The Origin of Coal

The rich plant growth in the coal age forests provided the organic material that would become coal. On the forest floor, roots, stems, tree trunks and leaves accumulated faster than they decomposed. This thick deposit of plant material is referred to as peat. Coal, in essence, is fossilised peat.

Eventually, some change to the environment (such as a flood) buried the peat. Beneath the ever-deepening layers of sand, silt and mud, the peat was slowly transformed. Initially, the weight of the sand and mud compressed the peat, squeezing out much of the moisture. As temperature and pressure both rose under the increasing thickness of the sediments above, the deposit was chemically changed into the black, lusterous material that we know as coal.

Looking at coal today, we can see evidence of the history of the forest swamp and the layers of plant material that accumulated long ago. The bark and wood of large trees form bright, almost glassy layers. Thin layers of sooty, soft material, called fusain by coal geologists, are actually fossil charcoal, evidence of fires in the primeval forest. Dull, heavy layers give clues to the presence of tiny grains of sediment washed into the ancient swamp by floodwaters. This inorganic matter is left behind as ash when coal is burned.

A thick layer, or seam, of coal tells us that the ancient forest swamp existed for a remarkable length of time. Because peat is compressed as it is transformed to coal, it may take between 5 and 10 m (16 - 32 ft) of peat to form just 1m (3 ft) of coal. Therefore, a coal seam that is just one metre (three feet) thick may represent 2,500-5,000 years of plant accumulation beneath the ancient forest swamps.

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