What Is Open-Pit Mining? Definition, Examples, and Environmental Impact

Open-pit mining is one of several non-tunnel approaches to mining that gives miners ready access to minerals and stone near Earth’s surface. Explosives help create massive, canyon-like holes. Heavy machinery refines the holes into workable pits and extracts the valuable materials that large trucks then cart away. Solid and liquid waste are usually kept at disposal sites near the pit. 

Open Pit Mining Definition

No government agency publishes information about the number of open-pit mines domestically or globally. Nor is there a reliable public source of information about the dollar size of the open-pit mining industry. This may be because open-pit mining is only one type of mining in which the soil and rock covering the material to be mined are removed. Together, those types of mines are called surface mines.

An additional problem with pegging the size of the open-pit mining industry is that many open-pit mines have underground tunnel components. 

More useful is to look at the mining industry in general. The Centers for Disease Control and Prevention (CDC) report that, in 2020, the United States had the following active mines: 6,251 sand and gravel pits, 4,281 stone quarries, 1,009 coal mines, 895 nonmetal mines, and 278 metal mines. Sand, gravel, stone, coal, and many of the metal and nonmetal mines are likely to be of the open pit type.

The classic open-pit mine is impressively deep and far wider at the top than at the bottom. One example is the particularly massive Bingham Canyon mine near Salt Lake City, Utah. It is nearly three-quarters of a mile deep and about 2.5 miles wide.

Pits are dug so that the walls ("batters") slope downward. The slope helps reduce the pull of gravity on the rocks and therefore minimizes the risk of boulders tumbling down and causing injuries. Flat, dirt terraces called “benches” or “berms” periodically extend out from the batters. They are wide enough to firmly support dinosaur-sized trucks and other heavy machinery as they pass each other. A system of ramps allows trucks and other dirt movers to drive between benches. 

Open-pit mining is commonly used to extract metallic ores like aluminum, bauxite, copper, gold, copper, and iron as well as non-metallic ores like coal, uranium, and phosphate. Open-pit mining is also known as open-cast mining, open-cut mining, and mega-mining. 

Environmentally, open-pit mining is devastating. It consumes enormous amounts of water, heavily pollutes water and air, disfigures landscapes, and permanently destroys habitat. Even after pits are exhausted and sites are rehabilitated, the pit area retains elevated risks of erosion and flooding.

Despite its environmental drawbacks, there are a few reasons that open-pit mining remains popular. Relying on heavy machinery and explosives, it is from three to five times more expedient than deep-shaft mining. As much as 20,000 tons can be mined in a single day. It’s safer for the miners, as well, because at most pits tunnels aren’t necessary, which means that the risks of tunnel collapse, fire, and toxic gas release are minute. 

Air Pollution

Heavy clouds of dust form during mining operations. Blasting alone is an enormous piece of the problem. In 2018 an international team of scientists publishing in E3S Web of Conferences reported that about 10 billion cubic meters of rock are exploded annually. The resulting clouds transport about 2.0-2.5 million tons of dust. 

The dust from drilling and blasting at some mines is highly radioactive. This is the case, for example, at uranium mines. The problem is not limited to well-known radioactive ores, however, as all ores are radioactive to some degree. 

Even when it’s not radioactive, dust containing heavy metals can be very dangerous. When it is breathed it can create a variety of respiratory problems including black lung disease. 

The explosives used in blasting release fumes rich in smog- and acid rain-producing gases like highly toxic nitrogen dioxide. As far back as 1973 Soviet scientists reported that the smog can form in the pits themselves. In 2019 Chinese scientists reported that a mine-produced hydrochloric acid mist was caustic enough to corrode concrete.

When mining equipment malfunctions or when workers such as welders make mistakes, coal ignites. Mine fires release toxic gases and cause significant air pollution. 

The gargantuan heavy equipment used at mine sites generates exhaust and pollutes the air.

Water Pollution

One of the most significant problems in open pit mining is also endemic to underground mining. The mineral pyrite is often found in coal mines. It contains sulfur. When pyrite is exposed and sulfur reacts with air and water, it forms an acid. Acidic water as well as any rock-bound heavy metals that the acid has dissolved leach out of the mines and into nearby rivers, lakes, and streams, killing aquatic life and making the water unusable.

A 2021 study in the peer-reviewed journal Ecological Applications showed the elimination of 40% of the marine animal species in 93 bodies of water downstream from an area of Appalachia that has multiple open-pit mines. Especially problematic in relation to coal mining, acid mine drainage can continue for hundreds of years, even long after the mine has closed.

