Chemical weathering

Topic: Geomorphic Processes: Weathering, Mass Movement, Erosion and Soil Formation · NCERT: Class 11, Ch 5 "Geomorphic Processes"

Meaning

Chemical weathering is the breaking down of rock by chemical reactions with oxygen, surface water, soil water and acids. The rock's minerals change into new substances, dissolve, or turn into a fine clastic state (tiny loose grains such as clay and silt). It matters because it creates soil material and karst landscapes such as caves and sinkholes, and it explains the red colour of laterite and red soils. It is strongest in hot, humid climates.

Explanation

How it works: cause → process → result

• Cause: rock meets water, air and acids.
• Water is the medium in which almost every reaction takes place.
• Air supplies oxygen (for oxidation) and carbon dioxide (for carbonation).

• Acids such as carbonic acid attack certain minerals.

• Process: minerals react and change.

• Some minerals dissolve and are carried away.

• Some take in water or oxygen and become weaker new minerals.

• Result: the rock decomposes, dissolves or crumbles into fine grains.

• Physical weathering only breaks rock into smaller pieces of the same material. Chemical weathering makes new material.
• The two often work together. Physical cracking opens fresh surfaces, and chemicals then attack those surfaces.

The five processes

• 1. Solution: minerals dissolve in water or weak acids and are carried away.
• Rock salt and gypsum dissolve in plain water.
• Limestone dissolves only when the water is acidic.

• Result: the rock loses mass with little solid waste left behind. Pits and hollows form on rock surfaces.

• 2. Carbonation: carbonate rocks react with carbonic acid.

• Rainwater takes in CO₂ from the air and soil and forms weak carbonic acid (H₂O + CO₂ → H₂CO₃) [2].
• The acid reacts with limestone (calcium carbonate) and dolomite (calcium–magnesium carbonate).
• These turn into soluble bicarbonates (CaCO₃ + H₂CO₃ → Ca(HCO₃)₂), and flowing water carries them away.
• Carbonic acid is especially good at dissolving limestone [2].

• Landform result: karst topography (a landscape of caves, sinkholes and underground drainage, formed when soluble rock such as limestone or dolomite dissolves) [2][3].

• 3. Hydration: water is chemically added to a mineral and becomes part of the crystal.

• Water joins the mineral, so the mineral swells (gets bigger).
• The swelling stresses the rock. Its grains loosen and the rock weakens.
• Repeated wetting and drying (hydration and dehydration) makes the rock crumble and flake.

• Examples: anhydrite → gypsum, when groundwater binds to anhydrite [2]. Feldspar → clay. Feldspar is the most common mineral group in granite.

• 4. Oxidation: minerals combine with oxygen. It mostly affects minerals that contain iron.

• Iron-bearing minerals meet oxygen in water or air and form iron oxides. In everyday terms this is rust, which forms when oxygen and iron react in the presence of water [2].
• Iron oxides are weaker than the original minerals, so the rock breaks down more easily.

• Result: red, brown or yellow coatings on rock and soil.

• 5. Reduction: the reverse of oxidation. It happens where oxygen is missing.

• It takes place below the water table (the level below which the ground is fully soaked with water) and in waterlogged soils.
• Iron loses oxygen, and the material turns grey-green or bluish.
• The pale, mottled layers in waterlogged soils come from this.

Process	Agent	What happens	Typical rock or mineral	Visible result
Solution	Water / weak acid	Mineral dissolves and is removed	Rock salt, gypsum, limestone (with acid)	Pits, hollows
Carbonation	Carbonic acid	Carbonate turns into soluble bicarbonate	Limestone, dolomite	Karst: caves, sinkholes
Hydration	Water added chemically	Mineral swells and weakens	Anhydrite → gypsum; feldspar → clay	Crumbling, flaking
Oxidation	Oxygen	Iron oxides form	Iron-bearing minerals	Red, brown or yellow colour
Reduction	Lack of oxygen	Oxygen is removed	Iron compounds below the water table	Grey-green colour

Conditions that speed it up

• Water: more water means more reactions.
• Heat: chemical reactions run faster when it is warm.
• Rotting plants and animals: they release extra CO₂ into the soil and underground.
• This makes soil water more acidic than rainwater.

• Rain picks up this CO₂ from the soil and forms carbonic acid [3].

• The odd one out, carbonation: it is faster in cold water, because cold water holds more dissolved CO₂. So it is the only chemical process that stays active in cold regions.

