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
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