By Kevin Beck Updated Aug 30, 2022
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Weathering is the gradual breakdown of rocks and soils under natural forces. It shapes landscapes, influences ecosystems, and even affects human infrastructure.
Weathering is broadly divided into mechanical (physical) and chemical processes. Mechanical weathering changes the form of a rock without altering its chemistry—think of freeze–thaw cycles or salt crystal expansion. Chemical weathering, by contrast, alters the mineral composition through reactions with water, oxygen, acids, and other substances.
Chemical weathering refers to the alteration of a rock’s molecular structure by chemical reactions. Unlike the rapid biochemical reactions in living organisms, these reactions unfold over long periods, often driven by naturally occurring agents such as water, oxygen, and dissolved gases.
In hydrolysis, water molecules break down minerals, forming soluble salts and new mineral phases. This process can reduce the strength of rock and release ions that affect soil chemistry.
Hydration involves the addition of water to a mineral. For example, feldspar reacts with water to produce clay minerals, expanding the rock volume and making it more susceptible to further weathering.
Carbonation is driven by carbonic acid (H₂CO₃), formed when CO₂ dissolves in water. This weak acid reacts readily with calcium‑rich rocks like limestone, dissolving them and creating features such as caves and karst landscapes.
Oxidation occurs when iron-bearing minerals react with oxygen and water, forming iron oxides (rust). The volume expansion of these oxides can crack rocks and weaken structures.
Acidification, including acid rain, results from sulfur dioxide and nitrogen oxides reacting with atmospheric moisture to form sulfuric and nitric acids. These acids accelerate the breakdown of building materials, soil, and vegetation.
