Chemical weathering

Detail

What chemical weathering is

• Chemical weathering is the breaking down of rock by chemical reactions. The rock does not just crack. Its minerals change into new substances.
• Rocks react with oxygen, surface water, soil water and acids.
• The reactions decompose rock (break it into new compounds), dissolve it, or reduce it to a fine clastic state (tiny loose grains such as clay and silt).

• Class 9 puts it simply: minerals react with water, air or acids and form new substances.

• Five named processes: solution, carbonation, hydration, oxidation and reduction.

• Speed-up factors:
• Water is the medium in which almost every reaction takes place.
• Air supplies oxygen (for oxidation) and carbon dioxide (for carbonation).
• Heat makes chemical reactions run faster.

• Rotting plants and animals release extra CO₂ into the soil and underground. This makes soil water more acidic than rainwater. Rain picks up CO₂ from the soil and forms carbonic acid [3].

• How it differs from physical weathering: physical weathering breaks rock into smaller pieces of the same material. Chemical weathering makes new material. Physical cracking, however, opens fresh surfaces, so the two processes often work together.

1. Solution

• Solution means minerals dissolve in water or weak acids and are carried away in the water.
• Rock salt and gypsum dissolve in plain water.

• Limestone dissolves only when the water is acidic (see carbonation).

• Result: the rock loses mass without leaving much solid waste behind. Hollows and pits form on rock surfaces.

2. Carbonation

• Carbonation is the reaction of carbonate rocks with carbonic acid.
• Step by step:
• Rainwater takes in CO₂ from the air and soil, which forms weak carbonic acid (H₂O + CO₂ → H₂CO₃) [2].
• The acid reacts with limestone (calcium carbonate) and dolomite (calcium–magnesium carbonate).
• It turns them into soluble bicarbonates (CaCO₃ + H₂CO₃ → Ca(HCO₃)₂), which flowing water carries away.

• Carbonic acid is especially good at dissolving limestone [2].

• Odd temperature rule: carbonation is faster in cold water, because cold water holds more dissolved CO₂. For this reason it is the only chemical process that stays active in cold regions.

• Landform result: karst topography
• Karst topography is a landscape formed when soluble rock such as limestone or dolomite dissolves. It has caves, sinkholes and underground drainage [2][3].
• Acidic water seeps down cracks, widens them and hollows out networks of caves [2].
• Most solution caves form at fairly shallow depths, made by water rich in carbonic acid from recent rainfall [4].
• Sinkhole: a funnel-shaped hollow in the ground surface, formed where rock below has dissolved [2].

• Tower karst: steep limestone hills separated by deep gorges. It is mainly found in the tropics, where dissolving along cracks in thick limestone cuts deep gorges [5].

• India example: Meghalaya

• Meghalaya's underground caves formed because four things come together there [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].

3. Hydration

• Hydration means water is chemically added to a mineral. The water becomes part of the crystal.
• Step by step:
• Water joins the mineral → the mineral swells (increases in volume).
• The swelling puts stress on the rock → the grains loosen and the rock weakens.

• Repeated hydration and dehydration (wetting and drying) → the rock crumbles and flakes.

• Examples:

• Anhydrite → gypsum, when groundwater binds to anhydrite [2].

• Feldspar → clay. Feldspar is the most common mineral group in granite.

• Exam point: clay is the material most affected by hydration (Class 11 exercise).

4. Oxidation

• Oxidation means minerals combine with oxygen. It mostly affects minerals that contain iron.
• Step by step:
• Iron-bearing minerals meet oxygen dissolved in water or air.
• They form iron oxides. In everyday terms this is rust, made when oxygen and iron react in the presence of water [2].

• The oxides are weaker than the original minerals, so the rock breaks down more easily.

• Colour result: red, brown or yellow coatings.

• This oxidation explains the red colour of laterite soils and red soils.
• In India, red soils cover the eastern and southern parts of the Deccan plateau. Laterite is found in high-rainfall, high-temperature areas such as Kerala, Karnataka and the hills of Odisha.

5. Reduction

• Reduction is the reverse of oxidation. It happens where oxygen is missing, for example below the water table (the level below which the ground is fully soaked with water) and in waterlogged soils.
• Result: iron loses oxygen and the material turns grey-green or bluish. Pale, mottled layers in waterlogged soils come from this.

Comparison of the five processes

Process	Agent	What happens	Typical rock or mineral	Visible result
Solution	Water / weak acid	Mineral dissolves, 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, 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

Where chemical weathering is strongest

• Hot, humid climates: a lot of water plus high heat means reactions are fast and go deep.

• This creates deep tropical weathering profiles: thick layers of rotted rock (regolith) below the soil (NCERT Class 11, Figure 5.2).

• Deserts: slow, because there is too little water.

• Cold regions: slow, because the low heat slows the reactions.
• Exception: carbonation, which is faster in cold water because cold water holds more CO₂.

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

Prelims Hooks

• Clay is the material most affected by hydration (NCERT Class 11 exercise).
• Carbonation is faster in cold water than in warm water, because cold water holds more CO₂.
• Anhydrite → gypsum is hydration. It is not carbonation or oxidation.
• Feldspar → clay is hydration.
• Red colour of laterite and red soils → oxidation of iron. Grey-green colour below the water table → reduction.
• Process–landform pair: carbonation → karst topography (caves, sinkholes, tower karst).
• Rock salt and gypsum dissolve in plain water (solution). Limestone needs acid to dissolve (solution or carbonation).
• Meghalaya's limestone caves lie in the Khasi, Jaintia and Garo hills [6].
• Trap: "Chemical weathering is weak in all cold regions." This is wrong for carbonation.
• Decaying organic matter adds CO₂ to soil water, which speeds up carbonation underground.

Mains Points

• Climate decides the type of weathering. Hot, humid tropics such as India's Western Ghats and North-East have deep chemical weathering profiles. The result is thick, leached laterite and red soils that are poor in nutrients. This affects farming and the need for fertiliser (GS-I and GS-III).
• Karst terrain is a hazard and a fragile resource. In limestone areas, rock dissolves along cracks and forms caves and sinkholes [2][4]. This weakens the ground under roads and buildings. Water also moves quickly underground through open channels, so pollutants spread easily. Meghalaya, with its very high rainfall and good limestone [6], shows how cave conservation can clash with limestone mining.
• Links to resources: long chemical weathering in the tropics concentrates insoluble iron and aluminium oxides (in laterite). Hydration of feldspar produces clay deposits. Both are economic minerals, which links weathering to the distribution of minerals in GS-I.
• Links to carbon and climate: carbonation consumes atmospheric and soil CO₂ and carries it away in rivers as bicarbonate [2][3]. This makes chemical weathering part of the long-term carbon cycle, a useful link in GS-I and GS-III answers on climate.