The water pollution created by open-pit mining is typical throughout the mining industry. Coal and other minerals are transported by rail, truck, or boat to “mills” where the mineral product is sorted out and the rock is crushed, ground, washed, and filtered. Then, depending on the mineral, the mining product is put through a variety of water- and solvent-intensive purification processes. The solvents, other industrial chemicals, and metals that remain in the water are collectively called “tailings.”

On-site accidents can send the tailings directly into the environment. This is what happened near Vancouver, Canada, at the Mount Polley Tailings Storage Facility on August 4, 2014. The collapse of an on-site dam sent eight million cubic meters of tailings into Polley Lake, Hazeltine Creek, and Quesnel Lake.

According to the official environmental impact report, the polluted water overwhelmed the creek and carved a new much wider and deeper valley. The surrounding wetlands became thick with metallic sludge. About 336 acres of topsoil around Polley Lake were washed away and deposits of tailing as thick as 11.5 feet blocked the lake’s outlet. Restoration efforts are ongoing. 

Water Consumption

Water consumption rates are tracked by the United States Geological Survey. During 2015, an estimated 4 trillion gallons per day were pumped out of the earth for the washing necessary for the milling process of mining. (This figure includes both surface mining and tunnel mining.) Groundwater was the source for 72% of that. The remainder was surface water, 77% of which was freshwater.

Waste and Habitat Destruction

Open-pit mines are dug directly into mountain tops. The vegetation is gone. The soil is gone. The habitat is completely destroyed. 

Until 1977, Federal law did not require that open-pit mines be remediated or rehabilitated in any way after mining operations had ceased. Since that year, the Federal Office of Surface Mining Reclamation and Enforcement has governed reclamation in concert with various state agencies. Regulations vary by state but, in general, mining companies need to clean up the site. They are not obligated to rebuild mountain tops. They don’t need to restore habitat. They must merely return the land to some usable form.

Regarding the word “usable”: The California Department of Conservation, for example, merely insists that pits be put to beneficial use. That Department lists open space, wildlife habitat, agriculture, or residential and commercial development as appropriate ways in which pit land can be reclaimed.

A portion of the enormous Beckman Quarry in San Antonio, Texas, became a Six Flags amusement park and a shopping center. The Big Brown Mine near Fairfield, Texas, is now a wildlife area and private lake. Bridgeport, West Virginia’s Pete Dye Golf Course, rated No. 9 on Golfweek’s ranking of the Best Modern Courses, is on the site of a former open-pit mine. 

Noise and Light Pollution

To maximize the use of the expensive machinery, many open-pit mines operate seven days a week, 24 hours per day. This creates untold noise and light pollution that is disturbing to humans and nearby wildlife. 

Long-Term Effects (Remediation and Reclamation)

Obligated to clean up open-pit sites, mining companies sometimes flatten solid waste that is rich in heavy metals and place it inside the pit that is to be backfilled. If there was pyrite in the mine, a layer of clay is deposited over the entire pit so that the pyrite and any sulfur it contains won’t immediately interact with water and air and create further acid mine drainage. (Unfortunately, there are no long-term studies on the success of clay layers.) 

The mine itself can be filled with waste rock. Then it is re-contoured. Topsoil is added and vegetation is planted.

The difficult truth is that, at remediated open-pit mines, the mountain top is forever gone. Meanwhile, when a mine closes the pumps keeping water out of the pit are turned off. The nearby topology may cause rainwater to always flow into the remediated pit. Sometimes the area becomes a lake—albeit one with particularly toxic waters.

Is Open-Pit Mining Safe?

For miners, open-pit mining is safer than tunnel mining—but it's not exempt from risk.

Mining tunnels can collapse or burn calamitously, killing many hundreds of miners at a time. For example, in 1942 a mixture of gas and coal dust exploded in the Honkeiko coal mine near Benxi in the Liaoning province of China. As tunnels collapsed and fire blasted throughout the mine, 1,549 miners died.

Even when mine gases don’t explode, they can kill, either because they are toxic when inhaled or because they take up a large percentage of the breathable gas that’s available in the narrow space. This deprives miners of oxygen and silently smothers them. 

The dangers to miners of open-pit mines are far fewer. According to the CDC, falling rock, machinery problems, electrocution, and various other accidents common across industries are also endemic to open-pit mining. In 2020, 29 miners were killed in the United States.