Maarg Manthan · Topic 3.14

Weathering: Chemical, Physical/Mechanical and Biological Processes

Geography › Geomorphic Processes and Landform Evolution · Topic 3.14

Weathering is the breakdown and decay of rocks in place, by the action of weather, water, chemicals and living things, without any movement of the material by an agent. It is of three kinds. Physical or mechanical weathering breaks rock into pieces without changing its minerals, as in frost wedging and thermal expansion. Chemical weathering changes the minerals by processes such as solution, carbonation, hydration, oxidation and hydrolysis. Biological weathering is done by roots, burrowing animals and microbes. Weathering produces regolith and soil.

5 min read · UPSC Prelims · Geography · Geomorphic Processes and Landform Evolution

At a Glance

Physical Breaks rock without chemical change
Chemical Changes minerals
Biological Plants, animals, microbes
Fastest in Warm, wet climates (chemical); cold, dry (physical)
Product Regolith and soil
Exam link UPSC Prelims, Geography: Geomorphic Processes and Landform Evolution

Where Does It Fit?

This topic follows the one on exogenic processes and takes the first step of that chain, weathering. It leads into the topic on its visible effects and the topic on mass movements of the weathered debris. Previous topic: Exogenic Processes: Denudation, Weathering, Mass Movements, Erosion, Transportation, Deposition. Next topic: Effects of Weathering: Exfoliation Domes and Tors.

What Is Weathering?

Weathering is the mechanical disintegration and chemical decomposition of rocks in place, without much movement of the material. Rocks are broken into smaller pieces, and weathered material, called regolith, is the raw material for soil. It is therefore a precondition of erosion and soil formation. Weathering is controlled by the nature of the rock, climate, relief, vegetation and time. It is fastest in warm, humid climates for chemical processes, and in cold or dry climates with large temperature swings for physical processes.

Type	Main processes	Climate favouring it
Physical (mechanical)	Frost wedging, thermal expansion, exfoliation, salt crystal growth, pressure release	Cold and dry regions
Chemical	Solution, carbonation, hydration, oxidation, reduction, hydrolysis	Warm and humid regions
Biological	Root growth, burrowing animals, organic acids, human activity	Most places with vegetation

What Is Physical (Mechanical) Weathering?

Process	How it works	Where common
Frost wedging	Water enters cracks, freezes and expands by about 9 per cent, widening the cracks	Cold, mountainous regions with daily freeze-thaw
Thermal expansion (insolation weathering)	Heating by day and cooling at night makes the rock expand and contract unevenly, so it cracks	Hot deserts
Pressure release (unloading)	Removal of overlying rock lets the rock below expand and crack in sheets parallel to the surface	Granite areas
Salt crystal growth	Salts crystallise in pores and push the grains apart	Deserts and coasts
Wetting and drying	Clay-rich rocks swell and shrink repeatedly and flake	Dry seasons after rain

What Is Chemical Weathering?

Solution: minerals such as rock salt and gypsum dissolve in water.
Carbonation: carbon dioxide in rainwater forms weak carbonic acid, which dissolves limestone and makes caves and sinkholes.
Hydration: minerals take up water and swell, and anhydrite changes into gypsum.
Hydrolysis: minerals react with water, and feldspar changes to clay minerals such as kaolinite.
Oxidation: iron-bearing minerals combine with oxygen and rust, giving red and brown colours.
Reduction: in waterlogged soils with little oxygen, iron changes to a greyish-green form.

Chemical weathering is fastest where it is warm and wet, as in the tropics, and slow in cold and dry places. Carbonation underlies the karst landforms in Karst (Groundwater) Topography: Sinkholes, Dolines, Uvalas.

What Is Biological Weathering?

Living things also break down rock. Roots grow into cracks and pry the rock apart. Burrowing animals such as earthworms, termites and rodents bring fresh rock to the surface. Lichens and mosses secrete acids that dissolve rock minerals, and are among the first colonisers of bare rock. Bacteria add to the chemical breakdown. Human activities such as mining, quarrying, ploughing and construction speed up weathering and soil loss. Organic acids from decaying plants also help to dissolve minerals.

Why Is Weathering Significant?

