Mechanical & Chemical
Fracture Physics · Reaction Kinetics · Salt & Frost Pressures · Spheroidal Synergy & Saprolite (28 Micro-Realms)
The Coupled Physics & Chemistry of Rock Breakdown
Mechanical (Physical) Weathering fractures rock into smaller angular fragments without changing its mineral composition, exponentially multiplying the exposed surface area. Chemical Weathering attacks those newly exposed surfaces with water, $\text{O}_2$, and acids ($\text{H}_2\text{CO}_3$)—altering unstable primary silicates into secondary clay minerals (kaolinite), iron oxides, and soluble ions while weakening grain boundaries for further mechanical splitting.
I. Mechanical Disintegration: Unloading & Thermal Stress Physics
Physical vs. Chemical
- Mechanical: Physical size reduction
- Retains original mineral identity
- Chemical: Alters crystal lattice
Surface-Area Multiplier
- Halving cube length doubles area
- Mechanical cracks open conduits
- Speeds up chemical attack 10x
Unloading Sheeting Joints
- Denudation removes overburden load
- Elastic upward rock relaxation
- Sheet joints parallel to surface
Exfoliation Domes (NCERT)
- Massive unjointed granite plutons
- Spalls thick curved rock slabs
- Bhongir (Telangana) & Half Dome
Block Disintegration
- High diurnal $\Delta T$ in hot deserts
- Homogeneous jointed rocks (Quartzite)
- Splits along orthogonal joints
Granular Disintegration
- Coarse polymineralic rocks (Granite)
- Dark biotite expands more than quartz
- Crumbles grain-by-grain into Grus
Thermal Tors (NCERT)
- Differential thermal expansion
- Heaving of outer rock layers
- Smooth rounded boulders (1–10 m)
II. Crystal Growth Pressures: Frost Wedging, Salt & Slaking (NCERT)
Frost Wedging (+9% Ice)
- Water freezes in joints at $0^\circ\text{C}$
- Expands ~9% (exerts up to 210 MPa)
- Exceeds tensile strength of granite
Ice-Lens Segregation
- Unfrozen pore water migrates
- Feeds growing sub-surface ice lens
- Shatters porous chalk & shale
Felsenmeer & Talus Cones
- Felsenmeer: Periglacial blockfield
- Talus/Scree: Angular cliff apron
- Ladakh & Spiti high mountain passes
3 Salt Weathering Modes
- 1) Thermal expansion ($30\text{–}50^\circ\text{C}$)
- 2) Hydration volume swell
- 3) Supersaturated crystallization
Halite & Gypsum Splitting
CaSO₄
- Ca, Na, Mg, K & Ba salts (NCERT)
- $\text{NaCl}$ & $\text{CaSO}_4$ most effective
- Splits desert chalk, sandstone, gneiss
Tafoni & Honeycomb Pits
- Granular foliar salt fretting
- Individual grains fall off
- Carves coastal/arid honeycomb pits
Wetting-Drying Slaking
- Clay-rich shales absorb water
- Ordered water molecules push layers
- Crumbles upon drying (Slaking)
III. Chemical Decomposition: Reaction Kinetics & Mineral Alteration
Van't Hoff's Thermal Law
2x TO 3x REACTION RATE
- Reaction speed doubles per $+10^\circ\text{C}$
- Warm + wet humid tropics maximize
- Dry deserts & ice caps inhibit
Solution Kinetics (NCERT)
- Polar $\text{H}_2\text{O}$ dissociates ionic salts
- Nitrates, sulphates, potassium, halite
- Leaves zero solid residue behind
Carbonation Equation
- Calcite + Carbonic acid reaction
- Yields soluble Calcium Bicarbonate
- Soil $\text{CO}_2$ is 10–100x atmospheric!
Hydration Lattice Swell
- Reversible physical-chemical bridge
- Anhydrite $\to$ Gypsum (+38% volume)
- Hematite $\to$ Limonite yellow rust
Silicate Hydrolysis
- $\text{H}^+$ ions replace $\text{K}^+, \text{Na}^+, \text{Ca}^{2+}$
- Converts Feldspar into Kaolinite clay
- Primary producer of clay minerals
Oxidation ($\text{Fe}^{2+} \to \text{Fe}^{3+}$)
- Ferrous iron oxidizes to Ferric
- Collapses pyroxene/amphibole lattice
- Forms red Hematite & yellow Goethite
Reduction & Gleying
- Waterlogged anaerobic bog/paddy soils
- Insoluble $\text{Fe}^{3+}$ reduces to soluble $\text{Fe}^{2+}$
- Leaves bluish-grey gley horizon
IV. Mechanical–Chemical Synergy, Spheroidal Corestones & Duricrusts
Mutual Feedback Loop
- Mechanical cracks let water enter
- Chemical hydration swells cracks wider
- Never operate in isolation (NCERT)
3D Corner Attack (Spheroidal)
- Corners attacked from 3 sides
- Edges from 2 sides, faces from 1
- Turns cubic joint block into sphere!
Basal Weathering Front
- Ruxton & Berry tropical profile
- Sharp boundary at fresh bedrock
- Stripping exposes Bornhardt inselbergs
Peltier's Climate Diagram
FROSTHOT/WET:
CHEMICAL
- Plots Mean Annual $T$ vs Rainfall $P$
- Periglacial: Peak mechanical wedging
- Equatorial: Peak chemical rotting
Desilication & Laterite
- Intense monsoon wet-dry leaching
- Silica washes out (Desilication)
- Leaves hard iron-alumina Laterite brick
Duricrusts & Kankar
- Capillary rise in semi-arid climates
- Calcrete ($\text{CaCO}_3$ — Indian Kankar)
- Silcrete ($\text{SiO}_2$) & Ferricrete ($\text{Fe}_2\text{O}_3$)
Master Comparison Key
- Mechanical $\to$ Angular clasts & sand
- Chemical $\to$ Clay, oxides & ions
- Together build Earth's soil mantle
Core Distinctions
• Mechanical Weathering: Driven by unloading, thermal $\Delta T$, frost wedging, and salt crystal growth; breaks rock into smaller fragments without changing its chemical/mineral composition; dominates in cold periglacial and hot arid climates.
• Chemical Weathering: Driven by water, $\text{O}_2$, and $\text{CO}_2$; decomposes primary minerals into new secondary minerals (clays, oxides); dominates in warm, humid tropical climates.
When chemical water penetrates intersecting rectangular rock joints, corners are attacked simultaneously from 3 sides, edges from 2 sides, and flat faces from only 1 side—causing corners to weather fastest and transforming a cube into concentric spherical shells around a rounded corestone.
3. Laterite (Tropical Humid) vs. Kankar Calcrete (Semi-Arid)In heavy-rainfall regions (Western Ghats/Odisha), downward leaching removes silica and bases, leaving an iron/aluminium Laterite (Ferricrete) crust. In semi-arid regions (Bangar plains/Rajasthan), upward capillary evaporation precipitates calcium carbonate nodules called Kankar (Calcrete).
Mechanical vs. Chemical Matrix
Process → Product Conversion Key