Weathering

Weathering is the in situ physical breakdown (mechanical disaggregation) and chemical alteration (decomposition) of rocks and minerals at or near Earth's surface upon exposure to the atmosphere, hydrosphere, and biosphere. In geological terminology, weathering is distinguished from erosion: weathering degrades and transforms parent bedrock in place to produce unconsolidated regolith, soil horizons, secondary clay minerals, and dissolved aqueous ions, whereas erosion involves the kinetic detachment and downslope or downstream mass transport of those weathered products by flowing water, wind, ice, or gravity within the broader rock cycle.

From a thermodynamic perspective, weathering occurs because igneous and metamorphic mineral assemblages—such as olivine, pyroxene, amphibole, and plagioclase feldspar—crystallize at elevated subsurface temperatures (400–1,200 °C) and lithostatic pressures (0.1–3.0 GPa) under relatively anhydrous, low-oxygen conditions. When tectonic uplift and exhumation bring these rocks into the near-surface Critical Zone (characterized by ambient temperatures of −20 to 40 °C, atmospheric pressure of 0.1 MPa, abundant liquid H₂O, dissolved O₂, and biologically elevated partial pressures of CO₂), the primary minerals are far from chemical equilibrium and spontaneously react toward lower Gibbs free energy states represented by hydrous phyllosilicates (clays), ferric oxides, and aqueous solutes.

Stress Fracture Propagation and Surface-Area Amplification

Physical (mechanical) weathering fractures coherent bedrock into progressively smaller clasts without altering the internal crystal chemistry of the constituent minerals. Its primary geochemical consequence is the geometric amplification of reactive specific surface area: cleaving a single 1-meter cube of granite (surface area = 6 m²) into 1-millimeter cubes (10⁹ particles) increases the total exposed surface area by a factor of 1,000 to 6,000 m², accelerating the kinetics of surface-controlled chemical dissolution. Five principal mechanical stress regimes drive physical weathering:

• Frost Wedging and Ice Segregation: When liquid water freezes into hexagonal ice Ih within rock joints and pores, its specific volume increases by approximately 9.05%, capable of generating theoretical confining pressures up to 207 MPa in sealed cavities. Even more effective in natural fractured rock is ice segregation (cryosuction): between −3 °C and −8 °C, thin films of unfrozen adsorbed water migrate along chemical potential gradients toward growing ice lenses, exerting tensile stresses of 2 to 15 MPa that exceed the tensile strength of most crustal rocks (typically 3–12 MPa) and produce angular alpine talus slopes.
• Unloading and Pressure-Release Exfoliation: Deep plutonic bodies such as granitic batholiths crystallize under 10 to 25 km of overburden (lithostatic pressures of 300 to 700 MPa). As erosion strips away the overlying crust, elastic decompression occurs perpendicular to the land surface, generating sub-parallel sheet joints spacing from centimeters near the surface to several meters at depth and producing dome landforms such as Half Dome in Yosemite and Sugar Loaf in Rio de Janeiro.
• Salt-Crystal Growth (Haloclasty): In arid deserts and coastal spray zones, saline pore waters containing NaCl, CaSO₄, or Na₂SO₄ evaporate within rock microcracks. Crystallization pressure and subsequent hydration expansion (such as the transition of anhydrous thenardite Na₂SO₄ to mirabilite Na₂SO₄·10H₂O, which increases molar volume by >300%) disaggregate sandstone and granite grain by grain, forming honeycomb tafoni cavities.
• Thermal Insolation Fatigue and Biotic Wedging: Diurnal solar heating and nocturnal radiative cooling create steep thermal gradients in desert boulders; because minerals such as quartz and feldspar possess differing coefficients of thermal expansion, millions of micro-strain cycles induce intergranular fatigue cracks. Simultaneously, penetrating plant roots exert radial turgor pressures of 0.5 to 1.5 MPa, physically prying apart pre-existing fractures.

Hydrolysis, Dissolution, Oxidation, and Biological Chelation

Chemical weathering decomposes primary minerals through aqueous proton (H⁺) attack, electron transfer (redox), and ligand complexation. Rainwater equilibrating with atmospheric carbon dioxide (*p*CO₂ ≈ 420 μatm) forms weak carbonic acid (H₂CO₃) with a pH of ~5.65; within vegetated soils, root and microbial respiration elevate soil-gas *p*CO₂ by 10- to 100-fold (10^−2.5 to 10^−1.5 atm) alongside humic, fulvic, and oxalic acids, lowering soil water pH to 4.0–5.0:

CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq)

Four principal chemical reactions govern mineral alteration:

1. Acid Hydrolysis of Silicates: The most volumetrically important weathering reaction on continents is the proton-promoted hydrolysis of feldspars (which constitute >50% of Earth's crust). In the hydrolysis of potassium feldspar (orthoclase, KAlSi₃O₈), H⁺ ions exchange with K⁺ cations in the crystal lattice, disrupting the aluminosilicate framework to form insoluble kaolinite clay, dissolved silicic acid, and potassium bicarbonate:

