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The Carboniferous coal forests

Most of the world’s coal was laid down during the Carboniferous period, 360 million years ago. And huge amounts of oxygen were released for the animals of the time to breathe.

Trees had actually appeared some 20 million years earlier. But they didn’t get their big chance until after the Late Devonian extinction event had set the scene. Little seems to be known about what might have caused the extinction. There’s an impact theory – there’s always an impact theory. But on this occasion the evidence doesn’t seem to be terribly convincing.

On balance an earthly cause looks more likely. There are signs of major changes in sea level at around this time. There were climatic changes and changes in ocean chemistry as well. So it’s tempting to blame plate tectonics and the super-continent cycle.

However as soon as the extinction was over, massive forests appeared. And the Devonian period gave way to the Carboniferous. This forest scene comes from ‘History of Life’ by Richard Cowen.

The trees were quite different from modern trees. A major player was the ‘clubmoss’ tree (colour picture: Hans Steur). It could grow to more than 30 metres. Another was the ‘horsetail’ tree which grew up to 20 metres. These trees are clearly related to their modern brethren. But the actual species died out at the end of the Carboniferous when conditions became colder and much drier.

The sea level had risen, and large areas of low-lying land had been covered by shallow seas and freshwater swamps. The climate was also warm and wet.

These were ideal conditions for the laying down of coal seams. Trees and other vegetation die and fall into the water; where they are quickly covered. Oxygen is excluded. So bacteria, bugs and fungi can’t attack the dead remains and rot them away.

More falls on top; and before you know it you have a good thick deposit.

Eventually conditions change, and gravel and sand get deposited on top. Over tens and hundreds of millions of years, other adventures befall the deposits. And we end up with a coal seam, perhaps several kilometres underground.

During the Carboniferous – and as a result of all this ‘carbon burial’ – the amount of oxygen in the atmosphere grew to crazy heights, around 35% (more). At these levels, dry wood combusts spontaneously. Fortunately the coal forests were wet. This was also the time of the giant dragonflies (see above link), with wingspans up to 2 feet.

The conditions changed again at the end of the Carboniferous, and as far as I know only limited amounts of coal have been laid down since.