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24 Coal

coal to come out, and for ventilation so that fresh air is brought in and any dangerous gases, like methane, are removed. Many shaft mines are less than 1,000 feet (300 meters) underground, but some go much deeper. In longwall or panel mining, a machine moves along a wall-like coal face, digging or gouging or scraping off the coal. Usually this falls onto a conveyor that runs alongside the face and takes it to one end for removal. For safety, the rock surface above the face is held up by table-like metal plates on hydraulic rams that press on the roof. These roof supports are put up in stages, with the whole system advancing as the block-like panel is cut back. Left behind is the unsupported roof over a chamber-like empty area called the goaf, where coal once was. Here the roof is usually allowed to collapse in a safe, controlled way. The whole process hardly pauses and is known as continuous mining.

An open-pit coal mine in the Rocky Mountains.