4.5 Global distribution of coal
Figure 35 shows the global distribution of coal deposits. The major areas are principally in the Northern Hemisphere; with the exception of Australia, the southern continents are relatively deficient in coal deposits.
This relatively uneven distribution is the result of peat formation at different times in the geological past in predominantly tropical latitudes, and the subsequent drift of the continents to their present-day positions. As Figure 36 shows, the oldest coals of any economic significance date from the Middle Carboniferous Period — the earliest geological strata in which coal has been identified are of Devonian age but they are of little economic significance. With the exception of parts of the Triassic Period, major coal deposits have been forming somewhere in the world throughout the last 320 Ma. Sedimentary sequences of the last 2-3 Ma do not contain coals, simply because there has been insufficient time for them to develop from plant debris. Possible future coals exist today in the form of peats that are accumulating in modern mires.
A broad chain of large coalfields of Carboniferous age extends from the eastern USA, through Europe, the Russian Federation and south into China. A second chain of Permian coalfields is found in the southern continents — South America, India, southern Africa, Australia and Antarctica (the latter is not shown on Figure 35). The vast coalfields of the western USA (see Box 1) and Canada are of Cretaceous-Tertiary age (Figure 35). Mesozoic-Cenozoic lignites are also important global sources of coal.
Figure 35 shows that not all regions have coal reserves. They either do not possess coal-bearing rocks, or they do, but they are not considered reserves.
There are several geological reasons for this:
Regions where post-Devonian sediments were not deposited or have been completely eroded have no coal whatsoever, such as Scandinavia and much of Africa.
Regions may have no coal-bearing rocks — although they were once deposited they have now been eroded away. Ireland once contained a substantial sequence of Upper Carboniferous coals; probably as much coal as the UK, but only a tiny fraction now remains.
Some regions have coal-bearing rocks that are buried so deeply under younger sedimentary sequences that they cannot be worked economically. The Amazon basin is a good example.
Few preserved sedimentary rock sequences formed under terrestrial conditions with which mires were associated. Some terrestrial sediments formed at high palaeolatitudes and others in arid climatic belts, neither of which are conducive to peat formation, whilst other sediments formed on the sea floor. The Carboniferous sediments of South Africa and the Permian of the UK (dominated by terrestrial glacial and desert deposits respectively) are good examples, as neither is coal bearing, whereas Permian terrestrial sediments of South Africa and those of the Carboniferous of the UK both contain substantial coal sequences formed in a humid climate.