Kim Harris, age 13, of Danville, I11., for her question:
HOW IS STONE QUARRIED?
Quarrying is a method of taking large solid blocks or broken masses of stone from the earth and preparing them for construction projects. A quarry is a large pit in the earth from which the stone is taken. Stone is quarried by the plug and feather method, the explosive method or channeling by machinery.
With the plug and feather method, rocks can be split along smooth lines by exerting constantly increasing pressure evenly on all parts of the rock's surface. With this pressure workmen can break the rock into any size and shape. Evenly cut blocks are called dimension stone.
The main tools in the plug and feather method are a plug or wedge, which is flat on its two opposite surfaces, and two pieces of steel, called feathers, which are rounded on one side and flat on the other. The first step is to drill a series of small holes about three quarters of an inch in diameter into the rock in a straight line.
Next the wedge is placed between the two feathers and all three are inserted in a hole. When all the holes are filled, the workmen drive the wedges and feathers downward to split the rock.
The explosive method is generally used to break off huge masses of rock from their place in the earth. Strong explosives are best if crushed rock is desired. A milder explosive is used when more regular stones are desired.
Most large quarries use a channeling machine to make the first cut into a solid bed of rock. It looks like a small locomotive with long chisels on the sides. The machine moves along a track on top of the rock and forces the chisels downward.
Gradually, the chisels cut the rock to any depth from one to 10 feet. The rock is broken off below by blasting or by the plug and feather method.
Almost 6,000 quarries are in operation now in the United States.
Some quarries are dug into the sides of mountains. Most are open at the surface. A quarry may be over 100 feet deep and many times that in width.
Kinds of stone taken from quarries include basalt, granite, limestone, marble, sandstone, slate and travertine.
When the explosive method is used, the explosive is put into holes and set off by slow burning fuses or electric firing. Sometimes hundreds of tons of stone are forced out of the earth in a few huge pieces.
About 1 billion tons of stone are quarried annually in the United States. Crushed stone makes up most of the total production.
Crushed stone is widely used in paving roads and making concrete.
In blasting, dynamite is used most often, but there are many other kids of explosives including TNT. Some blasting operations use a mixture of ammonium nitrate and fuel oil as the main explosive charge.