Coal is a sedimentary rock, created as layers of plant material such as wood builds up. As the layers increase, the pressure increases and with it heat. In time, the heat and pressure create anaerobic conditions allowing peat to form. Further heating and pressure will form lignite, a kind of moist brownish coal. Continued heat and pressure then forms coal. The greater the pressure and heat, the greater the percentage of the coal that becomes pure carbon. Smokeless coal has under gone longer high pressures and temperatures than more smoky coals. Ultimately forming graphite, which has a high carbon content but is no longer used for a fuel.

Most of the coal in the world was made at the same time. Between 360 and 300 million years ago. Before and since this time period little new coal has been created. Nor will coal ever form on a large scale on this planet ever again.

Around 360 million years ago, some plants evolved Lignans. This fibrous material is a fundamental part of wood. This type of plant gained a height advantage over other plants and so thrived and spread around the world.

At that time when these plants died, the bacteria and fungi that could decompose all the other types of plants could not break down this new type of woody plant.

The wood from these early trees did not decompose and in time eventually transformed into coal.

Then around 60 million years later, bacteria began to evolve that could decompose wood and these spread rapidly halting the production of coal as the wood was now being decomposed.

It's not accurate to say that coal is fossilised wood. Strictly speaking, a fossil is formed by minerals replacing the molecules of the plant or animal, thus preserving the structure and features of the original organism but none of the original material. Coal is wood that has been under immense pressure and converted in to pure carbon. You will not often see any wood markings or structure in coal.

You can get fossilised tree wood, but this is made of rock, not carbon.

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Aba Selassie
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