Coal is the fossilized remains of plants that have been put under high pressure beneath the ground for millions of years. Ancient trees and plants became buried in swampy areas, where the process of decay was very slow. The first level of decay produced a soft, earthy material called peat. As the material became covered with more and more sediment, the pressure gradually transformed it into coal. The type of coal varies according to the amount of water and carbon that it contains. The more deeply coal is buried, the more carbon and the less water it contains, forming a drier, better-quality coal. Coal is found in layers called seams. The lower a seam is found, the narrower it tends to be.
The basic recipe for any good fossil fuel is simple: Mix peat with acidic, hypoxic water, cover with sediment and cook on high for at least 100 million years. When these conditions occurred on land en masse during the Carboniferous Period — especially in the vast tropical peat swamps that gave the period its name — they launched the long, slow process of coalification.
Coal formation begins when lots of plants die in dense, stagnant swamps like the Carboniferous ones. Bacteria swarm in to eat everything, consuming oxygen in the process — sometimes a bit too much for their own good. Depending on the amount and frequency of bacterial feasting, the swamp’s surface waters can become oxygen-depleted, wiping out the same aerobic bacteria that used it all up. With these decomposer microbes gone, plant matter stops decaying when it dies, instead piling up in mushy heaps known as peat.
“Peat was buried quickly enough and buried in an anaerobic environment, which happens fortuitously here and there,” says USGS research geologist Paul Hackley. “An anaerobic environment prevented bacterial degradation. As the peat swamp continues to grow, you may have hundreds of feet of peat.”
Peat itself has long been used as a fuel source in some parts of the world, but it’s still a far cry from coal. For that transformation to happen, sediment must eventually cover the peat, Hackly explains, compressing it down into the Earth’s crust. That sedimentation can occur in a variety of ways, and it swept over many peat swamps when the Carboniferous Period ended about 300 million years ago. As continents drifted and climates shifted, the peat was shoved down even deeper, with rock crushing it from above and geothermal heat roasting it from below. Over millions of years, this geological Crock-Pot pressure-cooked peat deposits to create coal beds.
While Appalachia’s mountainous mines tap into some of the country’s oldest, largest and most iconic coal beds, American coal didn’t all form at once, Ruppert points out. The Carboniferous Period, which pre-dated dinosaurs, was peat bogs’ heyday, but new coalification continued long into and after the age of the dinosaurs.
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