The term weathering refers to the breakdown of rocks and other earth materials by natural processes. There are two major types of weathering: physical weathering and chemical weathering. According to Michael Ritter, professor of geology at the University of Wisconsin, there are four types of chemical weathering. Does this Spark an idea?
Oxidation
Oxidation, often referred to as rusting, occurs when oxygen breaks down rocks and other earth materials. Oxygen in water combines with metallic elements in rocks and soil to form softer earth materials that are easier to break apart into smaller pieces. Common iron-bearing minerals in rocks that are easily oxidized include olivine, pyroxene, amphibole and biotite. This process produces iron oxides that are much softer, such as limonite, hematite and goethite. It is often easy to tell when oxidation has occurred because iron oxides have recognizably "rusty" colors of red and orange.
Hydrolysis
Hydrolysis occurs when free hydrogen and hydroxide ions in water replace mineral ions in rocks and soil. You can tell where hydrolysis has occurred because what is left behind is clay, which is much easier to break apart and erode than most rocks. Substances such as feldspar and iron-bearing silicates easily undergo hydrolysis when exposed to water and acids.
Hydration
Hydration occurs when water is absorbed by stone; one example is the conversion of hematite to limonite. Hydration increases the size of stone, which causes stress and zones of weakness to develop, and can cause pieces of the rock to flake off. However, according to professor Michael Ritter, there is some argument among geologists as to whether or not hydration is truly a form of chemical weathering, simply because it does not produce a chemically different product. Some geologists would consider hydration to be a mechanical or physical method of weathering.
Carbonic Acid
Sometimes called dissolution, the actions of carbonic acid break down materials, such as marble and limestone, over time. This occurs when carbon dioxide from the atmosphere combines with rainwater to produce rainwater that is more acidic than usual, commonly referred to as "acid rain." The actions of carbonic acid are of particular concern to many who wish to preserve ancient monuments and statues, many of which are composed partly or entirely of marble or limestone.
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