Chemical and Physical Weathering

Introduction

Breaking rocks down: the physical and chemical forces that turn solid rock into sediment and soil

Weathering is the in-place breakdown of rock at or near Earth's surface, and it is the essential first step in producing sediment, soil, and the landforms that define every landscape. Two broad categories of weathering operate simultaneously and reinforce each other: physical (mechanical) weathering, which disintegrates rock into smaller pieces without changing its mineral chemistry, and chemical weathering, which decomposes minerals through chemical reactions, transforming rock from the inside out.

Physical weathering processes are most effective wherever temperature fluctuates across critical thresholds or where confining pressure changes. Freeze-thaw cycling (frost wedging) is the most powerful mechanical agent in alpine and periglacial environments. Water is anomalous in that it expands approximately 9% on freezing, exerting pressures of up to 207 MPa in confined rock pores and joints. The process is most effective between 0°C (32°F) and −5°C (23°F), because in that range water remains liquid in larger pores but begins to freeze in smaller ones, maximising the hydraulic pressure differential. Below −5°C (23°F), most pore water is already frozen and volumetric change is small; above 0°C (32°F) no freezing occurs. Thermal expansion and contraction acts primarily in deserts and on bare rock surfaces, where diurnal temperature swings of ~40°C (72°F) stress mineral grain boundaries through differential thermal expansion — a mechanism called insolation weathering or thermoclasty. Salt crystallisation (haloclasty) is critical in coastal, arid, and urban environments: halite and gypsum crystals grow in rock pores as saline solutions evaporate, exerting disruptive pressures that exceed the tensile strength of many lithologies. Pressure unloading or exfoliation occurs when deep plutonic rocks — granite, gneiss — are exposed by erosion: removal of overlying rock (overburden) releases confining pressure and the rock expands outward, generating curved sheeting joints parallel to the topographic surface and producing the characteristic exfoliation domes seen at Yosemite's Half Dome.

Chemical weathering attacks the mineral lattice itself, converting thermodynamically unstable primary minerals (crystallised at high temperatures and pressures deep in Earth) into secondary minerals stable under low-temperature, water-rich surface conditions. Hydrolysis is the most important chemical weathering reaction globally: water molecules dissociate and H⁺ ions attack silicate mineral frameworks, releasing cations and producing clay minerals. The reaction of potassium feldspar (orthoclase) illustrates the process: K-feldspar + water → kaolinite + K⁺ in solution + dissolved SiO₂. Hydrolysis rates are controlled by temperature (following the Arrhenius equation — rates approximately double per 10°C (18°F) increase) and by mineral surface area, which is why physical weathering and chemical weathering are synergistic: fracturing increases surface area and accelerates subsequent chemical attack. Carbonation dissolves carbonate rocks: CO₂ dissolves in rainwater to form carbonic acid (H₂CO₃), which reacts with calcium carbonate as CaCO₃ + CO₂ + H₂O → Ca²⁺ + 2HCO₃⁻, carrying dissolved ions into groundwater and rivers. This reaction underpins all karst topography. Oxidation converts ferrous iron (Fe²⁺) in minerals such as biotite and pyroxene to ferric iron (Fe³⁺) in goethite and hematite, producing the red and orange colours of lateritic soils and ferricretes. Hydration incorporates water molecules directly into mineral structures, expanding volume and weakening mineral bonds.

Climate exerts master control over weathering intensity. Chemical reaction rates roughly double with each 10°C rise in temperature; humid tropical regions with year-round warmth and abundant rainfall therefore develop the deepest and most chemically advanced weathering profiles on Earth. Tropical laterites and bauxites can extend 30–100 m (98–328 ft) below the surface, whereas polar and high-alpine areas experience shallow weathering dominated by physical processes. The balance between the rate of rock exposure by erosion (supply-limited) and the rate of chemical transformation (weathering-limited) determines the nature of regolith: transport-limited landscapes accumulate deep weathering mantles, while weathering-limited landscapes expose near-fresh bedrock at the surface.

Weathering products depend on the intensity of leaching. Moderate hydrolysis produces smectite and illite clays; intense, prolonged tropical leaching produces kaolinite and ultimately gibbsite (Al(OH)₃) as silica is progressively removed. Quartz is highly resistant and concentrates as a residual mineral in soils and sand deposits worldwide. Iron oxides (hematite, goethite) are nearly universal in oxidising, well-drained tropical soils. Together these secondary minerals constitute the regolith — the mantle of altered, unconsolidated material covering bedrock — which is the foundation for soil formation and the primary medium for terrestrial ecosystems.

Key Terms