physical geography: chemical weathering

Weathering is this crazy concept that makes things change. We already went over some basics of physical weathering, now let’s review chemical weathering, which involves compositional changes or dissolution. Thus, the pH is the crucial factor here. Simplified, pH is the concentration of H+ ions. Most of what is going to be weathered chemically are metal cations, Silicates, Carbonates, etc. Chemical weathering primarily happens in the following three processes:

  • Acid Attacks
  • Oxidation
  • Chelation

Acid Attacks

Acid attacks involve water and acidity to act upon the soon-to-be weathered material (eg rock). Foremost, acidity is mostly generated via biota. Plants and animals release a lot of CO2 into the ground. This happens because plants breakdown animals as they decompose, which releases a lot of CO2. The climate connection here is that with greater tropical climates and flora/fauna, the greater the soil’s acidity. There are a few types of acid attacks. Solution is essentially dissolving (think salt into water, sugar into coffee, etc). This is mostly important for carbonate rocks, as their solubilities change with pH. Hydrolysis is the replacement of H+ ions with other metal cations. Cations have a positive charge, making them easy to substitute H+ ions. However, this greatly weakens rocks structure. It turns silicates (granite, basalts, etc) into clays. As a byproduct, it also releases OH. Carbonation mixes water with carbon dioxide to make carbonic acid. Fundamentally, that is the release of metal cations and HCO3- (bicarbonate). It converts silicates to clays and also dissolves the carbonate. This process is important in the formation of caves.

Oxidation

This occurs via dissolved oxygen in water. This reaction with oxygen causes the cation to give up an electron to oxygen. In example, Fe+2 –> Fe+3 as it bonds with oxygen. The product of this is that oxide minerals are formed. A common oxidation is rusting. For rocks, clays are also produces with oxides.

Oxidation potential is measured in Eh. Eh = log{oxidants/reductants}. Simplified, this is a measure of the prevalence of O2.

Chelation

Organic chemistry allows a large and complex range of decomposing reactions to take place. These chelation agents are organic compounds that do the job. They form ring-like structures around the metal ion in order to make it soluble. These agents come from leaf litter and other organic compounds.This make the metal ions soluble. The metal ions are carried away from the upper soil.

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