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Click here to return to the Earth Systems Revision Page Types of Weathering What is Weathering? All rocks are vulnerable to weathering, this is "the breakdown and decay of rocks in-situ related to elements of the weather (e.g. temperature, rainfall, frost etc.)" (although biological weathering - does not directly involve elements of the weather, it is still classed as a weathering process). Do NOT confuse with erosion (erosion - involves the breakdown and removal of material - weathering occurs "in-situ") Weathering is the first stage in the denudation of the landscape. Rocks are weakened and loosened by weathering processes. This weakened material is then removed by agents of erosion (e.g. ice, water etc.) Weathering can be classified into Physical, Chemical and biological Weathering) Physical Weathering Physical weathering is also known as mechanical weathering and it involves the physical breakdown of rock - it does not involve chemical change. Freeze Thaw This is the breakdown of rocks due to the expansion of water during freezing, a process common in upland Britain where evening temperatures often fluctuate around 0oC. Freeze thaw is most effective in jointed rock (e.g. granite). During freezing, water expands by 9% in volume. Water freezing in cracks in rocks, exerts pressure. Alternating freeze-thaw cycles gradually force the rock to split or cause rock fragments to break off. Where this process occurs on steep slopes, rock fragments collect at the base of the slope due to gravity in the form of a scree slope (see notes on scree) 
Pressure Release (also known as dilation) Rocks such as granite, formed as Igneous intrusions are formed under pressure. When weathering and erosion removes overlying rocks, the pressure is released and the underlying rock expands. This expansion results in the fracturing of the rock, which weakens it by making it susceptible to other weathering agents. If cracks develop parallel to the surface, sheeting of rock layers may occur. Thermal Expansion (insolation weathering) This process results from large diurnal temperature ranges which result in heating and cooling of the rock. When heated, expansion of the rock occurs, whilst during cooling the rock contracts. This expansion and contraction during cycles of temperature change results in stresses in the rock layers. Outer layers of rock heat and cool quicker than inner layers and over time the upper layers flake / peel off (exfoliation). It should be noted that the effectiveness of this process is heavily debated and some believe that it is only really effective when water is also present. 
Salt Crystallisation Water passing through crevases and joints in rocks, may be saline (carrying salts in solution). As the water evaporates, the dissolved salts precipitate and crystalise froming salt crystals. This may also take place where in rocks such as chalk, the rock is decomposed by solution to form salt solutions such as sodium carbonate which will then crystalise upon evaporation of the mosture. The salts may expand up to 3x their original size, and therefore the crystals put stresses upon the rock as they grow, resulting in granular disintegration (gradually breaking off individual grains of rock). Chemical Weathering Chemical weathering is where rocks are decomposed by chemical reaction between elements of the weather and rock minerals, resulting in either the alteration of a rock's internal mineral structure or the formation of new minerals (e.g. feldspar forming Kaolin in the process of hydrolysis). Weakened rock or the consequent deposits are then more easily removed by erosion processes. Water plays a key role in most chemical reactions and also provides a transport mechanism for other elements that carry out weathering. Chemical weathering is most dominant in hot and humid areas such as equatorial zones and least effective where there is little rain such as in desert or polar regions (where most water is held as ice). The susceptibility of rocks to chemical weathering is determined by the types of minerals they contains and their mineral structure. There are a number of different types of chemical weathering. Oxidation The exposure of rocks to oxygen in air or water can result in a reaction between the oxygen and iron-based minerals in the rocks. Iron readily oxidises and during oxidation, blue grey ferrous iron (Fe2+) is transformed to red ferric iron (Fe3+). This causes a weakening of the rock structure enabling them to crumble easily and making them more susceptible to other weathering processes. Carbonation Rainwater contains dissolved CO2 which forms a weak carbonic acid (H20 + C02 = H2C03). Carbonic acid is able to react with calcium carbonate (common in rocks such as limestone and chalk) to form calcium bicarbonate which is then easily removed in solution in water. Limestone is gradually dissoved in this way as the calcium carbonate is converted to calcium bicarbonate and carried away in solution by running water. Solution Water can act as a solvent by breaking down chemical bonds in minerals causing them to dissolve in a process known as solution - carbonation is therefore a form of solution although it is mineral specific in relation to calcium carbonate. Solution rates tend to increase with an increased acidity of water. Hydrolysis This is where acidic water reacts with rock forming minerals such as feldspar. This is a common process in the weathering of granite. Hydrogen ions in the water displace potassium ions in the feldspar. This causes the feldspar to break down into a secondary mineral, Kaolin (China Clay). Whilst the feldspar in granite decomposes, the quarz and mica remain relatively unaffected but the structure weakened. Hydration This occurs as the addition of water causes minerals in rock to swell (by about 0.5%) due to a chemical reaction as the mineral absorbs water ('hydrates'), thus involving both chemical and physical (mechanical weathering). The formation of gypsum when water combines with anhydrite (CaS04 (anhydrite) + 2H20 (water) = CaS04.2H20 (gypsum)). Gypsum is fairly soluble and can then be fairly easily removed by solution. Biological Weathering Usually consists of a combination of physical (growth of roots into joints in rocks) and chemical (e.g. impact of organic acids) processes. Tree Roots As roots of plants and trees grow downwards, they often enter and exploit cracks joints in rock. As they grow they are able to gradually wedge the joints further apart, eventually resulting in detatchment of rock fragments (simillar to freeze-thaw) Organic Acids As roots as well as surface litter decayse, organic acids are released into the ground. Percolating rainwater, moves these acides further down and the organic acids may react with minerals in the rock through a process called chelation. The combination of rainwater and organic acids combines with aluminium and iron which are washed out of the soil. Respiration of bacteria and tree roots also releases C02 which when becomes dissolved in water forms a weak carbonic acid which can increase the chemical weathering process, carbonation. | 
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ANIMAL ACTIVITY Burrowing animals help to open up joints in rock and also help to bring rock fragments to the surface, where they are exposed to further weathering. At the coast, animals such as limpets increase the rate of chemical weathering through the acids secreted as they cling to rock surfaces. 
See also the following links / resources...
A pdf download - providing a simple mind-map summarising the different types of weathering. A good summary online tutorial from Kesgrave School (Suffolk) on the different types of weathering Excellent overview of weathering types with associated photographs from Pamela Gore of Georgia Perimeter College. See also this good overview of weathering from Wikipedia If you are stuck on any of these key concepts or want to develop your understanding further, try out some of the link from the Earth Systems Weblinks section. For some more summary notes and some excellent weblinks see also the Weathering section on Geography Pages (with thanks to Alan Parkinson from King Edward VII School, Kings Lynn). |
