• October 4, 2010 /  No Comments

    Weathering refers to the process by which rocks are broken apart or chemically altered to become sediment. This process can be further subdivided into two categories: physical/mechanical weathering and chemical weathering. Chemical weathering refers to the processes by which rocks react with the atmosphere to form new substances. These reactions can alter a rock and transform the rock into sand, clays, and other minerals.

    CHEMICAL WEATHERING

    Like mechanical weathering, there are several different types of chemical weathering. One type is oxidation, the reaction of oxygen with iron-bearing minerals in rock. In an oxidation reaction, the iron atom loses electrons and precipitates as another mineral. Perhaps the most well-known oxidation reaction is the formation of rust. Consider another reaction, the oxidation of pyroxene into limonite and dissolved silica, below:

    4FeSiO3 +         O2 +      H2O     →     4FeO(OH)  +  4SiO2

    (pyroxene)   +  (oxygen) +   (water)    →    (limonite)  +   (silica)

    Uluru, also known as Ayers Rock, is a sandstone inselberg which gets its coloring from the oxidation of iron-bearing minerals. http://www.flickr.com/photos/mrcs1/3146166191

    Chemical weathering can also result from exposure to water. Hydrolysis occurs when silicate minerals react with water so that the mineral recombines with the water molecule to form a new mineral. For example, consider the mineral potassium feldspar. Potassium feldspar is a fairly common mineral and can be found in igneous, metamorphic and sedimentary rocks. When potassium feldspar reacts with slightly acidic water, it can be transformed into kaolinite, a clay mineral:

    4KAlSi3O8 +      4H+ +      2H2O        →       4K+ +     Al4Si4O10(OH)8 +    8SiO2

    (potassium feldspar) +  (hydrogen ion) +  (water)  →   (potassium ion)  +   (kaolinite) +    (silica)

    Granitic pebbles within the rock have weathered, partly through hydrolysis, into kaolin (seen as white spots in the picture above). http://academic.emporia.edu/aberjame/struc_geo/appalach/appalach.htm

    Minerals in rock can also react with water in a process called hydration. Some minerals can absorb water into their structure, causing their volume to expand and put stress on the rock. Over time, this chemical reaction can lead to a physical weakening of the rock.

    Lastly, rocks can also be subjected to dissolution, the process by which a mineral completely dissolves in water. Dissolution is an especially effective method of chemical weathering in rocks that contain either magnesium carbonate or calcium carbonate, two substances which are easily dissolved by water or other acidic solutions. (Most commonly, the weak acid in question is carbonic acid, the result of a reaction between carbon dioxide and water. However, other weak acids are possible. For example, the decomposition of organic material can produce humic acid which acts much like carbonic acid.) In a dissolution reaction, the mineral is broken into its constituent ions in solution:

    CaCO3 +      H2CO3 →      Ca2+ +    2HCO3

    (calcite) +   (carbonic acid)   →   (calcium ion) +  (bicarbonate ion)

    Rainwater falling on this rock, and flowing along fractures in the rock, has slowly dissolved some of the limestone to create pits and channels. http://www.earth.ox.ac.uk/~oesis/field/medium/pavement1_1647.jpg

    Dissolution is the most easily observed kind of chemical weathering. Over time, the action of slightly acidic solutions on the rock can leave pits and holes, and it can act to slowly enlarge and widen preexisting fractures. On a large scale, dissolution can result in a very distinct type of topography- karst topography. Such areas can feature sinkholes, springs, caves, caverns and other features related to the dissolution of underlying bedrock.

    This photo, of the Ohio Caverns, shows the results of large-scale dissolution as water flowed through subsurface limestone. http://www.panoramio.com/photo/22535678

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