---
title: "Volcanism; chemical weathering"
subject: "Engineering Geology I"
chapter: 6
key: "semester_3/engineering_geology_i:volcanism-chemical-weathering"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/volcanism-chemical-weathering
site: BCT Engineering
---

# Volcanism; chemical weathering

Engineering Geology I · Chapter 6: Physical Geology

- Asked in: 2074 Asoj, 3 marks
- Video: [Chemical Weathering Processes](https://www.youtube.com/watch?v=JzsmkVUEy0Q) — 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/volcanism-chemical-weathering
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-6

## Question

What is volcanism? Describe chemical weathering.

## Short answer

Volcanism is the process where magma, gases, and rock fragments rise from Earth's interior, erupting on the surface or intruding into the crust. Chemical weathering alters rock minerals into stable clays and soluble salts via processes like solution, carbonation, hydrolysis, oxidation, and hydration, fastest in warm, wet climates.

## Answer

**Volcanism** (vulcanicity) is the set of processes by which magma, gases and fragmented rock rise from the earth's interior and are erupted on the surface through a vent or fissure, or are intruded into the crust.

- Volcanism produces lava, gases and pyroclasts, and builds cones, calderas and lava plateaus. Associated features are hot springs, geysers and fumaroles.

#### Chemical weathering
**Chemical weathering** changes the minerals of the rock into new, more stable minerals (mostly clay) and soluble salts. It is fastest in warm, wet climates. Main processes:

- **Solution:** soluble minerals such as rock salt and gypsum dissolve in water.
- **Carbonation:** CO$_2$ in rain forms carbonic acid, which dissolves limestone: $CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2$. This produces karst.
- **Hydrolysis:** feldspar reacts with water to form clay: $2KAlSi_3O_8 + 2H^+ + H_2O \rightarrow Al_2Si_2O_5(OH)_4 + 4SiO_2 + 2K^+$ (orthoclase to kaolinite).
- **Oxidation:** iron minerals combine with oxygen and turn into rust-coloured oxides, e.g. $4FeO + O_2 \rightarrow 2Fe_2O_3$.
- **Hydration:** minerals take up water and swell, e.g. anhydrite to gypsum, hematite to limonite.

## Check yourself

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

1. Volcanism involves:
   A. Wind erosion
   B. Eruption or intrusion of magma, gases and fragments
   C. Rock folding
   D. Glacier motion
   Answer: B. It builds cones, calderas and lava plateaus. (Key point: Volcanism)

2. Which are features associated with volcanism?
   A. Moraines
   B. Ox-bows
   C. Hot springs, geysers, fumaroles
   D. Dunes
   Answer: C. They result from volcanic heat. (Key point: Associated)

3. Chemical weathering is fastest in:
   A. Cold dry climates
   B. Deserts
   C. Polar regions
   D. Warm, wet climates
   Answer: D. Water drives reactions. (Key point: Chemical weathering)

4. What does oxidation of iron minerals produce?
   A. Rust-coloured oxides such as Fe2O3
   B. Clay
   C. Calcium bicarbonate
   D. Gypsum
   Answer: A. 4FeO + O2 -> 2Fe2O3. (Key point: Oxidation)

## Related in Engineering Geology I

- [Weathering and erosion; volcanism](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aweathering-and-erosion-volcanism) — Engineering Geology I, chapter 6
- [Physical and chemical weathering](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aphysical-and-chemical-weathering) — Engineering Geology I, chapter 6, asked 5×
- [Chemical versus physical weathering; factors](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Achemical-versus-physical-weathering-factors) — Engineering Geology I, chapter 6