Chemical weathering constantly reshapes the Earth’s surface by altering the chemical composition of rocks and minerals, breaking down materials through chemical reactions to create new, softer, and often more soluble compounds. Unlike physical weathering, which simply breaks rocks into smaller pieces without changing their makeup, chemical weathering fundamentally changes the minerals themselves. This process is a critical part of the rock cycle, leading to the formation of sediments and soil.
Carbonation
Carbonation is a form of chemical weathering driven by the interaction between carbon dioxide, water, and specific types of rock. This process begins when atmospheric carbon dioxide dissolves in rainwater, forming a weak acid called carbonic acid. Although mildly corrosive, its persistent action over long periods can be highly effective at dissolving rock.
This weak carbonic acid readily reacts with carbonate minerals, most notably calcite, the primary component of limestone and marble. The reaction changes the solid, insoluble calcium carbonate into soluble calcium bicarbonate. Since calcium bicarbonate is soluble, flowing water carries the dissolved material away in a process called dissolution. This action is responsible for the formation of extensive cave systems, sinkholes, and other features in limestone landscapes.
Oxidation
Oxidation occurs when minerals lose electrons upon exposure to oxygen, typically in the atmosphere or dissolved in water. This causes a change in the element’s valence state, making the mineral structure less stable. The most common example involves iron-bearing minerals, such as pyrite or olivine, found within many rock types.
When the iron in these minerals reacts with oxygen, it forms iron oxides, commonly known as rust. The reddish-brown color seen on many weathered rocks, including basalt and granite, is a direct result of this iron oxidation. The resulting iron oxide compounds are significantly softer and weaker than the original iron-containing silicates, causing the rock to crumble.
Hydrolysis
Hydrolysis involves the direct reaction between a mineral and the hydrogen (\(\text{H}^+\)) and hydroxide (\(\text{OH}^-\)) ions that make up a water molecule. In this reaction, the water molecule splits, and its components chemically interact with the mineral’s internal structure. This process is particularly important in the breakdown of silicate minerals, the most abundant minerals in the Earth’s crust.
A prime example is the weathering of feldspar, a common mineral in igneous rocks like granite. Through hydrolysis, the hydrogen ions replace the potassium, sodium, or calcium ions within the feldspar’s structure. This chemical exchange converts the relatively weather-resistant feldspar into new, stable, and much softer compounds known as clay minerals, such as kaolinite. The formation of clay and the release of dissolved ions into the water are the defining outcomes of hydrolysis, fundamentally transforming hard rock into the parent material for soil.
The Role of Living Organisms
Living organisms significantly contribute to chemical weathering by acting as natural chemical factories that produce corrosive agents, a process often called bioweathering. Plant roots and soil microorganisms, such as bacteria and fungi, release various organic acids (including citric and oxalic acid) directly onto rock surfaces. These acids enhance weathering in two ways: they lower the \(\text{pH}\) of the surrounding water, making it more acidic, and they participate in chelation.
Chelation involves the acid molecules binding to and isolating metal ions, such as iron and aluminum, effectively pulling them out of the mineral structure. Lichens, which are symbiotic associations of fungi and algae, secrete these acids directly onto bare rock, etching tiny pits and accelerating the decay process.