Weathering Mechanisms
Micro-Crack Physics · Pore-Fluid & Lattice Chemistry · Rhizosphere Chelation · Regolith Architecture (28 Micro-Realms)
How Solid Crystalline Bedrock Yields at the Grain Boundary
At the microscopic scale, weathering is governed by four interlocking mechanisms: 1) Sub-critical tensile crack growth driven by unloading, thermal anisotropy, cryosuction, and salt crystallization pressure, 2) Pore-fluid mineral kinetics driven by soil-respired $\text{CO}_2$, proton exchange, and biotite oxidation expansion, 3) Biogeochemical rhizosphere chelation where organic ligands strip metallic cations, and 4) Two-stage regolith evolution that converts jointed rock into corestones, saprolite, and graded soil profiles.
I. Sub-Surface Stress & Micro-Crack Propagation Mechanics
Griffith Micro-Crack Law
- Stress concentrates at crack tips
- Rock tensile strength is 1/10th compressive
- Sub-critical stress corrosion cracking
Confining Pressure Release
- Erosion strips vertical load ($\sigma_v$)
- Horizontal stress ($\sigma_h$) remains locked
- Triggers upward tensile sheeting
Thermal Anisotropy ($\alpha_v$)
3.7x
1.0x
- Quartz expands $3\times$ more than feldspar
- Dark mafic grains absorb more heat
- Inter-granular shear snaps boundaries
Thermal Shock & Fire Spall
- Rock is a poor thermal conductor
- Outer 2–5 cm heats/cools rapidly
- Wildfires & desert rain spall shells
Cryosuction Ice Lenses
- Walder–Hallet frost-cracking model
- Capillary suction draws unfrozen water
- Feeds growing ice lenses at $-3^\circ\text{C}$
Correns' Salt Pressure
- Supersaturated brine in micro-pores
- Halite/Gypsum crystal growth pressure
- Exerts 10–200 MPa against pore walls
Double-Layer Clay Swelling
- Smectite/Montmorillonite clay lattices
- Adsorb polar water between sheets
- Osmotic swelling wedges shales apart
II. Pore-Fluid & Crystal-Lattice Chemical Mechanisms
Water Dipole Solvation
- Asymmetric V-shaped polar molecule
- $\delta^-$ oxygen pulls $\text{Na}^+, \text{Ca}^{2+}$ cations
- $\delta^+$ hydrogen pulls $\text{Cl}^-, \text{SO}_4^{2-}$ anions
Soil $\text{pCO}_2$ Acid Amplifier
- Atmosphere $\text{CO}_2 \approx 0.04\%$
- Root/microbe soil air $\text{CO}_2 = 1\text{–}5\%$
- Supercharges $\text{H}_2\text{CO}_3$ carbonation!
Proton ($\text{H}^+$) Exchange
- Tiny high-charge-density $\text{H}^+$ proton
- Penetrates silicate crystal frame
- Ejects $\text{K}^+, \text{Na}^+, \text{Ca}^{2+}$ into solution
Polynov's Ion Mobility
- $\text{Cl}^-, \text{SO}_4^{2-} > \text{Ca}^{2+}, \text{Na}^+ > \text{Mg}^{2+}, \text{K}^+$
- Silica ($\text{SiO}_2$) has moderate mobility
- $\text{Fe}_2\text{O}_3$ & $\text{Al}_2\text{O}_3$ stay as residual cap
Biotite Oxidation Wedge
- $\text{Fe}^{2+}$ in biotite mica oxidizes to $\text{Fe}^{3+}$
- Expands cleavage sheets by 40%
- Micro-fractures entire granite rock!
Eh–pH Redox Boundary
RUST
GLEY
- Water table marks redox boundary
- Vadose zone (+Eh): Insoluble $\text{Fe}^{3+}$
- Phreatic zone (-Eh): Mobile $\text{Fe}^{2+}$
Clay Neoformation Sequence
- Dry/poor drainage $\to$ 2:1 Smectite
- Moderate leaching $\to$ 1:1 Kaolinite
- Extreme monsoon $\to$ Gibbsite (Bauxite)
III. Biogeochemical, Rhizospheric & Microbial Mechanisms
Organo-Metallic Chelation
- Greek "Chele" = Crab's claw
- Humic, fulvic & citric organic acids
- Mobilizes otherwise insoluble Fe & Al!
