Overview
The processes of erosion are the distinct ways in which a moving agent (water, wind, ice or waves) wears rock down and carries it away. They are four: hydraulic action (the force of moving water), corrasion or abrasion (the load grinding the rock), attrition (carried fragments wearing each other down), and corrosion or solution (water chemically dissolving soluble rock). The first three are mechanical and the last is chemical.
Erosion and its processes
What erosion is and how it differs from weathering
Erosion is the wearing away of rock and soil and the carrying of that material to a new place by a moving agent such as water, ice, snow, wind, waves or gravity. The key word is transport: erosion not only loosens material but moves it away.
This is what separates erosion from weathering. Weathering breaks rock down in situ, on the spot, with little or no movement, whereas erosion involves the actual removal and transport of the debris. Erosion acts through a small set of distinct physical and chemical processes.
The four processes of erosion
Erosion works through four classic processes. Three are mechanical, wearing rock down by physical force, and one is chemical, dissolving rock away. The same four act whether the agent is a river, the sea, the wind or a glacier.
The three mechanical processes are hydraulic action, corrasion and attrition, while the chemical process is corrosion, also called solution. Understanding each, and not confusing the two similar names, is the heart of this topic.
The mechanical processes of erosion
Hydraulic action, corrasion and attrition
Hydraulic action is the sheer force of moving water. When waves or a river strike a rock face, water is driven into cracks and compresses the air inside, prising the rock apart over time without any tool.
Corrasion, also called abrasion, occurs when the sediment carried by the agent, sand, pebbles and rock fragments, grinds against and batters the rock surface. Attrition is the related process by which those carried fragments collide with each other, chipping and grinding until they become smaller, smoother and rounder.
How the mechanical processes act together
In practice these three physical processes work side by side. Hydraulic action loosens and dislodges rock, corrasion uses that loosened load as a natural sandpaper, and attrition steadily reduces the load itself into finer particles.
None of them changes the chemistry of the rock; they simply break it apart by force and friction. This is why they are grouped together as the mechanical, or physical, processes of erosion.
Chemical erosion and the key distinction
Corrosion: the chemical process
Corrosion, also known as solution, is the one chemical process of erosion. Here water reacts chemically with soluble rock and dissolves it, rather than wearing it down by force.
It is most effective where water is slightly acidic, with a pH below seven, acting on soluble rocks such as limestone and chalk. The dissolved material is carried away in solution, which is how features like limestone caves are slowly hollowed out.
Corrosion versus corrasion: the crucial difference
The two terms that trip up most candidates are corrosion and corrasion, separated by a single letter but opposite in nature. Corrosion is a chemical process, in which water dissolves soluble rock such as limestone.
Corrasion, by contrast, is a mechanical process, in which the load carried by the agent physically grinds and batters the rock, and it acts on rock of all kinds. In short, corrosion dissolves chemically while corrasion wears down physically.
Agents and landforms of erosion
Erosion by rivers, sea, wind and ice
These four processes operate wherever an agent is at work. In rivers and along coasts all four are active, with hydraulic action and corrasion usually the most powerful, while corrosion dominates in limestone country.
Wind erosion in deserts relies chiefly on corrasion and attrition as sand is blasted against rock and worn down, and glacial ice erodes mainly by corrasion as embedded debris scours the bedrock beneath the moving ice.
The landforms these processes create
Acting over long periods, these processes carve a wide range of landforms. Rivers cut valleys, gorges and waterfalls; the sea shapes cliffs, sea caves, arches and stacks; and corrosion alone produces caves and sinkholes in limestone.
Recognising which process dominates therefore helps explain why a landscape looks as it does. Reading the balance of physical and chemical erosion is a basic skill of physical geography.
How erosion processes appear in the UPSC exam
Erosion processes in GS Paper I and Geography Optional
Erosion processes are a reliable Prelims topic, tested above all on the corrosion-corrasion distinction. The high-yield points are few and precise.
- Erosion transports material; weathering breaks rock in place.
- Hydraulic action, corrasion and attrition are mechanical processes.
- Corrosion (solution) is the chemical process, dissolving soluble rock.
- Corrosion is chemical; corrasion (abrasion) is mechanical.
| Process | Type | How it works |
|---|---|---|
| Hydraulic action | Mechanical | force of moving water in cracks |
| Corrasion (abrasion) | Mechanical | the load grinds the rock surface |
| Attrition | Mechanical | carried fragments wear each other |
| Corrosion (solution) | Chemical | water dissolves soluble rock |
A precise answer names the process, states whether it is mechanical or chemical, and never confuses corrosion with corrasion, the discipline this article builds throughout.
Prelims MCQ practice
Each question below tests one specific concept on the topic. Click to reveal the answer and a full option-wise explanation.
Q1. Which of the following processes of erosion is chemical in nature?
- Hydraulic action
- Corrasion
- Attrition
- Corrosion
Show answer and explanation
Answer: Corrosion
Explanation.
Corrosion (solution) is the chemical process, in which water dissolves soluble rock. The other three are mechanical. Hence (d).
Q2. The process by which the load carried by a river or the sea grinds against and wears down the rock surface is called:
- Attrition
- Corrasion (abrasion)
- Corrosion
- Hydraulic action
Show answer and explanation
Answer: Corrasion (abrasion)
Explanation.
Corrasion, also called abrasion, is the grinding of the rock surface by the sediment load carried by the agent. Hence (b).
Q3. With reference to the processes of erosion, consider the following statements:
- Hydraulic action is the erosive force of moving water itself.
- Attrition reduces the carried fragments into smaller, smoother pieces.
- Corrosion acts only on insoluble rocks such as granite.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Statements 1 and 2 are correct. Statement 3 is wrong: corrosion acts on soluble rocks such as limestone and chalk, not on granite. Hence 1 and 2 only.
Q4. Which rock type is most affected by corrosion (solution)?
- Granite
- Basalt
- Limestone
- Quartzite
Show answer and explanation
Answer: Limestone
Explanation.
Corrosion is most effective on soluble rocks such as limestone and chalk, which slightly acidic water dissolves. Hence (c).
Q5. What is the key difference between corrosion and corrasion?
- Corrosion is mechanical; corrasion is chemical
- Corrosion is chemical; corrasion is mechanical
- Both are chemical
- Both are mechanical
Show answer and explanation
Answer: Corrosion is chemical; corrasion is mechanical
Explanation.
Corrosion dissolves soluble rock chemically, while corrasion grinds rock down mechanically. Hence (b).
Q6. Consider the following statements about erosion and weathering:
- Weathering involves the breakdown of rock in place, with little movement.
- Erosion involves the transport of the broken material to a new place.
- Attrition and corrasion are processes of weathering, not erosion.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Statements 1 and 2 are correct. Statement 3 is wrong: attrition and corrasion are processes of erosion, not weathering. Hence 1 and 2 only.
Sources and Further Reading
Editorial Disclaimer
This article explains the processes of erosion for UPSC preparation, drawing on standard physical-geography sources. Definitions reflect the cited authorities.
