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Weathering is the disintegration and decomposition of rocks in situ (Figure 1). It is an important set of processes: it not only prepares the land to be worked upon by other agents of , it also affects human activities. Weathering is affected by rock type (jointing pattern), climate and soil. Increasingly, weathering processes are affected by human use of the landscape. Concurrently, weathering also influences the way humans interact with the landscape. The production of soil depends, in part, on weathering. Certain economic resources (such as china clay) are produced by weathering processes and the creation of landforms such as caves become tourist attractions (Figure 2). This case study explores some of the interactions between the weathering of the land and human activities.
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Figure 1. Different
types of weathering.
Figure 2. Harrison’s Cave, Barbados: the island’s premier inland tourist destination.
Commercial uses of weathered rocks and landscapes
The product ofdepends on the composition of the original rock. For example:
Figure 3. Partially
Of the numerous clay minerals, two of the most common are kaolinite and montmorillonite:
Granite is a good example of a rock used commercially for its kaolinite. This is the result of . Weathered shales also produce good brick clays, whereas basalt produces fertile soils based on montmorillonite. In central Cornwall, clay-bearing ground covers approximately 64 square kilometres. Nine tonnes of waste rock and debris are produced for every one tonne of china clay used. 87 per cent of the china clay produced in Cornwall is exported and some 60 grades are produced. The major markets are western Europe, with the Nordic countries being the most important. Over 2,500 people are directly involved in the mining and distribution of china clay to the ports. The value to the local economy is immense, with over £120 million going directly into the Cornish economy. About half this figure is paid in wages, with the remainder being used to buy goods and services.
Uses of kaolin (china clay)
· Paper – as a filler or loading as a coating pigment
Type of paper
Typical kaolin content (%)
Board up to 10
Printing up to 30
Newsprint up to 8
Other economic aspects of weathering
Groundwater fills many of the voids in weathered rocks, and under normal conditions it accumulates at the base of the weathering profile. On weathered rocks, weathering often improves the grade of economic deposits by concentrating desirable elements such as copper around the water table. Oxidised ores are usually found above the water table, reduced ores (usually sulphides) below the water table.
Another economic aspect of weathering is the alteration of prepared stones. The most resistant stones are quartzite, quartz-rich sandstone, and fine-grained rocks such as slate. In contrast, marble, sandstone and granite will all begin to weather in a matter of decades/centuries.
Soils are the product of weathering as well as processes such as translocation (movement of water within the soil), gleying (waterlogging) and organic changes. The speed of soil formation depends on many factors. Weathering depends on the local environment and the type of rock. For example, an iron nail buried in the ground will only take a year or so to decay to the point that it is easily snapped in two. Iron nails rust much more slowly in drier environments. Aluminium cans decay very slowly, even in humid climates. Glass decays even more slowly, while plastic is considered essentially non-biodegradable. It is somewhat ironic that a plastic tombstone will endure much longer than one made in marble!
Rates of weathering of clean rock surfaces
Some hard crusts (duricrusts) are associated with weathering. Laterite (a hardened layer within a soil) may form as a result of the concentration of iron or aluminium in layers within the soil. Under wet conditions, iron and aluminium are carried down through the soil. In contrast, in hot dry conditions silica may be concentrated at the surface to form an impermeable, toxic crust.
The pace of weathering
An adverse health effect of weathering is the concentration of aluminium in groundwater. Under very acidic conditions (i.e. < 4.5 pH) iron and aluminium may be flushed down through the soil. An alternative idea is that iron bacteria and other micro-organisms are particularly effective at mobilising minerals. Research at the Great Bauxite Deposit in northern Australia suggests that microbiology plays an important part in bauxite formation.
Weathering and climate
Climate is closely associated with all aspects of , i.e. breaking up the land, transporting the debris and farming new landforms.
