Weathering
In-Situ Breakdown · Chemical (5) · Physical/Mechanical (4) · Biological · Exfoliation & Ore Enrichment (28 Micro-Realms)
The In-Situ Disintegration & Decomposition of Crustal Rocks (NCERT)
Weathering is defined as the mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate. Because very little or no motion of materials takes place, it is strictly an in-situ (on-site) process—operating via Chemical (Solution, Carbonation, Hydration, Oxidation, Reduction), Physical (Unloading, Thermal, Frost, Salt), and Biological pathways to create the regolith mantle essential for soils and economic ore enrichment.
I. In-Situ Fundamentals & Chemical Weathering Processes (NCERT)
In-Situ On-Site Process
- Action of weather & climate elements
- Very little or no motion of rock
- Strictly an on-site process (NCERT)
Disintegration vs. Decay
- Mechanical: Physical disintegration
- Chemical: Mineral decomposition
- Work together simultaneously
3 Major Weathering Groups
- 1. Chemical weathering processes
- 2. Physical / Mechanical processes
- 3. Biological weathering processes
4 Conditioning Factors
- Geological structure & rock type
- Climatic regime & topography
- Natural vegetative cover (NCERT)
Chemical Catalyst Triad
- Water + Air ($\text{O}_2, \text{CO}_2$) + Heat
- Must be present to speed reactions
- Plant decay boosts underground $\text{CO}_2$
1. Solution (Dissolution)
- Solids dissolve in water or acid
- Nitrates, sulphates & potassium
- Rock salt ($\text{NaCl}$) & gypsum leach out
2. Carbonation ($\text{H}_2\text{CO}_3$)
- Reaction of carbonate & bicarbonate
- $\text{CO}_2 + \text{H}_2\text{O} \to \text{H}_2\text{CO}_3$ (carbonic acid)
- Dissolves Limestone/Dolomite into caves
3. Hydration (Swelling)
- Chemical addition of water molecule
- Anhydrite ($\text{CaSO}_4$) $\to$ Gypsum
- Volume expansion fatigues rock
4. Oxidation (Rusting)
- Combination of mineral with $\text{O}_2$
- Iron, manganese & sulphur affected
- Red rock colour turns brown/yellow
5. Reduction (Anaerobic)
- Occurs in absence of oxygen
- Below water table / stagnant water
- Red iron reduces to greenish-grey
Goldich Stability Series
- Inverse of Bowen's Reaction Series
- Olivine & Ca-Plagioclase rot first
- Quartz survives as sand grains
II. Physical / Mechanical Weathering Processes (NCERT)
Physical Driving Forces
- 1) Overburden pressure (unloading)
- 2) Thermal expansion & contraction
- 3) Water/ice/salt crystal pressures
1. Unloading & Expansion
- Removal of overlying rock load
- Upper layers expand & fracture
- Produces large Exfoliation Domes
2. Temp Changes & Tors
- Diurnal desert heating & cooling
- Differential mineral expansion
- Smooth rounded Tors in granites
3. Frost Wedging (+9% Vol)
- Water in cracks freezes into ice
- Volume expands by ~9% (30 MPa)
- Pries rock apart into angular blocks
Freeze-Thaw & Talus Scree
- Rapid in mid-latitude high elevations
- Where freeze & melt alternate daily
- Builds angular Talus/Scree aprons
4. Salt Weathering (Tafoni)
- Salt crystals expand at $30\text{–}50^\circ\text{C}$
- $\text{NaCl}$ & $\text{CaSO}_4$ granular splitting
- Creates honeycomb polygonal pits
Wetting & Drying (Slaking)
- Repeated hydration & desiccation
- Coastal cliffs & tropical shales
- Outer layers flake & crumble off
III. Biological Weathering & Special Effects (Exfoliation — NCERT)
Burrowing Fauna Mixing
- Earthworms, termites, rodents
- Wedge & excavate fresh surfaces
- Expose deeper rock to air & moisture
Plant Roots & Humic Acids
- Tree roots exert tremendous pressure
- Algae, mosses & lichens secrete acids
- Decaying litter releases humic/$\text{H}_2\text{CO}_3$
Human Disturbance
DEFORESTATION
- Ploughing & cultivating soils
- Quarrying, road-cuts & mining
- Accelerates rock-water contact
Exfoliation (Result, Not Process)
- Exfoliation is a RESULT, not a process!
- Curved onion-skin sheets strip off
- From unloading OR thermal changes
Domes vs. Tors
- Exfoliation Domes: From unloading
- Tors: From thermal expansion
- Corners round into spheroidal boulders
IV. Significance of Weathering: Soils, Erosion & Ore Enrichment (NCERT)
Regolith & Soil Formation
- First step in pedogenesis (soil)
- Depth of regolith mantle controls
- Biome & biodiversity richness (NCERT)
Aid to Erosion & Relief
- Erosion cannot be significant if
- Rocks are not weathered first
- Accelerates reduction of high relief
Supergene Ore Enrichment
- Leaches away unwanted silica/salts
- Concentrates valuable residual ores
- Bauxite (Al), Iron, Manganese, Copper
Biogeochemical Carbon Sink
- Releases mineral ions to biosphere
- Silicate weathering consumes $\text{CO}_2$
- Earth's long-term climate thermostat
Indian Weathering Signatures
- Bhongir (Telangana): Exfoliation dome
- Hyderabad/Hampi: Granitic Tors
- Odisha/W. Ghats: Laterite Bauxite caps
Core Distinctions
Exfoliation is a result, not a process. Large, smooth, rounded Exfoliation Domes result from unloading and pressure release as overlying rock is stripped away (e.g., Bhongir/Bhuvanagiri fort rock in Telangana). Smooth-surfaced, rounded small-to-big boulders called Tors result from differential thermal expansion and contraction in granites.
2. Oxidation vs. Reduction Colour Shift (NCERT)• Oxidation: Occurs where atmosphere and oxygenated water are readily available; red iron minerals turn to brown or yellow rust.
• Reduction: Occurs in anaerobic conditions (below the water table or in stagnant waterlogged areas); red iron is chemically reduced to a greenish or bluish-grey colour.
Without chemical weathering and groundwater leaching away unwanted soluble materials (like silica and salts), many low-grade rock formations would never concentrate into economically viable deposits of Aluminium (Bauxite), Iron, Manganese, Nickel, and Copper.
Master Weathering Matrix
NCERT Chapter 6 Classification