Open-Pit Mining

Open-pit mining, also called opencast, open-cut, strip mining, or quarrying, is a mechanical extraction method that dates back to over 3,000 years. Ancient mines include the Tongling copper mine of China's mid Shang Dynasty 3,300 ± 60 years ago and some potential United Nations Educational, Scientific and Cultural Organization World Heritage sites—Mitterberg in Austria and the Timna mine from ancient Egypt (now located in Israel). Quarrying refers to opencast mining that extracts building materials, such as granite, limestone, marble, and slate. The method extracts resources occurring close to the surface or deeper formations that are structurally unstable or too friable to make tunneling possible. Commonly mined minerals include bauxite, copper, diamonds, gold, gypsum, kaolin, lead, manganese, nickel, phosphate, and zinc. The largest resource mined is coal, which also produces the greatest amount of solid waste.

With the rise of the Industrial Revolution, there was an urgent need for coal and iron ore, which fueled massive growth of mining. As technology advanced, there was recognition that other metals could be of use, such as titanium, aluminum, and platinum. Peasant populations moving to cities to take advantage of new industrial jobs required greater coal extraction for electricity generation and iron ore for the building of urban infrastructure. Human wealth increased, creating a demand for gold and diamonds. As a consequence of this growth, mines expanded into huge operations, such as the Bingham Canyon copper mine in Utah, which is the largest excavation in the world, measuring 790 m (meters; 2,590 ft. [feet]) deep and 3.7 km (kilometers; 2.3 miles) wide. The mined-out, open-pit Big Hole diamond mine in Kimberley, South Africa, reached a depth of more than 1,000 m (3,300 ft.). Some of the largest opencast mines in the world are the Cerrejon coal mine in Colombia and the Grasberg-Erstberg copper mine in Indonesia, which extracts 0.24 million tons of copper ore every day.

Surface vegetation, soil material, or overburden covering the ore-bearing horizon is stripped off by earth-moving equipment, such as bucket wheel excavators, stripping shovels, and walking draglines. Rock material is broken up by explosives, and mechanical shovels separate minerals from gangue. Blasting may be preceded by drilling holes into rock to implant explosives. In coal mining in hilly terrain, overlying material is stripped following the contours around the hillside or removed off the mountaintop to expose the underlying coal seams. Deposits are mined along benches, steps, or terraces cut along the wall of the excavation. Minerals are transported by truck or train to undergo further processing. This may involve the use of chemicals such as in the cyanide leach process in gold mining. The method causes marked environmental degradation.

Environmental effects of open-pit mining include erosion, sedimentation, and landslides. It has been estimated that solid waste produced by metal mining is 8,000 million tons/year. The “shoot-and-shove” mining practices of coal mining in the Appalachian Mountains of the United States have caused widespread environmental devastation. The rock overburden above the coal seams is blasted away and then pushed into the valleys, infilling streams, and destroying river ecosystem. Hydraulicing, in which high-pressure water is aimed at hillslopes to loosen the sediments [Page 2087]in which the gold is contained, is commonly practiced in gold mining in developing countries. This technique leads to unstable hillslopes and enhanced erosion and sedimentation of nearby streams. Tailings from gold ore processing end up in artificial ponds and may be toxic due to the use of hazardous chemicals or associated gangue minerals such as arsenopyrite (see the second photo). Mercury is also commonly used in the extraction of gold, and approximately 3,000 tons of this toxic chemical has found its way in the past 15 years into the Amazon River due to mining activities. This has led to contamination of the environment, and biomag-nification and bioaccumulation increased the toxic-ity of the mercury for the wildlife in the basin.

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