Lichen Bioweathering: Etch-Pit Formation

Bioweathering Controls

Organic acid secretion 1.0×

Surface temperature 18 °C

Thallus radial growth rate 0.8 mm/yr

Time acceleration 40 yr/s

Weathering State

Elapsed time

0 yr

Colonized area

0.0%

Mean etch depth

0.0 µm

Mineral mass lost

0.00 g/m²

How it works

Crustose lichens weather silicate rock chemically: fungal hyphae secrete organic acids — chiefly oxalic acid — that chelate metal cations (Al³⁺, Ca²⁺, Fe³⁺, K⁺) directly out of the feldspar lattice, breaking Si–O–Al bonds far faster than plain water or CO₂-driven weathering would. The freed cations often re-precipitate under the thallus as calcium-oxalate crystals (whewellite), a mineral signature geologists use to detect lichen weathering in the rock record.

Each mineral grain's etch-pit depth follows chelation-limited dissolution kinetics with Arrhenius temperature dependence:

dD/dt = k₀ · S · f(age) · exp[ −Ea/R · (1/T − 1/T_ref) ]

D      etch-pit depth (µm)
k₀     base dissolution-rate constant
S      acid secretion control (relative units)
f(age) thallus maturity, 0→1 as the colony establishes
Ea     activation energy ≈ 50 kJ/mol (oxalic-acid/feldspar)
R      gas constant, 8.314 J/(mol·K)
T      surface temperature (K), T_ref = 298.15 K

Lichen colonies expand radially at a roughly constant linear rate once established — the basis of real-world lichenometry, used to date rock surfaces (moraines, tombstones, rockfalls) from thallus diameter:

r(t) = min( r_max, v_growth · t )

Organic acid secretion — scales the chelation rate S; higher values pit the rock faster.
Surface temperature — enters the Arrhenius factor; warmer surfaces weather faster, colder ones far slower.
Thallus growth rate — sets v_growth, how quickly each colony's colonized radius expands.
Time acceleration — simulated years advanced per real second, so multi-century weathering is visible in minutes.
+ New Colony — seeds an additional lichen thallus at a random uncolonized grain.

Lichen thallus Fresh feldspar Etched mineral

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