---
title: "Volcanic materials"
subject: "Engineering Geology I"
chapter: 6
key: "semester_3/engineering_geology_i:volcanic-materials"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/volcanic-materials
site: BCT Engineering
---

# Volcanic materials

Engineering Geology I · Chapter 6: Physical Geology

- Asked in: 2068 Baisakh, 2 marks
- Video: [Volcanoes: Formation, Types, and Activity](https://www.youtube.com/watch?v=sd8cVCtIRO4) — 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/volcanic-materials
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-6

## Question

What are the different volcanic materials?

## Short answer

Volcanic materials are gases (mainly water vapour, CO₂, SO₂), liquid lava (basaltic or rhyolitic), and solid pyroclasts. Pyroclasts include ash (<2 mm), lapilli (2–64 mm), bombs and blocks (>64 mm), pumice, scoria, tuff and volcanic breccia.

## Answer

Volcanic materials ejected through the vent are of three kinds.

1. **Gases and vapours:** water vapour (most), CO$_2$, SO$_2$, H$_2$S, HCl, nitrogen.
2. **Liquid:** **lava** (molten rock at the surface). Basic (basaltic) lava is runny; acid (rhyolitic) lava is thick.
3. **Solids (pyroclasts):**
   - *Volcanic dust and ash:* less than 2 mm
   - *Lapilli:* 2 to 64 mm
   - *Volcanic bombs and blocks:* greater than 64 mm
   - *Pumice, scoria, tuff, and volcanic breccia*

## Check yourself

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

1. Which gas is most abundant among volcanic gases?
   A. Carbon dioxide
   B. Nitrogen
   C. Water vapour
   D. Hydrogen sulphide
   Answer: C. Water vapour is the largest component, with CO2, SO2, H2S, HCl and nitrogen. (Key point: Main gas)

2. Which statement about lava is correct?
   A. Basic lava is thick; acid lava is runny
   B. Both are equally runny
   C. Lava is a gas at the surface
   D. Basic (basaltic) lava is runny; acid (rhyolitic) lava is thick
   Answer: D. Silica content makes acid lava viscous. (Key point: Lava types)

3. Pyroclasts of size 2 to 64 mm are called:
   A. Lapilli
   B. Volcanic dust and ash
   C. Volcanic bombs and blocks
   D. Pumice
   Answer: A. Dust and ash are below 2 mm; bombs and blocks exceed 64 mm. (Key point: Pyroclast sizes)

4. Volcanic materials are grouped as:
   A. Soil, rock and water
   B. Gases and vapours, liquid lava, and solid pyroclasts
   C. Ash, mud and ice
   D. Magma, minerals and fossils
   Answer: B. The answer lists these three kinds. (Key point: Three kinds)