Chemical weathering is the process by which changes take place in the very chemical structure of rocks themselves. Chemical weathering represents a second stage of rock disintegration in which small pieces of rock produced by physical weathering are then further broken apart by chemical processes.
Three chemical reactions in particular are effective in bringing about the weathering of a rock: acid reactions, hydrolysis, and oxidation. Acids form readily in the soil. One of the most common such reactions occurs when carbon dioxide in the air reacts with water to form a weak acid, carbonic acid. Carbonic acid has the ability to attack many kinds of rocks, changing them into other forms. For example, when carbonic acid reacts with limestone, it produces calcium bicarbonate, which is partially soluble in water. Caves are formed when underground water containing carbonic acid travels through blocks of limestone, dissolves out the limestone, and leaves empty pockets (caves) behind.
Acids produced by human activities can also produce chemical weathering. For example, the conversion of metallic ores to the pure metals often results in the formation of sulfur dioxide. When sulfur dioxide combines with water, it forms the weak acid sulfurous acid and, eventually, the stronger acid sulfuric acid. Both of these acids are capable of attacking certain kinds of rocks in much the way that carbonic acid does.
Hydrolysis is a chemical reaction by which a compound reacts with water to form one or more new substances. A number of rock-forming minerals readily undergo hydrolysis, especially in acidic conditions. For example, the common mineral feldspar will undergo hydrolysis to produce a clay-type mineral known as kaolinite and silicic acid. Both of these new compounds are much more soluble in water than is feldspar. Hydrolysis of the mineral results, therefore, in the degradation of any rocks in which it may occur.
Oxidation occurs when the metallic part of a mineral reacts with oxygen in the air (or from some other source) to produce a new substance that is different in structure or more soluble than the original mineral. The spectacular red, orange, and yellow color of certain natural rock formations—such as those in Utah's Bryce Canyon—are an indication that an oxide of iron has been produced during the chemical weathering of the rock formations.
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