Climate	Chemical weathering	Main reason
Hot, humid (equatorial, tropical monsoon)	Very high; deep profiles	Plenty of water and heat
Hot desert	Low	Too little water
Cold (polar, high mountain)	Low, except carbonation	Low heat; cold water holds more CO₂

Where it occurs

• Hot, humid tropics (world): fast, deep reactions create deep tropical weathering profiles. These are thick layers of rotted rock (regolith) lying below the soil (NCERT Class 11, Figure 5.2).
• Tropical limestone areas (world): tower karst, which means steep limestone hills separated by deep gorges. It forms mainly in the tropics, where rock dissolving along cracks in thick limestone cuts deep gorges [5].
• Limestone areas in general: solution caves mostly form at fairly shallow depths. They are made by water rich in carbonic acid from recent rainfall [4]. Acidic water seeps down cracks, widens them and hollows out cave networks [2]. Sinkholes (funnel-shaped hollows in the ground surface) form where the rock below has dissolved [2].
• Meghalaya (India): underground caves formed here because four things come together [6]:
• the world's highest rainfall
• good-quality limestone
• low elevation
• a hot, moist climate

• Limestone of varying quantity and quality occurs in all three hill ranges: Khasi, Jaintia and Garo [6].

• Deccan plateau (India): oxidation of iron gives the red colour of the red soils of the eastern and southern Deccan.

• Kerala, Karnataka and the hills of Odisha (India): laterite forms in these high-rainfall, high-temperature areas, and its red colour also comes from oxidation.
• Western Ghats and North-East India: hot, humid regions with deep chemical weathering profiles.
• Deserts: chemical weathering is weak because there is too little water.
• Cold regions (polar, high mountain): chemical weathering is weak, except carbonation.

Don't confuse with

• Physical (mechanical) weathering: breaks rock into smaller pieces of the same material. Chemical weathering makes new substances.
• Hydration vs carbonation: anhydrite → gypsum and feldspar → clay are hydration (water joins the crystal). Carbonation is the reaction of carbonic acid with limestone and dolomite.
• Oxidation vs reduction: oxidation adds oxygen to iron and gives red, brown or yellow colours, as in laterite and red soils. Reduction takes place where oxygen is missing, below the water table, and gives grey-green colours.
• Solution vs carbonation: rock salt and gypsum dissolve in plain water (solution). Limestone needs acidic water to dissolve (carbonation).

Prelims Hooks

• Clay is the material most affected by hydration (NCERT Class 11 exercise). Anhydrite → gypsum and feldspar → clay are both hydration, not carbonation or oxidation.
• Carbonation is faster in cold water than in warm water, because cold water holds more CO₂. Trap: "Chemical weathering is weak in all cold regions" is wrong, because carbonation stays active there.
• Process–landform pair: carbonation → karst topography (caves, sinkholes, tower karst) [2][3].
• Colour matches: red colour of laterite and red soils → oxidation of iron. Grey-green colour below the water table → reduction.
• Meghalaya's limestone caves lie in the Khasi, Jaintia and Garo hills [6].
• Decaying organic matter adds CO₂ to soil water. This makes soil water more acidic than rainwater and speeds up carbonation underground [3].

Mains Points

• Climate decides the type of weathering, and that shapes soils and farming (GS-I, GS-III).
• Hot, humid regions such as India's Western Ghats and North-East have deep chemical weathering profiles.
• Heavy rain washes nutrients out of the soil (leaching), leaving thick laterite and red soils that are poor in nutrients.

• Farmers in these areas therefore need more fertiliser and careful soil management.

• Karst terrain is both a hazard and a fragile resource (GS-I, GS-III).

• Rock dissolves along cracks and forms caves and sinkholes [2][4]. This weakens the ground under roads and buildings.
• Water moves quickly underground through open channels, so pollutants spread easily.

• Meghalaya has very high rainfall and good limestone [6], so cave conservation there can clash with limestone mining.

• Links to mineral resources and the carbon cycle (GS-I, GS-III).

• Long chemical weathering in the tropics concentrates insoluble iron and aluminium oxides in laterite. Hydration of feldspar produces clay deposits. Both are economic minerals.
• Carbonation uses up CO₂ from the air and soil, and rivers carry it away as bicarbonate [2][3]. This makes chemical weathering part of the long-term carbon cycle, which is a useful point in answers on climate.

Related concepts

• Solution
• Carbonation
• Hydration
• Oxidation

Read more

• Chemical weathering