Chemical erosion occurs when water transports dissolved minerals away from their source rocks. It follows chemical weathering, which results from the chemical alteration of rock by water. Chemical erosion is most common with limestone; slightly acidic rainwater dissolves calcium carbonate in the rock and redeposits it, sometimes far away, as in stalagmites and stalactites. Through oxidation, chemical erosion also occurs to some degree with unstable igneous minerals and iron-rich rocks.
Chemical weathering falls under three major types. Dissolution takes place with limestone and is famously responsible for limestone caves. In oxidation, the surfaces of iron-rich rocks are oxidized in the presence of water, altering the surface and giving it a ruddy appearance. Hydrolysis occurs when certain silicate minerals, such as feldspars, are chemically altered into clay, quartz and basic compounds in solution. Dissolution and hydrolysis are more likely than oxidation to cause erosion.
The running water that causes chemical erosion may also cause physical erosion in the same rocks. Rain is naturally slightly acidic, as water dissolves carbon dioxide in the atmosphere to create carbonic acid. The erosion process is exacerbated by acid rain, which results when sulfur and other atmospheric compounds combine with water to form more powerful acids.