Chemical Weathering

Chemical weathering is the process by which rocks and minerals undergo changes in their composition. Agents of chemical weathering include water, oxygen, carbon dioxide, and acid precipitation. The interaction of these agents with rock can cause some substances to dissolve, and some new minerals to form. The new minerals have properties different than those that were in the original rock. For example, iron often combines with oxygen to form iron oxide, such as in hematite.

The composition of a rock determines the effects that chemical weathering will have on it. Some minerals, such as calcite, which is composed of calcium carbonate, can decompose completely in acidic water. Limestone and marble are made almost entirely from calcite, and are therefore greatly affected by chemical weathering. Buildings and monuments made of these rocks usually show signs of wear as a result of chemical weathering. 

Temperature is another significant factor in chemical weathering because it influences the rate at which chemical interactions occur. Chemical reaction rates increase as temperature increases. With all other factors being equal, the rate of chemical weathering reactions doubles with each 10°C increase in temperature. Effect of water Water is an important agent in chemical weathering because it can dissolve many kinds of minerals and rocks. Water also plays an active role in many reactions by serving as a medium in which the reactions can occur. Water can also react directly with minerals in a chemical reaction. In one common reaction with water, large molecules of the mineral break down into smaller molecules. This reaction decomposes and transforms many silicate minerals.

For example, potassium feldspar decomposes into kaolinite, a fine-grained clay mineral common in soils. Effect of oxygen An important element in chemical weathering is oxygen. The chemical reaction of oxygen with other substances is called oxidation. Approximately 21 percent of Earth’s atmosphere is oxygen gas. Iron in rocks and minerals combines with this atmospheric oxygen to form minerals with the oxidized form of iron. A common mineral that contains the oxidized form of iron is hematite. 

Effect of carbon dioxide Another atmospheric gas that contributes to the chemical weathering process is carbon dioxide. Carbon dioxide is a gas that occurs naturally in the atmosphere as a product of living organisms. When carbon dioxide combines with water in the atmosphere, it forms a very weak acid called carbonic acid that falls to Earth’s surface as precipitation. Precipitation includes rain, snow, sleet, and fog. Natural precipitation has a pH of 5.6. The slight acidity of precipitation causes it to dissolve certain rocks, such as limestone. 

Decaying organic matter and respiration produce high levels of carbon dioxide. When slightly acidic water from precipitation seeps into the ground and combines with carbon dioxide in the soil, carbonic acid becomes an agent in the chemical weathering process. Carbonic acid slowly reacts with minerals such as calcite in limestone and marble to dissolve rocks. After many years, limestone caverns can form where the carbonic acid flowed through cracks in limestone rocks and reacted with calcite. Effect of acid precipitation Another agent of chemical weathering is acid precipitation, which is caused by sulfur dioxide and nitrogen oxides that are released into the atmosphere, in large part by human activities. Sulfur dioxide is primarily the product of industrial burning of fossil fuels. Motor-vehicle exhausts also contribute to the emissions of nitrogen oxides. These two gases combine with oxygen and water in the atmosphere, forming sulfuric and nitric acids, which are strong acids.

The acidity of a solution is described using the pH scale, as you learned in Chapter 3. Acid precipitation is precipitation that has a pH value below 5.6—the pH of normal rainfall. Because strong acids can be harmful to many organisms and destructive to humanmade structures, acid precipitation often creates problems. Many plant and animal populations cannot survive even slight changes in acidity.