Chemical Weathering Processes

Chemical weathering is not about breaking rocks into smaller pieces by force; it is about changing their very chemistry. The minerals that make up a rock react with substances around them — chiefly water, oxygen, and acids — and in doing so, they decompose, dissolve, or crumble into a fine, loose, clastic state. The rock is, in effect, chemically transformed.

A group of weathering processes acts together to bring this about: solution, carbonation, hydration, oxidation, and reduction. Each works through chemical reactions involving oxygen, surface water, soil water, and other acids. For these reactions to proceed at any meaningful speed, three things must be present together: water, air (supplying oxygen and carbon dioxide), and heat. Heat is the accelerator — without it, chemical reactions crawl; with it, they speed up considerably.

The carbon dioxide needed for these reactions does not come only from the atmosphere. Underground, the decomposition of plants and animals adds substantially to the quantity of carbon dioxide in the soil and rock environment. This extra supply makes the chemical environment beneath our feet far more reactive than the surface alone would suggest.

A useful way to think about all this is that the chemical reactions acting on minerals in the ground are very much like the reactions you would set up in a laboratory. The same principles of chemistry apply — the same reactants, the same products, the same need for favourable conditions. The earth is simply a vast, slow, natural laboratory.

The five processes, in brief:

• Solution — minerals dissolve directly into water.
• Carbonation — carbon dioxide dissolved in water forms a weak acid that attacks certain minerals.
• Hydration — minerals absorb water molecules into their structure, causing them to swell and weaken. …