Rates of Weathering (complete information)

The Role of Climate

How rapidly a particular exposure of rock will weather is dependent on several different interacting factors in the environment.

Climate is important because it controls temperature and rainfall. Warmer temperatures, of course, promote chemical reaction. Even an increase of as little as 10°C cans double reaction rates.

Similarly, high rainfall accelerates rates of weathering by increasing the possibilities for solution and reaction and by helping to wash away surface debris and expose fresh surfaces for chemical attack. Both higher temperatures and an abundance of moisture promote the growth of plants, and luxuriant plant growth assists in weathering extracting ions from minerals and adding chemically reactive compounds.

The importance of temperature and rainfall is at least partly evident in the thickness of loose material or regolith that develops over bed-rock under various climatic conditions. In temperate regions having level topography, the layer of regolith is usually only about a meter deep, whereas in similar topography of the tropics, the loose material may from a blanket tens of meters thick. Arctic areas are only covered by regolith.

Parent Rock

Depending on their composition, texture, and such features as fractures and joints, rocks exposed at the earth’s surface may exhibit a wide range of resistance to weathering. Limestones and dolostones, for example, are highly susceptible to solution.

In regions having high rainfall, these rocks weather more rapidly than in arid regions. This observation suggests that it is difficult to list rocks in the order of their resistance to weathering because a rock might be more durable than another under one set of conditions and the same rock less durable under different conditions. As a general rule, igneous and metamorphic rocks are more resistant to weathering than sedimentary rocks. Silica- cemented quartz sandstone, however, is among the most enduring of rock types.

Statues and other sculptured edifices that have actual dates carved into the stone permit one to assess rates of weathering. In cities, because of the higher levels of carbon and sulphur dioxides, chemical weathering rates are generally higher than in nonindustrialized areas. The bust of beethoven was erected in a St. Louis Park in 1884. In spite of many attempts to treat the Italian Carrara marble from which the bust was carved, it continued to decompose. The composer’s gruff expression has been so altered that St. Louisans refer to the statue as “poor Beethoven.”

The texture of the parent rock will also influence its susceptibility to weathering. In general, coarsergrained porous varieties of rocks of the same composition tend to weather more rapidly than dense finegrained varieties. Rocks that have a large number of fractures provide a greater amount of surface area for chemical attack and therefore are also more readily weathered.

Acid Rain

Rates of chemical weathering are, to be sure, affected by the acidity of the solutions that attack rocks and minerals. As described above even unpolluted rainwater contains carbon dioxide and is slightly acidic. In recent years, however, this weak natural carbonic acid has been augmented by strong acids formed where rain has fallen through air polluted with oxides of nitrogen and sulphur.

This acidic precipitation includes both nitirc and sulphuric acid. It is called acid rain. Acid rain commonly exceeds the natural acidity of rainwater by more than 200 times and in a few instances by over 1000 times. If not neutralized by alkaline rocks and soils, the acid accumulates in lakes and streams where it is often lethal to fish and other forms of aquatic life. Acid rain also damages the foliage of plants and reduces the quality of soil by leaching away nutrients.

The acid solutions attack limestone, marble, and concrete building materials and damage valuable works of art. Presently, millions of dollars are being expanded in acid rain research in the hope that a solution to the problem can be found. Aside from local chemical treatment of particular areas, however, a long-term solution must involve reduction in atmospheric pollutants that form acid rain.