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13 Chapter One: How Coal Formed

First, the newly dead plants piled up into a squishy mass called peat. This happens today in peat bogs, mires, marshes, and swamps around the world. Depending on growing conditions, it took between 5,000 and 100,000 years to make a layer of peat one yard (0.91 meters) deep. Peat contains lots of water, along with many plant parts, such as roots, leaves, shoots, and bark, all in various stages of rotting or decay. But the water tends to be stagnant and low in oxygen. Without plentiful oxygen, the usual process of decay slows down. One of the products is the gas methane, familiar as the main substance in natural gas—and as deadly “firedamp” in coal mines, as discussed in chapter five. As more great plants grew and died, the peat got buried by more peat, and by other layers, too, including mud or silt from floods. The peat was changed by being compressed, which also generated heat. Its water was driven off. It altered in the anaerobic conditions due to the action of microbes called bacteria. The soggy, squashy peat changed into a drier, harder, more crumbly substance called lignite, or brown coal. This is a “young” or “immature” form of coal. Middle Stages: Toward Bituminous Coal Many millions of years passed. Greater compression from layers collecting on top, and higher temperatures from the deeper burial, caused more change. The lignite gradually transformed into a harder, darker material known as sub-bituminous coal, or black lignite. With yet more time, more pressure, more heat, and more chemical changes, sub- bituminous coal continued to change into the next form, bituminous coal. This is what many people picture when asked to think of coal: hard, dark brown or black chunks. Formation of bituminous coal needs temperatures of about 250–500°F (120–260°C), which usually means burial to 10,000 feet (3,000 meters) or more. On average, it took a layer of peat 10 feet (3 meters) deep to make a layer of bituminous coal 1 foot (30 centimeters) deep. Sometimes lumps of coal split to reveal fossil shapes or impressions of fern fronds, scale tree bark, and similar plant parts from those distant times. More than 90 percent of all coal mined in North America is bituminous and sub- bituminous. America’s eastern and mid-Atlantic coalfields are mainly bituminous,