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Factors affecting the rate of Weathering

Climate

Weathering processes are particularly dependent on certain climatic conditions which determine temperature and the availablity of water.

Water is important both in chemical reactions (for example in directly reacting with minerals e.g. in hydration, or providing elements (such as hydrogen) for chemical processes (e.g. hydrolysis)) as well as in physical weathering (e.g. freeze-thaw). Temperature is also very important in controlling the rates of weathering. Where there are large diurnal ranges in temperature, both freeze-thaw and thermal expansion are more effective. Thermal expansion may be particularly effective in deserts, with expansion of rock during the hot daily temperatures and contraction of rock, due to the significant fall in temperatures at night. Temperature is also important in determining the rates of chemical weathering. An increase in temperature of 10⁰C can cause a 2.5 x increase in the rate of a chemical reaction which explains the high rates of chemical weathering in warm, moist tropical environments (as shown on the Peltier diagram). Chemical weathering is therefore limited in cold, dry locations and ineffective where average temperatures are way below 0^oC. It is noted (e.g. in Palmer and Yates, 2005 - see suggested reading) however, that carbonation can be more effective in colder conditions, as CO₂ is more readily taken in by cold water.

Such is the control of climate on weathering that Peltier suggests that weathering rates can be calculated by the annual temperature and rainfall or a place. You must make sure that you are familliar with the famour Peltier diagram which shows the relationship between mean annual rainfall, mean annual temperature and weathering rates. (see diagram below)

Examples:

Freeze thaw (frost shattering) - is dominant in the Tundra - in these periglacial conditions, the spring thaw and the autumn freeze, along with the likelihood of frost throughout the year, provide the seasonal and diurnal freeze-thaw cycles which are perfect for this physical weathering process.

In temperate, humid regions which are cool and wet, regular freeze-thaw cycles are also able to take place - for example many areas of upland Britain.

In equatorial climates e.g. Brazil, chemical weathering is most dominant due to the hot and humid conditions.

Geology

The mineral composition and the physical structure of rock (presence of joints, bedding planes etc.) can greatly affect the rates of weathering. Rocks such as limestone and granite are particularly susceptible to weathering processes due to their mineral composition and thus experience relatively high rates of weathering. Calcium carbonate in limestone for example is greatly affected by carbonation whilst feldspar in granite is turned into Kaolin by the process of hydrolysis. The presence of joints and bedding planes in rocks also provide lines of weakness which can increase weathering. Plant roots are able to exploit cracks and joints in rocks through biological weathering, whilst freeze-thaw processes also exploit cracks, gradually forcing rock fragments apart. In turn, the presence of joints, can make rocks, which are otherwise impermeable (e.g. granite), pervious, allowing water to enter the rocks and thus increasing the rate of chemical weathering within the rock. The permeability of rocks is also affected by grain size and thus coarse grained rocks tend to experience greater rates of weathering.

Relief

The relief of the land may affect weathering in a number of ways. Slope processes, such as landslides, slumps and solifluction can result in the exposure of previously unexposed, bare rock which then becomes susceptible to weathering. In lowland areas, unweathered rock may be protected by thick layers of soil and weathered material (although organic acids from denser vegetation may increase chemical weathering). Rainfall totals tend to be higher in upland areas and temperatures colder - again increasing rates of physical weathering such as freeze-thaw. The accumulation of water at the base of slopes may also provide more water for chemical processes to take place. Temperature variations with respect to the aspect of different slope faces may also impact upon rates of weathering. In the northern hemisphere, rates of physical weathering are greater on north facing slopes, which experience more freeze-thaw cycles due to the lack of direct sunlight.

Soil / Vegetation Cover

The presence of vegetation helps to increase rates of weathering. This includes direct biological weathering, through the growth of plant roots into joints and along bedding planes, wedging rock apart. Vegetation can also increase rates of chemical weathering through the release of organic acids, important in processes such as chelation and also through the increased levels of carbon dioxide from plant respiration which forms weak carbonic acid when dissolved in water and increases rates of carbonation. The insulating effects of vegetation can however in some instances reduce weathering rates, for example by maintaining more constant temperatures (reducing the likelihood of thermal expansion) and therefore also reducing the temperature fluctuations which may have lead to freeze-thaw cycles.

Human Activity

Humans have greatly increased rates of weathering by influencing concentrations of chemical pollutants in the atmosphere by industry, power stations, vehicle emmisions etc. The increase in gases such as carbon dioxide, suluphur dioxide and nitrogen oxide has lead to the formation of acid rain, where these gases have formed acid in solution with water - e.g. carbonic acid. This acid rain, is able to more readily attack rocks such as limestone through carbonation and increases chemical weathering processes such as hydrolysis. Removal of vegetation (e.g. through deforestation) has conversely resulted in a decrease in chemical and biological weathering - for example through a reduction in organic acids.

Excellent summary from the USGS Summary CMG InfoBank on Rates of Weathering - brief and succinct!

Weathering and Factors affecting Rates of Weathering - Well worth a read - this starts with a summary of weathering processes but then focuses on the factors that can affect the rates of weathering!

Factors that Influence Weathering (adapted lecture notes from Prof. Stephen A. Nelson Tulane University)

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).

Also see p. 22-24 in Palmer, A and Yates, N - Advanced Geography (Core Edexcel A text)