---
title: "How soils are formed"
subject: "Soil Mechanics"
chapter: 1
key: "semester_4/soil_mechanics:soil-formation-process"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/soil-formation-process
site: BCT Engineering
---

# How soils are formed

Soil Mechanics · Chapter 1: Introduction

- Asked in: 2079 Asoj, 1 marks; 2077 Chaitra, 2 marks (asked 2 times)
- Video: [Soil and Soil Dynamics](https://www.youtube.com/watch?v=mg7XSjcnZQM) — Bozeman Science
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/soil-formation-process
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-1

## Question

Briefly describe how soils are formed (process of soil formation).

## Short answer

Soil forms by weathering of parent rock via physical disintegration (temperature, frost, abrasion), chemical decomposition (oxidation, hydrolysis), and biological action. Weathered particles remain as residual soil or are transported by water, wind, ice, or gravity to form transported soils.

## Answer

**Soil** is formed by the **weathering** (breakdown) of parent rock by physical, chemical and biological agents. The rock breaks into smaller particles that are either left in place or carried away and deposited elsewhere.

#### 1. Physical (mechanical) weathering
Rock is broken into smaller pieces without change in its mineral composition. Causes:
- Temperature changes (expansion and contraction, daily heating and cooling).
- Freezing of water in cracks (frost wedging).
- Abrasion by wind, running water, glaciers and waves.
- Plant roots growing in cracks, and unloading (release of pressure).

This produces coarse-grained soils such as gravel, sand and silt.

#### 2. Chemical weathering
Minerals of the rock are altered into new minerals by chemical reactions: **oxidation, hydration, carbonation, hydrolysis and leaching**. Feldspar, for example, changes to clay minerals (kaolinite, illite, montmorillonite). This produces fine-grained clayey soils.

#### 3. Biological action
Lichens, bacteria, burrowing animals and roots add organic acids and break rock, adding organic matter to the soil.

#### Fate of the weathered material
- **Residual soils** stay at the place of formation over the parent rock.
- **Transported soils** are carried by water (alluvial), wind (aeolian), ice (glacial) or gravity (colluvial) and deposited elsewhere.

```text
Parent rock -> Weathering -> Soil particles
                 |-> stays in place: residual soil
                 |-> moved by agents: transported soil
```

## Check yourself

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

1. Soil is formed mainly by which process?
   A. Cooling of molten magma only
   B. Weathering of parent rock by physical, chemical and biological agents
   C. Compaction of sand by machines
   D. Deposition of cement dust
   Answer: B. Soil is the product of weathering (breakdown) of parent rock. (Key point: Soil from weathering)

2. Physical weathering differs from chemical weathering in that it:
   A. Changes the mineral composition and makes clay
   B. Produces only montmorillonite
   C. Only occurs in warm humid climates
   D. Breaks rock into smaller pieces without changing mineral composition, giving coarse soils
   Answer: D. Physical weathering is mechanical breakdown with no change in minerals, producing gravel, sand and silt. (Key point: Physical weathering)

3. Which is an example of chemical weathering and its product?
   A. Frost wedging producing gravel
   B. Feldspar altering to clay minerals such as kaolinite
   C. Abrasion by wind producing sand
   D. Root growth producing silt
   Answer: B. Chemical reactions such as hydrolysis change feldspar into clay minerals, giving fine-grained soils. (Key point: Chemical weathering products)

4. What is the difference between residual and transported soil?
   A. Both stay in place but differ in colour
   B. Residual soil is carried by water; transported soil is not
   C. Residual soil is always sandy; transported soil is always clay
   D. Residual soil stays at the place of formation; transported soil is carried by water, wind, ice or gravity and deposited elsewhere
   Answer: D. Residual soils remain over the parent rock while transported soils are moved by agents and deposited elsewhere. (Key point: Residual vs transported)

## Related in Soil Mechanics

- [Types of soil by formation, size](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asoil-types-formation-grain-size) — Soil Mechanics, chapter 1
- [Field identification of fine-grained soils](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afield-identification-clay-silt) — Soil Mechanics, chapter 3, asked 4×
- [Residual and transported soils](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aresidual-transported-soils) — Soil Mechanics, chapter 1, asked 2×
- [USCS classification of soils A and B](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-soils-a-b) — Soil Mechanics, chapter 3
- [Adsorbed water; diffuse layer formation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aadsorbed-water-diffuse-layer) — Soil Mechanics, chapter 4
- [Average permeability of layered soil](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Alayered-soil-average-permeability) — Soil Mechanics, chapter 6

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- [Foundations on black cotton soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ablack-cotton-foundation) — Building Technology, chapter 2