Coal forms in swamps from the decomposition of plant debris. As plants die, leaves, stems and bark litter the swamp bottom in great profusion. All plants contain oxygen, hydrogen and carbon. After death, bacteria break down these chemical elements. In fresh, well-aerated water, the plant debris is totally destroyed but stagnant water will only allow the bacteria to cause partial breakdown of the elements. The amounts of oxygen, nitrogen and hydrogen are all reduced but the carbon content is virtually unaffected. This is formed the first and most primitive form of coal, which is peat.
Peat is roughly 50 percent carbon and 50 percent of the other three elements plus ash, ash being composed of sand and clay fragments which inter-mixed with the plant debris in the swamp. Peat is dark in color and very fibrous in nature. This forerunner of coal is a cheap source of fuel but does not contain enough carbon compared to the other elements to be an efficient energy producer.
Year after year, layer upon layer, the peat accumulates at a rate of approximately 28.5 cm per year. It appears that throughout geologic history swamp bottoms were prone to subsidence at a rate roughly equal to the rate of peat accumulation, thus peat beds of thicknesses in excess of 30 m and more were able to form. Roughly 20 m of peat is needed for each metre of coal that will eventually form.
Inevitably, the life of a swamp comes to an end. A river may change course burying the swamp under vast layers of sediments or, as often happens, a transgressing sea covers the swamp with layers of limestone, sandstone or shale. Whatever the cause of the swamp's demise, the layers of peat are subjected to compaction and slight heat from overlying sediments. With continued burial, the peat dries out, the hydrogen, oxygen and nitrogen levels are further reduced while the carbon and ash remain intact. When the carbon level reaches approximately 60 percent, lignite is formed and with continued compaction and reduction of the other three elements the coal progresses through the ranks of sub-bituminous, bituminous and anthracite. Most coals never progress beyond the bituminous rank.
Coal-bearing rocks have been formed since land plants evolved about 400 million years ago. The Pennsylvanian Period of 300 million years ago was dominated by large land plants, which flourished in warm swampy environments. Eighty percent of the world's coal deposits were laid down in peat beds during this time. Alberta's coal deposits are of much later origin, the Pennsylvanian deposits in the province having been removed by erosion. Only the Cretaceous and Tertiary Periods escaped erosion in Alberta and it is in the rocks of that age that we find the great coal deposits of the province.
This article is extracted from Alex Johnston, Keith G. Gladwyn and L. Gregory
Ellis. Lethbridge: Its Coal Industry
(Lethbridge, Lethbridge: City of Lethbridge, 1989), Occasional
Paper No. 20, The Lethbridge Historical Society. The Heritage Community Foundation and the Year of the Coal Miner
Consortium (of which the City of Lethbridge is the lead partner)
would like to thank the authors for permission to reprint this