Coal

If a tree falls in an ancient forest, but there are no wood-eating bacteria or fungi there to decompose it, what happens? You get the beginnings of coal formation…

Coal is the name given to a whole group of carbonaceous (carbon-based) substances that are quite different in their chemical makeup, character and geographical origins. What almost all coal has in common, however, is that it was formed during the same period of time: the Carboniferous period. 

Between around 360 million and 300 million years ago, ancient lignin-rich plants (trees) first appeared, but the bacteria, fungi, and other microbes that decompose woody plants today didn’t exist yet. When these early trees died, they just piled up on top of each other on the swampy ground. The weight of all the heavy wood falling on top eventually compressed the trees at the bottom into peat. Over time and under the heat and pressure of deep burial, this peat turned into massive seams of the hard, crumbly black mass we now burn so readily to power human life.

Over the course of 60 million years of evolution, bacteria and fungi developed the ability to break down the carbon-carbon bonds in the lignin into carbon and oxygen, drastically slowing the formation of new coal deposits at around 300 million years ago. Some small amounts of coal have formed since then: when plant matter accumulates in waterlogged, oxygen-poor swamps and mires that halt its decay, new peat is formed. Under the right conditions this can eventually transform into coal over millions of years, but palaeontologist Peter Ward and geologist Joe Kirschvink estimate that ninety percent of the coal we burn today comes from the Carboniferous period. When we burn coal today, we are abruptly returning to our atmosphere carbon that was captured very slowly by ancient organisms over 300 million years ago, and that is effectively irreplaceable. 

If you look at the various samples of coal we have in our collection you might notice a difference in colour and texture between them, which corresponds directly with their ‘heating value’ or energy density. Coals are ranked from low energy density lignites (of which jet is one) with their earthy brown colour and woody texture, through to brittle and shiny grey-black anthracites, the most energy dense form of coal. The quality of coal is dependent on the type of vegetation it is made of and the depth, temperature and pressure it was subjected to when formed.