ZIMSEC O Level Geography Notes: Main types of weathering: Chemical weathering
- Chemical weathering is the breaking down or decomposition of rocks as a result of various chemical processes and reactions altering the chemical components of the rock.
- Some types of rocks decompose when they come into contact with water, oxygen,carbon dioxide and other naturally occurring acids such as in the soil.
- These acids include humic acids, carbonic acids, acid rain and smog( which is fog laced with toxic and sometimes corrosive materials).
- Some minerals in the rocks for example calcium are susceptible to chemical reaction.
- They undergo chemical changes and fall of the rock leading to the reduction of the rock’s size.
- Chemical weathering processes include oxidation, hydration, hydrolysis, carbonation, organic weathering and acid rain.
- Chemical weathering is most prevalent in humid zones, against reactive minerals in rocks at typically at the base of slopes where it is likely to be wet and humid for extended periods of time to allow the chemical processes to occur.
- Occurs when rocks or more specifically certain minerals within the rocks are exposed to and react with oxygen in the air or water.
- The most prevalent of these is when rocks containing iron compounds react with oxygen to in a form of oxidation called rusting as the rock is transformed from what is known as a ferrous state to a ferric state.
- During rusting rocks change their colour and become reddish-brown (the colour of rust).
- This compromises the integrity and structure of the rock making them crumble easily thus aiding other forms of weathering as well.
- Reduction (the opposite of oxidation) also occurs in waterlogged areas where a process called gleying takes place.
- Some types of rocks for example those that contain salt minerals have the capacity to absorb water into their structure causing them to swell and become susceptible to future breakdown.
- Hydration is in actual fact a physical-chemical weathering processes since the rocks swell and exert pressure in addition to changing their chemical structure.
- The rock swell during wet periods and contract during dry periods causing them to fracture and to develope joints and weaknesses.
- For example anhydrite absorbs water to become gypsum.
- Hydrogen ions in water react with minerals ions in the rock
- That is the water reacts with minerals in the rock instead of dissolving it.
- This gives rise to different compounds.
- This is very common in granite areas
- Where the felspar in the granite/igneous reacts with hydrogen to form clay.
- Mica another rock mineral can also be affected by hydrogen in acidic water solutions.
- This is when carbon dioxide dissolves with rain water to form carbonic acid.
- This weak acid reacts with rocks that are composed of calcium carbonate for example limestone.
- The calcium is dissolved and removed in as a calcium bicarbonate solution by running water.
- The may result in the formation of landform features such as Underground caves such as the Chinhoyi caves as well as other limestone features such as stalagmites and stalactites.
- Some minerals like rock salt are soluble in water
- When they come in contact with water they just dissolve in situ.
- As the rocks dissolve in water they may form clints and grikes/grykes which are collectively known as limestone pavements.
- Decomposing vegetation (humus) releases humic acid.
- This acid attacks calcium, magnesium and iron minerals within rocks in a process called chelation.
- The respiration of bacteria and plant root increases carbon dioxide levels within the soil thus accelerating the process of carbonation.
- Lichen extracts iron from rocks via reduction.
- Human and economic activities are realising more and more carbon dioxide into the atmosphere together with other chemicals such as sulphur dioxide and nitric oxide.
- These gases then dissolved into rain water to form acidic solutions.
- Acid rain attacks and corrode limestones and at a lesser rapid rate sandstones.
- Also the acidic solutions tend to free up oxygen ions thus fueling the process of hydrolysis.
Click here to learn about Physical/Mechanical weathering.
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