Chemical weathering

Chemical weathering is the process by which chemical reactions break down rocks. This kind of weathering varies greatly and depends on the type of minerals in the rock, the type of chemicals introduced, and other factors. Three of the main varieties of chemical weathering are oxidation, hydrolysis, and solution, all of which involve different chemical stimuli. Chemical weathering occurs naturally and often combines with other forms of weathering and other natural forces to change rocks and rock formations in many ways.

The Weathering of Rocks

Rocks form deep underground, where temperatures are high and pressure is intense. These conditions are perfect for rocks. However, many forces, both natural and artificial, frequently move rocks to Earth's surface. On the surface, environmental conditions are much different from those underground. Temperatures are cooler and more varied, and the pressure is greatly reduced. Rocks, no longer in their ideal condition, begin to decompose and disintegrate. The processes by which rocks on the surface of Earth break down are known as weathering.

Weathering may combine with other natural forces to significantly affect individual rocks, rock formations, and sometimes even entire landscapes. One of the most important forces often seen in conjunction with weathering is erosion, which occurs when water or wind moves materials away from their original locations. In some instances, weathering may cause rocks to break into small pieces, which are then swept away by erosion. Weathering and erosion can cause massive changes to rock and, in doing so, have created many unusual landforms. One such landform is Bryce Canyon in Utah, a natural rock formation that resembles hundreds of pointed towers.

Chemical Forms of Weathering

One of the most common forms of weathering is chemical weathering, a process by which rocks break down through chemical reactions. These chemical reactions may alter, create, or eliminate minerals in rocks. These changes ultimately cause rocks to disintegrate. In some cases, chemical changes destroy rocks by converting their minerals into clay or soluble salts that are subsequently incorporated into the soil.

Different rocks react differently to chemical change. According to a scale of weathering resistance known as the Goldich dissolution series, rocks containing minerals such as calcic plagioclase, magnesium pyroxene, or amphibole are very susceptible to chemical weathering. Alternately, rocks containing minerals such as potassium feldspar, muscovite, or quartz are extremely durable and highly resistant to the forces of chemical weathering.

The main environmental factors in chemical weathering are water and heat. Rainwater is the most active force in triggering chemical reactions with the minerals in rocks. For this reason, locations with warm, humid climates and plenty of rainfall are most likely to experience significant chemical weathering. Acidity in rain, often associated with pollution, can also greatly increase the rate of chemical weathering of rocks.

Chemical weathering occurs in many forms, but among the most common are oxidation, hydrolysis, and solution. Oxidation may occur when certain kinds of minerals in rocks are exposed to oxygen. Oxygen causes minerals such as iron and manganese to rust, which is a main product of oxidation. Rust is commonly found on both naturally occurring and human-produced metals.

Hydrolysis relates to a chemical reaction caused by water. In this kind of reaction, materials in water (oxygen and hydrogen) or acids in the water bond with minerals to create new substances. This kind of reaction often causes rock to break down into clay and salts, which help to form soil.

Solution, also known as carbonation, is generally caused by interaction among carbon dioxide, rainwater, and rock minerals. A weak acid created by water and carbon dioxide can, over time, dissolve some types of rock. Carbonation processes in caves, for instance, are responsible for the creation of stalactites and stalagmites.

Chemical weathering often combines with other forms of weathering, such as physical weathering. Physical weathering, also known as mechanical weathering, relates to changes to rocks caused by physical force rather than by chemical alterations. One of the most well-known examples of physical weathering is the damage done to rock and rock-like materials by frost. When water freezes into ice, it expands. This expansion, if taking place in a crevice in a rock, may be enough to push the rock apart. Frost damage frequently occurs on paved roads after harsh winters. In addition, other weathering on rocks may be organic or caused by the actions of humans, animals, or plants.

Factors in Weathering

Many factors affect the weathering of rocks. One of the clearest and most important factors in weathering is the environmental conditions to which a rock is exposed. Climates that are hot, cold, dry, wet, or polluted will have particular effects on the different kinds of rocks in that area.

The composition of rock is another important factor in weathering. Rocks may be composed of hundreds of materials and combinations of materials. All of these materials react differently to the conditions that cause weathering. Some materials are prone to disintegrate quickly in certain conditions, while others are much more durable and can last for long periods basically unchanged.

Time is also important when considering the weathering of rocks. Weathering usually is not a fast process; in fact, it may take months, years, or eras of thousands or millions of years to occur, depending on the materials and elements involved. Generally, the longer a rock is exposed to the elements, the more wear it experiences.

The final major factor in weathering is the surface area of the rock: the greater the surface area of the rock exposed, the more likely the rock will become weathered. Rocks that have cracks or other openings have more surface area exposed and, therefore, will be subjected to more weathering than smooth rocks with no openings.

Bibliography

"Chemical Weathering." The Rock Cycle. The Geological Society of London. Web. 12 Dec. 2014. http://www.geolsoc.org.uk/ks3/gsl/education/resources/rockcycle/page3564.html

Guo, Xiao. "Chemical Weathering Rates and Controlling Mechanisms of Glacial Catchments Within Different Climate Regimes in the Tibetan Plateau." Life & Environment, vol. 11, 5 July 2023, doi: 10.7717/peerj.15594. Accessed 20 Nov. 2024.

Mantei, Erwin J. "Weathering and Soils." Missouri State University. Web. 12 Dec. 2014. http://courses.missouristate.edu/emantei/creative/glg110/weathering.html

"Social Genesis and Development, Lesson 2 – Processes of Weathering." Plant & Soil Sciences eLibrary. University of Nebraska – Lincoln. Web. 12 Dec. 2014. http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1124303183&topicorder=4&maxto=7

Stimac, John P. "Mechanical and Chemical Weathering." Department of Geology/Geography. Eastern Illinois University College of Sciences. Web. 12 Dec. 2014. http://www.ux1.eiu.edu/~cfjps/1300/weathering.html