Weathering is the first step in the formation of soil and thus supports agriculture and vegetation. It enriches the concentration of some ores: bauxite, iron ore, manganese and copper accumulate when weathering removes other material. It breaks rock into material that can be eroded and shapes the surface, creating features such as tors, caves and rounded boulders. As an analytical point, the same processes are also hazards, since weathered slopes fail in landslides and heavy rainfall on loosened rock raises risk, so land-use planning and slope management in the hills are as important as the science itself. The relation of weathering to soil is developed in Stages, Processes and Profiles of Soil Formation.

What Controls the Rate of Weathering?

Climate: temperature and rainfall. Warm, humid climates have the deepest weathering, up to tens of metres of regolith.
Rock type: quartzite and granite resist, while limestone dissolves, shale crumbles and basalt weathers fast in the tropics.
Joints and cracks: let water in and increase the surface area.
Vegetation and relief: plants hold water and acids on the surface, and gentle slopes keep the regolith in place while steep slopes strip it.
Time: older surfaces are more deeply weathered.

The effects of weathering are in Effects of Weathering: Exfoliation Domes and Tors and the movement of the debris in Mass Movements: Heave, Flow, Slide; Slow, Rapid and Landslide Categories.

Why Is This Topic Important?

Terms Processes are asked
Soil Basis of soil formation
Ores Concentrates bauxite and laterite

What Came Next?

The next topic describes the landforms that result from weathering: exfoliation domes and tors. Next topic: Effects of Weathering: Exfoliation Domes and Tors.

Key People and Terms

Regolith Loose weathered mantle
Frost wedging Splitting of rock by freezing water
Carbonation Dissolving by carbonic acid
Hydrolysis Reaction with water forming new minerals
Unloading Pressure-release cracking
Lichen Algae-fungus partnership on rock

Exam Corner

Points to Remember

Carbonation dissolves limestone.
Frost wedging: water expands by about 9 per cent.
Hydrolysis turns feldspar into clay.
Chemical weathering is strongest in warm, wet areas.
Weathering involves no transport.

Do Not Confuse With

Hydration and hydrolysis Taking up water versus reacting with water
Weathering and erosion In place versus carried away
Exfoliation and frost wedging Peeling from heating and cooling or unloading versus freezing water

Memory Hook

Frost splits, heat cracks, acid dissolves, rust reddens, roots pry.

Mains Angle

Questions ask for types of weathering and their significance. These points add depth.

Types Physical, chemical, biological
Climate Controls the dominant type
Significance Soil, ores and landforms

A Question You May Face

An original practice question, not a past paper question.

“Describe the different types of weathering and examine the role of climate in determining them.”

How to Answer

Define Weathering.
Types Three kinds.
Climate Tropics versus cold regions.
Significance Soil and ores.

GS Relevance

Prelims: types and processes of weathering, factors, results. GS Paper 1: geomorphic processes.

Frequently Asked Questions

What is weathering?

Weathering is the breakdown and decay of rocks in place, at or near the surface of the Earth, by the action of the atmosphere, water, chemicals and living things. It does not involve movement of the material by an agent, which is the difference between weathering and erosion.

What are the types of weathering?

There are three types of weathering. Physical or mechanical weathering breaks rock without changing its minerals, chemical weathering changes the minerals through reactions such as carbonation, hydrolysis and oxidation, and biological weathering is caused by plants, animals and microorganisms, through root growth, burrowing and acids.

What is frost wedging?

Frost wedging is a physical weathering process in which water seeps into cracks in rock, freezes at night and expands by about nine per cent, widening the cracks. Repeated freezing and thawing eventually splits off pieces of rock, especially in cold mountain regions.

What is carbonation?

Carbonation is a chemical weathering process in which carbon dioxide dissolved in rainwater forms weak carbonic acid. This acid dissolves limestone and other carbonate rocks, forming caves, sinkholes and the landscape known as karst, and it is more active in humid regions.

Why is weathering important?

Weathering breaks rock into smaller pieces and forms regolith and soil, which support plant life. It prepares rock for erosion and creates many landforms, and in the tropics it concentrates ores such as bauxite, iron and manganese in the weathered layer, which become valuable mineral deposits.

PYQ Practice — Statement Analysis

1 Carbonation is an example of chemical weathering.

True

It dissolves limestone.

2 Frost wedging is a chemical weathering process.

False

It is a physical process.

3 Weathering involves transport of material by an agent.