It is possible that weathering plays a part in controlling climate as chemical weathering is strongly influenced by climate but also helps to determine it. Weathering consumes carbon dioxide leading to a reversal of the greenhouse effect and global warming. On the other hand, weathering locks up carbon dioxide which stimulates plant growth, releasing CO2 (Figure 4).
Figure 4. Rate
of weathering of different rock types.
The effects of weathering
Weathering can also produce landforms that offer important touristic opportunities. Many of these are the result of . These are normally caves and caverns but the spectacular tower karst of Guilan, China, is an example of a major tourist attraction. In Britain, up to 400,000 visitors visit the Cheddar Caves in Somerset each year and about 20 per cent of Cheddar’s population of 5,000 derive some income, directly or indirectly, from tourism.
In Barbados, Harrison’s Cave is one of the country’s most visited attractions (Figure 5). It is estimated to be 500,000 years old and is rich in dramatic formations of stalagmites and stalactites, pools and waterfalls. There are many caves in the limestone bedrock of Barbados, but Harrison’s is the only one developed so that visitors can tour it using carts pulled by electric trams.
Figure 5. Stalagmites,
Harrison’s Cave, Barbados.
The cave was first discovered in 1795 but it wasn’t explored until 1970. The explorers realised that the cave’s chambers were significant enough to be developed as a tourist attraction. Development began in 1974. Several million pounds were spent to dig tunnels, complete surveys, install underwater lighting in the most dramatic pools and devise a dam and drainage system to divert two streams from what would become the main passageways for the trams. The cave was finally opened to the public in 1981. Today, Harrison’s Cave is the island's largest single tourist attraction.
Weathering processes in urban environments
The main decay process is. This is related to the conversion of SO2 in the burning of fossil fuels to atmospheric sulphuric acid. This dissolves calcium carbonate to the more soluble (calcium sulphate). Gypsum is a salt, and within pores and fractures in rocks it expands, contracts and recrystalises. and dehydration of gypsum are controlled by small changes in temperature and humidity.
Urban weathering is not just related to raised SO2 levels. Lichen, fungi, algae and bacteria colonise surfaces and sub-surfaces of stonework and contribute to its decay. Human activities designed to reduce the impact of weathering can have adverse effects. For example, the cleaning of statues in London resulted in accelerated weathering, since the rocks were treated with sulphuric acid (see the case study on acid rain also in this series).
Salt weathering affects not only limestones (calcium carbonate) but also cement, concrete and mortar, which contain powdered limestone. For this reason, salt weathering has great impact on many cultural buildings (churches, mosques, libraries, etc.) throughout the world. In addition, salt gets into the urban environment through the de-icing roads in winter, as marine aerosols in coastal regions and also through groundwater. In Uzbekistan, the small towns of Khiva, Bukhara and Samarkand have seen significant buildings ruined by capillary rise, a rising water table (the result of over-irrigation) and an increase in the salinity of the groundwater.
Many rocks appear to weather faster in urban areas rather than rural areas. Recent studies on marble gravestones in Durham suggested rates of weathering of 2 microns per year in rural areas and up to 10 microns per year in urban-industrial areas. The weathering of St. Paul’s Cathedral in London, which is built of Portland limestone, occurs at a rate of about 0.62 microns/year. This represents a cumulative loss of 1.5 centimetres of the surface since St Paul’s was built.
Urban buildings often become excessively weathered due to atmospheric pollution. Increased CO2 levels are partly responsible, but the main culprit is SO2 (sulphur dioxide), which dissolves in water and acts as a dilute acid.
Air pollution is a relatively modern phenomenon. Thus many buildings, sometimes hundreds of years old, have experienced accelerated weathering in the last century. The Taj Mahal is a fine example from an LEDC – three decades of pollution caused by two coal-burning power stations, an oil refinery and some 250 iron foundries close to the building have reduced its gloss. In cities in Britain and the USA, legislation to combat air pollution has resulted in sharp decreases in sulphur dioxide and smoke pollution over the past 40 years, but not in nitrogen oxides.
