Weathering

1. Meaning and place in geomorphology

Weathering is an exogenic process driven by conditions at Earth’s surface. Rocks formed at depth or under different temperature and pressure conditions may become unstable when exposed to air, water and organisms. They then break apart, undergo mineral alteration, or dissolve. Weathering is essentially in situ: material need not move away from its parent rock for weathering to occur.

Erosion, by contrast, involves detachment and removal by moving agents. Mass movement is the downslope movement of rock or soil primarily under gravity, as in landslides and rockfalls. These processes interact: weathering weakens a rock face, a rockfall displaces its fragments, and a stream subsequently transports them. Denudation is the broader lowering and wearing away of the land surface through these interacting processes.

Weathered material contributes to regolith, the loose cover above bedrock. Saprolite is deeply weathered rock that retains aspects of the original rock’s structure. Soil formation additionally involves organic inputs, biological activity, movement of substances and horizon differentiation. Thus, weathering is essential to most soil development, but weathered rock and mature soil are not synonymous.

• Diagnostic question: Has the material merely changed where it lies, or has it been transported?
• Dissolved products may be carried away by percolating water; this does not make chemical alteration itself equivalent to erosion.

2. Physical or mechanical weathering

Physical weathering fragments rock without necessarily altering its mineral composition. Thermal expansion and contraction can generate stresses when rock surfaces heat and cool unevenly. Different minerals also expand at different rates. Repeated thermal stresses may promote granular disintegration or flaking, especially on exposed surfaces, although the outcome depends on moisture, mineral composition and existing fractures.

Unloading occurs when erosion removes overlying material and reduces confining pressure. Expansion of the exposed rock can produce sheet joints roughly parallel to the surface, especially in massive rocks such as granite. Detachment of curved sheets is termed exfoliation. However, exfoliation describes a form of peeling rather than a single cause: thermal stresses and chemical alteration can also contribute.

Frost weathering requires water in pores or cracks and suitable freezing conditions. Water expands by about 9 per cent when it freezes; in sufficiently confined openings this generates stress. Ice segregation, involving water migration towards growing ice lenses, is another important mechanism. Salt weathering occurs when salt crystals grow within pores or cracks and exert pressure. It is common on arid and coastal rock surfaces and on porous building stone.

• Cold alone is insufficient for strong frost weathering: moisture, permeability and the temperature regime matter.
• Fragmentation increases surface area, often accelerating subsequent chemical weathering.
• Honeycomb weathering and tafoni may involve salt action, but their origin should not automatically be assigned to one process.

3. Chemical and biological weathering

Chemical weathering changes mineral composition or transfers constituents into solution. Solution directly dissolves soluble minerals such as halite. Carbonation occurs when carbon dioxide dissolves in water to form weak carbonic acid, which attacks susceptible minerals. In limestone, calcite reacts with carbon dioxide-bearing water to produce dissolved calcium and bicarbonate ions. This supports the development of karst features such as sinkholes and caves where geological and drainage conditions are favourable.

Hydrolysis is especially important in silicate rocks. Water and hydrogen ions react with minerals such as feldspar, producing clay minerals and dissolved constituents. Oxidation involves electron loss, commonly when iron-bearing minerals react with oxygen in the presence of moisture. Iron oxides and oxyhydroxides often give weathered material red, yellow or brown colours. Hydration incorporates water into a mineral structure; conversion of anhydrite to gypsum is a standard example.

Organisms contribute mechanically and chemically. Roots enlarge existing cracks as they grow; burrowing animals disturb material and expose fresh surfaces. Lichens, microbes and decomposing organic matter release acids or increase soil carbon dioxide, assisting mineral dissolution. Biological weathering is therefore best understood as organism-mediated physical and chemical action rather than a wholly separate set of mineral reactions.

• Carbonation involves carbon dioxide-bearing water; hydrolysis involves reactions between water and mineral constituents.
• Rust-coloured rock commonly indicates iron oxidation, but colour alone cannot establish the complete weathering history.
• Roots normally exploit existing weaknesses rather than simply penetrating intact, unfractured rock.

4. Controls and characteristic weathering forms

Climate controls water availability and reaction rates. Warm, humid tropical environments generally favour intense chemical alteration and deep weathering profiles. Arid environments often favour salt crystallisation and thermal stresses, although chemical weathering still occurs during wet episodes. Frost processes are important in suitable high-latitude and high-altitude settings, including parts of the Himalaya. No climatic region experiences only one category of weathering.

Lithology and mineralogy are equally important. Minerals crystallised at high temperatures, such as olivine, are generally less stable under surface conditions than quartz. This broad relationship is expressed by the Goldich weathering stability series and broadly reverses the crystallisation sequence represented by Bowen’s reaction series. Nevertheless, actual weathering depends on drainage, acidity, temperature and mineral accessibility.

Joints, bedding planes, pores and faults permit water entry and expose more reaction surfaces. In jointed rock, chemical attack proceeds from several sides at corners and edges, rounding blocks through spheroidal weathering and sometimes leaving corestones. This differs from simple surface peeling. Relief and drainage also matter: stable surfaces can preserve thick weathering mantles, while rapid stripping on steep slopes may expose fresh bedrock.

• Differential weathering means unequal breakdown of rocks or minerals because their properties and exposure differ.
• Time allows alteration to accumulate, but an old surface need not retain a thick profile if erosion continually removes material.

5. Environmental and economic significance

Weathering supplies mineral particles and dissolved nutrients for ecosystems and soil development. It can increase porosity and groundwater storage in hard-rock terrain, although clay-rich alteration may locally reduce permeability. Weathered joints weaken slopes and foundations, making the thickness and character of the weathered zone important for roads, dams, tunnels and buildings.

Intense tropical leaching can remove mobile constituents and leave relative concentrations of iron and aluminium compounds. Under suitable parent-material, drainage and landscape conditions, this produces lateritic profiles and may contribute to bauxite formation. Laterite is not automatically an aluminium ore: an economically workable deposit requires adequate mineral concentration, quality and accessibility.

Weathering also affects the long-term carbon cycle. Silicate weathering consumes carbon dioxide, and subsequent transport and carbonate burial can produce a net geological carbon sink. Carbonate dissolution consumes carbon dioxide locally, but carbonate reprecipitation can release it; it should not be equated uncritically with the long-term silicate-weathering sink. At human timescales, polluted moisture can accelerate deterioration of limestone, marble and other building materials, making weathering relevant to heritage conservation.

• Agriculture: nutrient release and soil formation, alongside possible nutrient depletion under prolonged leaching.
• Engineering: reduced rock strength and greater susceptibility to slope failure.
• Resources: residual mineral enrichment and development of weathered aquifer zones.

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