Salt crystallisation occurs in hot deserts and on coasts. If water entering the cracks and pores in rocks is at all saline, then salt crystals are often left as it evaporates. The increasing size over time of these crystals puts the surrounding rock under pressure.
Thermal expansion, or insulation, is a very important type of physical weathering, which occurs particularly in desert type areas. It is essentially very similar to freeze-thaw weathering, without the water. The outer layer of the rock is heated greatly by the sun during the day, causing it to expand. At night, the cooling of the rock causes it to contract. After this expansion and contraction has been repeated many times, the outer skin of the rock peels away in a way similar to that of an onion. Exfoliation leaves rounded boulders and dome shaped rocky outcrops, which are features of many desert landscapes.
Tree roots may sometimes grow along bedding planes or joints in rocks, detaching blocks of rock. The removal of soil by burrowing animals such as rabbits can be another cause of biological weathering.
Chemical weathering happens when the minerals of which the rock is made are changed, leading to the disintegration of the Rock. It tends to affect certain minerals selectively, and although there are several different types of chemical weathering, including oxidation, hydration, hydrolysis, carbonation, solution, chelation and the effects of acid rain, the different forms tends to operate together.
Oxidation, as the name suggests, occurs upon contact of the rock with oxygen, from the air or from water. A common effect is the rusting of rocks containing iron, as blue-grey becomes reddish-brown when ferrous compounds are oxidised. The chemical structure of the rock is altered by oxidation, making it more susceptible to other forms of weathering.
Hydration simply involves the absorption of water into the existing minerals of the rock, causing the expansion of the mineral, leading to eventual weakening. It is less severe than hydrolysis, the most significant chemical weathering process, whereby H+ and OH ions in water react with the mineral ions.
Carbonation is a result of the reactions of rainwater and carbon dioxide to produce carbonic acid (H2CO3), which slowly dissolves any rocks made of calcium carbonate, such as limestone.
Gases produced from emissions of carbon dioxide, sulphur dioxide and nitrogen oxide from power stations and car exhaust dissolve in water droplets in the atmosphere, forming dilute acids solutions which fall as rain, dissolving limestone and sandstone’s in a similar way to carbonation.
Chelation is a complex biochemical process, in which chelating agents are released from the decaying humus in the soil and cause a change in the chemical structure of the surrounding rock.
The two main categories of weathering each have they’re own generalised needs to provide optimum conditions for weathering to occur quickly. Mechanical weathering requires more specific weather conditions than chemical weathering, because it is far more of a physical process, and almost all of the different types are based on the freeze-thaw, or expand-contract principle. They all need a constant fluctuation around 0"aC, and extreme temperatures either side of this which prevent the constant freezing and thawing will not allow for much mechanical weathering to occur. The presence of water is another importance; if the rock is too dry there will be no moisture to freeze in the pores, yet being too dry will lead to vegetation cover, the roots of which tend to hold the rock and soil together. The conditions required for physical weathering, with temperatures fluctuating around zero degrees and a medium level of rainfall, are those typically found in areas of high and middle latitude. This diagram shows how the level of mechanical weathering processes are dependent on temperature and rainfall, conditions affected by latitude:
Chemical weathering processes are all based upon a range of chemical reactions, each of which requires the presence of water. Since the rate of almost any chemical reaction is speeded up by an increase in temperature, (it is suggested that the rate of chemical weathering doubles with every 10"aC temperature increase), it follows therefore that the warmer and more moist the environment, conditions typically found in areas of low latitude, the greater the rate of chemical weathering there. The diagram below shows the effect of temperature and rainfall on the rate of chemical weathering:
A balance between temperature and rainfall are therefore required to ensure optimum conditions for both types of weathering, as the American Physicist Peltier discussed in his theories of climatic controls on weathering. Since the temperature and rainfall are affected by latitude, it would be safe to say that physical weathering processes are more efficient in areas of high and middle latitude, and that chemical weathering processes are more efficient in areas of low latitude. However, I would certainly not say that they are confined to these areas, rather that they occur at a higher rate and are therefore more noticeable in these areas.
