Fred Stone, age 13, of Barre, Vt., for his question:
WHAT IS PLACER MINING?
Mining is the process of removing mineral substances from the earth. Since some minerals are found deep within the earth while others are located near the surface, different types of mining must be used.
Among the methods of mining used when the mineral deposits occur at or near the surface of the earth are dredging, open pit mining, strip mining, quarrying and placer mining.
Placer mining is a way of obtaining tin, platinum, gold and other so called heavy minerals from gravel and sand deposits where nearby water supplies are plentiful. To start placer mining, mineral bearing gravel and sand are first shoveled into the upper end of a slanting wooden trough called a riffle box.
In the riffle box, sand and gravel are washed away. The mineral bearing minerals are heavier than the sand and gravel and settle in grooves on the bottom of the box. The noneconomic gravel and sand are washed away.
Sometimes in placer mining the mineral bearing gravel and sand may also be moved directly from a deposit into the riffle box by the force of water shooting out through a large nozzle called a giant. Placer mining done in this way is called hydraulicking.
Sometimes a form of placer mining called panning is used to get gold and other minerals from streams.
Dredging is used where mineral bearing sand and gravel layers are extremely thick. A pond or lake must be formed so that a large bargelike machine called a dredge can be floated. An endless chain of buckets is then attached to a long beam at the front of the dredge.
When the beam is lowered, the buckets dip into the water and dig up the mineral bearing sand and gravel and move the material to a bin on the deck of the dredge. By digging forward while depositing waste to the rear, the dredge moves ahead as the deposit is mined.
Open pit mining is often used to dig valuable minerals from large deposits in hard rock. Often whole mountains are removed or large pits dug by this method.
Miners make a series of connected, steplike ledges called benches when digging an open pit mine. Each bench is lower than the one above. The benches form a road around the sides of the mine so that the ore can be brought up from the pit by truck or train.
In open pit mining, explosives are often used to break up great masses of rock.
Strip mining is used to obtain coal and other minerals that lie flat near the earth's surface. Huge power shevels remove the soil and rocks that cover the mineral deposits. The shovels then transfer the material, called overburden, to piles of waste known as spoil. Smaller shovels then remove the coal or ore.
Strip mining provides an economical method of obtaining many vital minerals.
Quarrying is a method in which large blocks of rock are wedged loose or sawed out of a deposit that lies near the surface.