Surface mining

Surface mining is a broad category of mining used to extract minerals from the earth. Unlike underground mining, which uses shafts or tunnels to reach deposits, surface mining removes the soil and rock sitting on top. This overlying material is known as the overburden. This method is used globally to find many different types of minerals. In North America, surface mining began to be used in the mid-16th century. It became very popular throughout the 20th century. Today, surface mines produce most of the coal mined in the United States.

The process of surface mining usually follows a specific sequence of steps. First, heavy equipment like earthmovers are used to remove the overburden. Once the mineral deposit is exposed, large machines take over. These machines might include dragline excavators or bucket-wheel excavators to extract the mineral. After the minerals are collected, they must be transported away. Historically, workers used manual labor, horse-drawn vehicles, or mining railways. Modern mines typically use large haul trucks on roads built specifically for the mine.

There are five main types of surface mining, each suited to different landscapes. Strip mining is a common method used for coal and lignite, which is brown coal. It involves removing a long strip of overburden to reach a mineral seam. This is most practical when the ore is near the surface and mostly horizontal. There are two forms: area stripping on flat terrain and contour mining in hilly areas. Some machines used here, like bucket-wheel excavators, can move 12,000 cubic meters of earth per hour.

Another method is open-pit mining, which creates a large pit or borrow on the ground surface. This technique is best suited for accessing mineral deposits that are mostly vertical. A different, more intense method is mountaintop removal mining, or MTR. In MTR, explosives are used to break up the overburden above a coal seam. This material is then moved by haul trucks into fills in nearby valleys. This process can change a steep mountain into a much flatter landscape.

Highwall mining is a more specialized method that is considered a semi-surface and semi-underground technique. It evolved from auger mining to recover coal adjacent to an area that was already surface-mined. It uses a continuous miner propelled by a hydraulic pushbeam transfer mechanism, or PTM. The machine goes through cycles of sumping to push forward and shearing to cut the coal. Operators use tools like gamma-ray sensors or coal-rock interface detection sensors to see the seam. This method displaces very little soil compared to mountaintop removal.

Finally, dredging is a method used for mining below the water table, often for gold. Small dredges might use suction to pull material from the bottom of a water body. Large-scale historical operations used floating dredges, which are barge-like vessels. These vessels scoop material via a conveyor belt on the bow. They filter the desired mineral on board and return the unwanted material via a conveyor on the stern.

While surface mining is often cheaper and safer for workers than underground mining, it has many environmental impacts. It can cause habitat destruction, noise, and air pollution. Blasting at mountaintop mines can release dust containing sulfur compounds. This dust can settle on nearby property and may be a health hazard. The U.S. Environmental Protection Agency (EPA) found that valley fills can increase mineral levels in nearby streams. This can lead to decreased aquatic biodiversity.

Despite these challenges, some land is reclaimed after mining is finished. In the United States, the Surface Mining Control and Reclamation Act of 1977 mandates this process. In areas like Kentucky and West Virginia, reclaimed land has been used for many things. Some sites have become golf courses, small-town airports, or even prisons. Proponents say this provides much-needed flat land for use in hilly regions. However, critics argue that the changes to the ecosystem and topography are too profound.