Geology I

- [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×
- [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×
- [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
- [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
- [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×

## 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
- [Rock mass versus intact rock](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-mass-vs-intact-rock) — Engineering Geology II, chapter 6, asked 7×