Mining and urbanization involve the greatest transformations of the landscape through human activity. Mining may leave huge pits and waste heaps, while urban areas contain large stocks of materials brought in from other places. Minerals extraction is broadly divided into three basic methods: open-pit or surface, underground and solution mining. Open workings are the dominant form of extraction of the main commodities mined or quarried: coal and aggregates. Surface, or open-pit, mining requires rock, soil and vegetation removal to reach mineral deposits. The waste rock, or overburden, is piled near the mine. The workings have large energy requirements and produce emissions to the atmosphere and discharges to nearby water bodies. For any particular mine, these hidden flows are often greater in magnitude than the mass of mineral or ore extracted for processing.
Urban use of materials involves two broad strands of inputs, stocks and outputs. The buildings and infrastructure of the city can be described as the 'urban fabric' (Douglas 1983) while the materials (largely food) consumed by the people and all other organisms within the city can be seen as passing through the urban biosphere. The biospheric use of materials has a rapid turnover, expressed by the high proportion of food waste and packaging in the domestic waste stream. The biospheric consumption is largely biomass-derived food and clothing, water and energy mainly using fossil fuels, but an increasing amount of hydrocarbon synthesized materials are used in packaging and other short-life materials. The inputs to the urban fabric include wood from biomass, but are mainly from mining and quarrying, and thus in national assessments of domestic and imported mineral products. The urban fabric has a slower turnover as buildings last for decades, if not centuries and, in rare cases, millennia. The renewal of the urban fabric produces construction and demolition waste (C&D waste) much of which is used to level the original site for new construction. Over the centuries, this leveling gradually raises the level of city streets and building ground floors over the residues of past structures. The residues thus become part of the 'urban deposit' (Wilburn and Goonan 1998). The dumps of wastes from biospheric consumption, from the industrial transformation of materials and from disposal of C&D waste are also part of this urban deposit. The urban fabric, and all the materials housed and stored within it, and the underlying and surrounding urban deposit make up the urban materials stock. The outputs from the city are all the transformed, manufactured and processed materials and goods as well as the gaseous emissions and liquid discharges and solid materials released to the surrounding environment.
Despite having relatively static total populations, the industrialized economies are increasing their use of materials, particularly construction materials, as households become smaller but more numerous and individuals acquire more possessions. In the rapidly industrializing countries, urban and industrial building and the construction of roads and other infrastructure are proceeding apace. For example, in China, the production of aggregates and associated hidden flows more than doubled in seven years, rising from 2313 million metric tonnes (MMT) in 1989 to 5403MMT in 1996 (Chen and Qiao 2000). However, per capita flows remain highest in the wealthier countries, the flows associated with aggregates being approximately 7.8 metric tonnes (t) per person per year in the USA, approximately 4.2t per person per year in China and only 0.39t per person in India.
This rapid transfer of materials from the natural environment to urban and industrial areas has a twofold impact: a removal of material from the earth's surface (a change in geomorphology) and the accumulation of a stock of concrete and other materials elsewhere in cities and industrial zones (a change in urban morphology). Currently, in many places, waste flows also lead to morphological change as landfills occupy old quarries or parts of river floodplains, or develop new hills as a land raise (a landfill in which the deposited material rises above the general level of the surrounding area). Thus industrial ecology transforms natural landscapes and in so doing has to be considered as a geological and geomorphological agent.
The role of human activity in earth surface processes and geological transformations has long been acknowledged. Sherlock's excellent (1922) studies provide many illustrations of the quantities of material involved in mining, construction and urban processes. In the concern over making land use and urban life more sustainable in the 1990s, much greater attention than ever before has been paid to the ecological footprints of cities (Rees 1994) and the ecological 'rucksack' of mining (Bringezu and Schütz 1996). Already analyses of materials fluxes have been produced for China, the USA, Japan, Germany, the Netherlands and Italy (Chen and Qiao 2000; Adriaanse et al. 1997; de Marco et al. 1999). Girardet has estimated the ecological footprint of Greater London as 125 times the area it occupies (Sustainable London Trust 1996). Earth scientists concerned about the human dimensions of geological processes have established a program (ESPROMUD) to demonstrate the environmental footprint of cities and extractive industries and to define guidelines to reduce it which specifically aims to assess the effects of extractive and urbanization activities on geomorphic processes (Cendrero and Douglas 1996; Douglas and Lawson 1997a). Accounting for the flows of materials in the urban process is a key element of this program.
The key quantities to be addressed in establishing the materials flows due to mining and urbanization are the annual masses of rock and earth surface materials extracted, including overburden and mineral processing residues, the volumes of waste created by human activities and the excavation and earth moving involved in major construction projects such as tunneling and road building. In addition, account should be taken of pollutant releases to the atmosphere, water and the soil, as well as of the chemicals, fuels and materials in machinery used in the mining and mineral-processing activities.
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