Urban stone decay causes structural damage, loss of cultural heritage and high maintenance costs (cleaning and replacement). Building stones in urban areas are subjected to the same weathering processes as natural outcrops but with additional influences. Buildings in the urban environment are particularly vulnerable to decay because of the following factors:
These environmental conditions in urban areas produce the following effects on building and monument surfaces:
Figure 6 – Weathered statue,
Figure 7. Biological
weathering in cement.
Urban weathering in Venice
The city of Venice in Italy contains many historical
buildings and monuments that constitute a key part of our European cultural
heritage. They are, however, under threat of decay, which is accelerated by air
pollution and rising sea levels. There are also over 2,000 pieces of ‘outdoor
art’, mainly stone carvings and sculptures, within the city. Studies of old
photographs have revealed that most decay has occurred since the Second World
War. The cause seems to be the high sulphur dioxide levels resulting from rapid
post-war industrialisation of the surrounding area.
Since 1973, laws have banned the use of oil within the city itself, replacing it with methane. However, pollution still drifts in from elsewhere. Smoke and sulphur dioxide react with marble, limestone and calcareous sandstones to produce the blackened, gypsum crusts which now coat many buildings in Venice. These unsightly crusts are damaging the under-lying stone. A predominantly humid environment also encourages the transformation of calcium carbonate into gypsum in the presence of sulphur dioxide.
A local relative sea-level rise has been a problem over the past century in Venice. This has been caused by a combination of natural subsidence and extraction of groundwater (which has now ceased). As well as flooding, higher sea levels have had a less visible impact on Venice’s environment through encouraging the penetration of water and salts into vulnerable building materials.
Major research is currently under way into stone decay in Venice, coupled with many schemes to restore damaged buildings and ‘outdoor art’. Estimates of the costs of restoration suggest that all the sculptures and carvings in Venice could be restored at a cost of some $9.5 million. Grime accounts for 15 per cent of the damage requiring conservation, corrosion or decay accounts for 35 per cent, and structural problems for the remaining 50 per cent.
Urban building decay in Britain
In Britain, levels of SO2 and NO2 (nitrogen dioxide) have increased throughout the twentieth century (Figures 8 and 9). However, levels are now falling, particularly of SO2. Road transport is the largest source of NO2 emissions in Britain, contributing 49 per cent of total emissions in 2000. In London 68 per cent of NO2 emissions come from road transport. Emissions of NO2 have fallen by 34 per cent from 1990 to 2000 and are projected to fall by a further 25 per cent by 2010. This has been mainly due to reductions in emissions from road transport and public power generation. Improvements in car engine design, particularly the fitting of three-way catalysts to petrol cars in response to tighter European emission standards, have contributed to the decline. Little, if any, decline is expected in emissions from other sources in London.
Figure 8. Aerial
emissions of sulphur dioxide (SO2) by sector in Britain, 1970–2002
Figure 9. Annual
nitrogen dioxide (NO2) concentrations measured at all continuous
monitoring network sites in Britain, 1976–2004.
Sulphur dioxide emissions decreased by 75 per cent from 1987 to 2003 as power stations have invested in technologies that remove sulphur from fuel gases and use alternative, less sulphurous sources, such as natural gas.
Strategies for combating urban building stone decay include:
At first, we might think of weathering as having an impact on landscape evolution as it prepares the land to be worked upon by other processes. However, weathering also has a great impact on human activities. In particular the development of soil, and with it food supply, is dependent upon the weathering of bedrock. Weathering also produces valuable minerals, such as kaolin from feldspar (found in granite) and this is important for local economic development, and many unique landforms, important tourist attractions, are shaped by weathering. Equally, it can be seen that human activities are affecting the type and rate of weathering that occurs. This is particularly noticeable in urban areas, where additional sources of chemicals, such as SO2 and NO2, react in the atmosphere to form sulphuric and nitric acids and have a damaging impact on rocks, especially limestone. As urban growth and industrial growth continue, it is likely that these impacts will increase over time rather than decrease.