Weathering refers to the gradual breakdown of rocks, soils and minerals through exposure to water, atmosphere and plants; in contrast with erosion which involves transport of this weathered material away from its source.
Different kinds of rocks weather differently; more vulnerable igneous rocks tend to erode more quickly than more resistant sedimentary and metamorphic ones.
Water, ice and rain help shape rocks by altering their shapes or altering its minerals – this process is known as mechanical weathering.
Chemical weathering encompasses dissolution and oxidation processes. Water and acid are key chemicals involved, reacting well with limestone rocks that contain dissolved salts – these chemical reactions usually taking place in warm and damp climates.
Chemical weathering can also result in the creation of karst landscapes, where surface rock is pitted with holes and caves. Air pollution from burning fossil fuels accelerates this process by emitting nitrogen oxide and sulfur dioxide into the air; these substances then break down into acids that eventually fall back down as acid rain, rapidly dissolving stone surfaces.
Water can seep into cracks and crevices in rock, and when frozen it expands to take up more space by wedgeing apart rocks – an effect known as “ice wedging”. Ice wedging is often the cause of frost heave damage to house foundations and roadways.
Other mechanical weathering processes include burrowing animals, root growth in surface cracks and erosion by gravity or ice (see Chapter 10, Mass Wasting). Thermal stress weathering – another form of mechanical weathering – occurs when daily temperature fluctuations cause rocks to expand and contract as temperature shifts occur.
Students can simulate this type of weathering using Plaster of Paris and balloons to recreate this process in nature. When heated, it expands while when cooled it contracts, mimicking how water freezes and thaws naturally in nature.
H2O is remarkable in that it can dissolve an array of materials. Furthermore, its powerful chemical composition makes it one of the main agents for chemical weathering.
Some rocks dissolve when exposed to acidic water. This occurs when carbonic acid seeps through limestone to form caves.
Weathering occurs when chemicals alter the physical structure of minerals. For instance, iron oxides become rust when they react with oxygen in the air to form rust – this process is known as oxidation.
Symbiotic fungi that live on some tree roots have also been known to weather minerals, providing another mechanism to transfer inorganic nutrients between roots and trees.
Salt (NaCl) is an ionic compound which forms either through solution formation or mining from natural salt deposits, making up an essential component of Earth’s crust and essential to plants, animals and humans. Some solutions of salts conduct electricity while some also have an unpleasant odour.
Rock salt has long been used in road de-icing strategies, yet its environmental side effects can be damaging. Rock salt changes the temperature at which water freezes. New strategies are being explored that aim to reach anti-icing nirvana with reduced use of rock salt; such as using calcium and magnesium compounds that are safer for the environment as well as increasing effectiveness while decreasing needed amounts.
Plant roots such as those from trees or mosses can physically weather rocks by growing into cracks and crevices and widening them – this process is known as biological weathering.
Fungi are capable of chemically weathering rocks. Some fungi produce chemicals that erode minerals by altering their structures or releasing acids or chelating molecules that break them down further.
Human activities can hasten the process of weathering. Air pollution from burning fossil fuels, for instance, can change some chemicals found in rocks into acids which eventually fall back down onto Earth as acid rain – creating rapid weathering effects that degrade stones and materials rapidly.
Biological weathering involves plants and animals moving about. Rabbits digging into cracks in rocks to widen them widen, while Piddock shells drill into rock to protect themselves, releasing acids that dissolve minerals or break apart rocks. Plant roots also grow into rocks to exert pressure upon them; walking may trigger physical or chemical weathering effects as well.
Cracks or holes in rocks create the opportunity for biological weathering to occur, with any crack or hole becoming an opportunity for this form of weathering. Lichens – colonies of fungi and algae which live symbiotically on rock surfaces – also contribute to this form of weathering, as the chemicals produced by lichens weaken and disintegrate minerals within rocks making them more vulnerable to other forms of weathering.