Mining

Surface mining is less expensive and safer than underground mining. About 90% of the rock and mineral resources mined in the United States and more than 60% of the nation's coal is produced by surface mining techniques. Coal mining accounts for about half of all surface mining, extraction of sand, gravel, stone, and clay for another 35%, phosphate rock for about 5%, and all metallic ores, for about 13%.

There are four kinds of surface mines: open-pit mines, strip mines, mountaintop mines, and alluvial (or placer) mines. Open-pit mines consist of deep cone-shaped holes or pits that are excavated in rock that is first loosened by blasting. In order to prevent the sides of the pit from collapsing, open-pit mines must be continually widened as they are deepened. Open-pit mines are used when the ore is of low grade, meaning that the amount of metal per cubic meter or kilogram of rock is small, and disseminated, meaning that the metals are distributed throughout large volumes of rock rather than being concentrated in veins. The size of open-pit mines, which often take decades to excavate, makes it uneconomical to reclaim the pits by filling them with rock.

Strip mines consist of shallow excavations, perhaps 109 yd (100 m) or less, that are used to mine tabular bodies of rock such as coal. Soil and rock above the mined material, known as overburden, are removed and set aside. Because the strip mines consist of shallow excavations, the overburden can be economically replaced, re-contoured to resemble the original topography, and replanted.

Mountaintop mining came into widespread use in Appalachian coal fields following the 1997 passage of the Surface Mining Control and Reclamation Act by the United States Congress. Mountaintop mining is similar to strip mining in that the overburden above a tabular coal deposit is removed. Instead of being stockpiled and used to restore the original topography, however, the overburden is used to fill adjacent valleys. Although mountaintop mining is an inexpensive method of mining coal in mountainous areas, the filling of valleys can have negative environmental impacts. Mountain-top mining in West Virginia was halted by a court order in 2002 on the grounds that the practice violated the Clean Water Act, but changes to the act may again make the method legal.

Alluvial mining is a form of surface mining used to recover heavy minerals such as gold from sand and gravel beds, including stream beds, known as placer deposits. In some cases these deposits can be removed mechanically by agitating the sand and gravel in simple pans. A more sophisticated and efficient way of separating placer minerals from the sand and gravel is a sluice box, a long, shallow box with wooden separators placed along its bottom. As sand and gravel is shaken in the sluice box, lighter sand grains are washed away and heavier metals are left behind. A particularly destructive form of alluvial mining is hydraulic mining, in which pressurized water is used to wash away large amounts of sand and gravel. Dredges are used in other large-scale alluvial mining operations.