Root Turgor Wedging
- Root tips enter hair-thin fractures
- Osmotic cell turgor + secondary cambium
- Exerts 1–2 MPa radial prying force
Rhizosphere Acid Halo
- 1–2 mm zone around living root hairs
- Roots pump $\text{H}^+$ to absorb $\text{K}^+, \text{Ca}^{2+}, \text{Mg}^{2+}$
- Drops local pore pH to 3.5–4.5
Lichens & Mycorrhizae
- Pioneer lithophytes on bare rock
- Secrete oxalic & lichenic acids
- Fungal hyphae bore micron tunnels
Chemolithotrophic Bacteria
THIOBACILLUS
- Acidithiobacillus ferrooxidans
- Oxidizes pyrite ($\text{FeS}_2$) into $\text{H}_2\text{SO}_4$
- Accelerates sulfide rock decay $10^5\times$
Faunal Bioturbation
- Earthworm casts & termite mounds
- Ingest & grind mineral grains
- Aerate profile with fresh $\text{O}_2$ & water
Biogenic Silica Pump
OPAL
- Grasses & bamboo uptake dissolved Si
- Precipitate opaline Phytoliths
- Regulates global dissolved silica flux
IV. Architectural Weathering Products & Profile Evolution
3 Exfoliation Pathways
- 1) Macro unloading $\to$ Exfoliation domes
- 2) Thermal $\Delta T \to$ Spheroidal Tors
- 3) Chemical hydration $\to$ Onion-skinning
Linton's Two-Stage Tor Model
ETCHING
TOR
- Stage 1: Deep chemical corestone rot
- Stage 2: Erosion washes away soft grus
- Leaves freestanding Castle Koppie/Tor
Liesegang Redox Rings
- Ostwald supersaturation-diffusion
- Concentric $\text{Fe}_2\text{O}_3$ bands in sandstone
- Mimics exfoliation in cross-section
Weathering Rinds & Varnish
- Discolored oxidized outer rock crust
- Thickness dates boulder exposure age
- Desert Varnish: Mn-Fe clay patina
Case Hardening & Tafoni
- Evaporation cements outer shell hard
- Shaded damp interior rots via salt
- Hollows boulder from the inside out!
6-Zone Engineering Profile
- Grade I (Fresh) to Grade VI (Soil)
- Saprolite retains original rock fabric
- Critical for dam & tunnel foundations
Weathering vs. Transport Limit
LIMITED
LIMITED
- Weathering-Limited: Bare desert cliffs
- Transport-Limited: Thick soil slopes
- Bridges Weathering to Realm 42
Core Distinctions
• Unloading & Pressure Release: Removal of overlying rock load causes large-scale upward expansion and fracturing into Exfoliation Domes.
• Thermal Expansion & Contraction: Differential heating of surface layers produces smooth-surfaced rounded boulders called Tors.
• Hydration & Spheroidal Weathering: Chemical water penetration along intersecting joints swells biotite/feldspar, peeling concentric onion-skin shells off rounded corestones.
In Carbonation, weak carbonic acid ($\text{H}_2\text{CO}_3$) dissolves calcium/magnesium carbonates. In Chelation, complex organic acids secreted by plant roots, lichens, and humus wrap around metallic cations ($\text{Fe}^{3+}, \text{Al}^{3+}$) like a claw—allowing even normally insoluble iron and aluminium to be leached downward (Podzolization).
3. Weathering-Limited vs. Transport-Limited LandscapesIn Weathering-Limited regimes (arid deserts, steep alpine crags), transport removes debris faster than weathering can create it, leaving bare bedrock slopes. In Transport-Limited regimes (humid tropics), weathering outpaces removal, building $30\text{–}60\text{ m}$ thick saprolite and regolith blankets.
Micro-Mechanism Matrix
Grain-Scale Force → Landform Output