2 KAlSi₃O₈(s) + 2 H₂CO₃(aq) + 9 H₂O(l) → Al₂Si₂O₅(OH)₄(s) + 4 H₄SiO₄(aq) + 2 K⁺(aq) + 2 HCO₃⁻(aq)

1. Congruent Dissolution: Carbonate and evaporite minerals dissolve completely into aqueous ions without leaving a solid residue. Carbonic acid dissolves calcite in limestone according to CaCO₃(s) + H₂CO₃(aq) ⇌ Ca²⁺(aq) + 2 HCO₃⁻(aq), sculpting subterranean caverns, sinkholes, and tower karst topography.
2. Oxidation: Ferrous iron (Fe²⁺) bound within mafic silicates (olivine, pyroxene, biotite) and sulfide minerals (pyrite, FeS₂) reacts with dissolved O₂ and H₂O to form insoluble ferric (Fe³⁺) oxyhydroxides such as goethite (FeOOH) and hematite (Fe₂O₃), staining weathered outcrops yellow-brown to brick-red. Oxidation of pyrite additionally generates sulfuric acid (H₂SO₄), driving rapid acid rock drainage.
3. Biological Chelation: Lichens, ectomycorrhizal fungi, and plant roots secrete low-molecular-weight organic anions (oxalate, citrate, malate) and siderophores that form stable multidentate chelate rings around poorly soluble Al³⁺, Fe³⁺, and Ca²⁺ ions, accelerating mineral dissolution rates by factors of 2 to 50 over abiotic baselines.

Goldich's Dissolution Series and the Urey Carbon Thermostat

In 1938, the American geochemist Samuel S. Goldich analyzed soil profiles developed over crystalline rocks and discovered that the relative resistance of common rock-forming silicate minerals to chemical weathering is the direct inverse of Bowen's reaction series:

Goldich stability reflects crystal-chemical bond energetics and polymerization: olivine consists of isolated SiO₄⁴⁻ tetrahedra linked by weak ionic Mg²⁺–O and Fe²⁺–O bonds that are rapidly leached by protons, whereas quartz consists of a three-dimensional framework of strong, partially covalent Si–O–Si bonds (bond dissociation energy ~452 kJ·mol⁻¹) with no leachable interstitial cations and a Mohs scale hardness of 7.

On planetary timescales, chemical weathering of calcium- and magnesium-bearing silicates (such as plagioclase and pyroxene in continental basalt provinces) governs Earth's long-term carbon cycle via the Urey reaction (CaSiO₃ + CO₂ → CaCO₃ + SiO₂). Because silicate hydrolysis obeys an Arrhenius-type temperature dependence (rate proportional to exp(−*E*_a/RT), with activation energies E_a ≈ 40–80 kJ·mol⁻¹) and scales with continental runoff, episodes of elevated atmospheric CO₂ and global warming accelerate continental silicate weathering, drawing down CO₂ into marine carbonate sediments and stabilizing Earth's surface climate over 10⁵ to 10⁷ years.

Why is carbonate dissolution neutral for atmospheric carbon dioxide over millions of years whereas silicate weathering is a net sink?

Dissolving limestone (CaCO₃ + CO₂ + H₂O → Ca²⁺ + 2 HCO₃⁻) consumes one mole of atmospheric CO₂ on land, but when marine organisms later precipitate that calcium back into calcium carbonate on the seafloor (Ca²⁺ + 2 HCO₃⁻ → CaCO₃ + CO₂ + H₂O), the exact same mole of CO₂ is released back to the ocean–atmosphere system. By contrast, weathering calcium silicate (CaSiO₃ + 2 CO₂ + H₂O → Ca²⁺ + 2 HCO₃⁻ + SiO₂) consumes two moles of CO₂ and releases only one mole during marine carbonate precipitation, permanently sequestering one net mole of CO₂ into limestone.

How does climate control whether physical or chemical weathering dominates?

As synthesized in Louis Peltier's (1950) morphoclimatic weathering diagrams, chemical weathering requires both liquid water and thermal energy to drive reaction kinetics, reaching maximal intensity in hot, humid tropical rainforests (mean annual temperature >22 °C, precipitation >2,000 mm·yr⁻¹), where deep lateritic soils and bauxite (Al(OH)₃) form. Physical weathering dominates in cold periglacial and alpine environments (mean annual temperature −10 to 2 °C) with frequent freeze–thaw cycles, whereas hyper-arid deserts experience slow overall weathering dominated by salt crystallization and thermal fatigue.

What causes spheroidal weathering in jointed igneous bedrock?

When orthogonal tectonic joints divide a granite or basalt outcrop into cubic or rectangular blocks, meteoric water percolating along the fracture network attacks the flat faces along one plane, the edges from two intersecting planes, and the corners from three intersecting planes simultaneously. Because chemical hydrolysis and clay expansion proceed fastest where the surface-area-to-volume ratio is highest, the corners and edges weather inward more rapidly than the faces, progressively rounding angular blocks into concentric spherical boulders ("corestones").

1. Goldich, S. S. (1938). "A study in rock-weathering." The Journal of Geology, 46(1), 17–58. https://doi.org/10.1086/624619
2. Berner, R. A., Lasaga, A. C., & Garrels, R. M. (1983). "The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years." American Journal of Science, 283(7), 641–683. https://doi.org/10.2475/ajs.283.7.641
3. Murton, J. B., Peterson, R., & Ozouf, J.-C. (2006). "Bedrock fracture by ice segregation in cold regions." Science, 314(5802), 1127–1129. https://doi.org/10.1126/science.1132127
4. Brantley, S. L., Goldhaber, M. B., & Ragnarsdottir, K. V. (2007). "Crossing disciplines and scales to understand the Critical Zone." Elements, 3(5), 307–314. https://doi.org/10.2113/gselements.3.5.307