Allen McKinney, age il, of Chester, Pa., for his question:
WHEN WAS COAL FORMED?
Coal is a soft black or brown rock that has an ability to burn. It's most important use is to produce steam to make electricity. Baked coal, called "coke," is used in blast furnaces to produce steel. Coal is also burned to heat buildings.
Geologists tell us that some coal was formed 400 million years ago, but that most high rank coal, such as that found in Pennsylvania, was formed more than 250 million years ago, during a time that geologists call the Carboniferous Period.
A chunk of coal may seem black and smooth, but if you look at a piece under a microscope you will see that it is colored red and gold and arranged in layers.
During the time that most coal was formed, the air was damp and steamy. Most of the plants were huge, strange looking ferns and trees that had no flowers. As these plants died, others grew on top of them. This happened time after time for thousands of years. The plants rotted and turned into a substance called "peat," which looks like rotted wood.
After a while, wide areas of the earth's surface sank. Streams and oceans poured in, carrying mud and sand which covered the peat. The pressure of the water above squeezed down the mass of peat and gradually formed it into coal.
Geologists estimate that a thickness of from five to eight feet of rotted ferns and other plants was needed to form one foot of coal.
Occasionally, the earth's crust buckled or folded. This movement greatly increased both pressure and heat beneath the surface. The increased pressure and heat formed the higher rank of coal.
Deposits of coal vary in thickness from less than one inch to more than 100 feet. Most coal is mined from beds which are from two and a half to eight feet thick. These deposits are sometimes called "veins" or "seams," but geologists prefer to use the word "bed." Coal beds are sandwiched between layers of rock and dirt.
Coal deposits may be level, sloping or tipped on end, depending on whether they have been affected by the cracking and folding of the earth's crust. Many of the deposits in the mountain sections of the eastern part of Pennsylvania are tilted at various angles.
Coal beds may lie deep in the ground or near the surface. In hilly or mountainous country, the coal often is exposed on the hillside.
But coal is more likely to be covered with dirt or weathered from exposure and hard to recognize. Excavations for buildings or highways frequently uncover coal beds.
Sometimes coal is found in drilling for water, oil or gas. But the principal method of locating coal is to use cord drillings. This is done by drilling out a core or column from the earth's crust to indicate, layer by layer, what is underground.
Because coal has no fixed chemical formula, geologists classify it as a sedimentary rock.