Sixteen Tons Part 1: How Coal is Formed

Coal has been in the news a lot lately, what with climate change and Donald Trump's promise to restore coal mining jobs. Before we talk about that, though, let's rewind a bit to talk about where coal comes from. More than a bit, really. We're going back almost 360 million years, to the dawn of the Carboniferous.

An 1880s German illustration of the Carboniferous era. [Image source]

The Carboniferous (sometimes referred to as the Mississippian and the Pennsylvanian in America) was a hot, humid period, at least for the first half or so. It featured a thicker atmosphere and much higher oxygen levels than are present on Earth today, which in turn led to much, much windier conditions and far more powerful winds than we see today. Seas were generally shallower than today, the Appalachians were rising out of the ground, and Pangea was busy assembling itself.

These conditions both lead to and were caused by the growth of massive tropical rainforests, dwarfing anything we have on Earth today. (Or, I should say, dwarfing even the rainforests we had before we started cutting them down.) Like many things in nature, cause and effect relationships are hopelessly tangled. The warm conditions ample precipitation allowed trees to spread rampantly, which in turn resulted in the trees adding more and more oxygen to the atmosphere. This same oxygen allowed the growth of truly massive insects- the largest to ever exist.

A lycopsid fossil. Lycopsids were clubmosses that reached heights comparable to modern trees- one species from the late Carboniferous could reach 50 meters in height. Despite their height, they are mostly unrelated to modern trees. [Image source]

This was the rainforest's first appearance ever. Lignin, or wood fiber, and suberin, the waxy substance that helps seal bark, both evolved during the early Carboniferous. The forests, however, where vastly different than ours. The "trees" were, for the most part, more primitive plants like lycopsids (giant club mosses), Lepidodendrales (scale trees), giant ferns, and horsetails. Later in the Carboniferous more familiar trees started appearing, but that was millions of years and at least one large scale extinction later. The trees that did exist then made ample use of lignin. Many of them had bark to wood ratios of 8 to 1 or higher, compared to modern valued of 1 to 4.

There was a problem, however- the trees weren't decomposing. This was due to a couple of different factors. First of all, there weren't many decomposers that had evolved to handle wood and bark yet, and those that existed were overworked. Next, lignin is difficult for even modern decomposers to break down- very specialized species have evolved to handle it. For ancient decomposers it was next to impossible.

An exposed coal seam in Nova Scotia. [Image source]

This lead to the buildup of massive amounts of dead trees. There were some significant side effects of this. The combination of huge amounts of dead wood and much higher oxygen levels resulted in truly massive wildfires, dwarfing anything we have today. Not for lack of trying on our part, either- our wildfires are pretty severe, thanks to climate change and decades of forest mismanagement. Atmospheric CO2 levels dropped even lower as more and more was converted into tree trunks, and more and more oxygen was emitted. Chunks of dead wood drifted down streams and rivers in such quantities that it completely changed oceanic ecosystems- the bits of dead wood lead to a massively more productive ecosystem, including, among other things, the explosion of new ammonite and other cephalopod species.

More importantly for our immediate purposes, however, it piled up in lowland swamps and forests. These areas are commonly found in 'downward escalator' geological zones. This means that the area is undergoing subsidence- essentially, it's sinking downwards under the weight of all the sediment being piled up. In this case, trees. Peat bogs were formed and buried. At no point since have as many peat bogs existed or as many trees been buried as during the early Carboniferous. Of course, the early Carboniferous doesn't have a monopoly on coal- it's been formed in most time periods since. There's coal forming today. By far the biggest deposits are from the Carboniferous, however. Ironically, the same conditions that lead to the impressive coal formation of the early Carboniferous bore the seeds of its own destruction. The massive amounts of carbon dioxide being taken out of the atmosphere eventually lead to a breakdown of the greenhouse effect and a death of much of the Carboniferous rainforest in a minor extinction event known as the Carboniferous rainforest collapse.

Peat bogs, like this fairly typical one in Canada, tend to just look like soggy flat areas with few to no stunted trees. In actuality, they're waterlogged masses of vegetation up to hundreds of feet deep. You can walk on them, but any heavy vehicle or building is just going to sink straight into them. [Image source]

As these peat bogs were carried down into the ocean by subsidence and other 'downward escalator' geological processes, they were quickly covered up by other layers of sediment. As these layers above began to build up, more and more pressure was applied to the coal. It slowly began to convert into coal through metamorphic processes- the application of heat and pressure underground. There was a specific sequence that this followed. There are a number of individual grades of coal out there. With each increasing grade, the coal had undergone further metamorphosis, starting out as sediment, moving through sedimentary rock, and eventually some moving towards becoming metamorphic. As the grades get higher, the coal produces fewer harmful toxins- though it's never free of them- and more greenhouse gases.

Peat was the precursor to coal- basically flammable mud. Lignite, brown coal, is the next rank. Interestingly, the gemstone jet is just highly compacted lignite. After that you get bituminous coal- still a sedimentary rock, but much denser. Anthracite is the highest grade of coal. It's a partially metamorphosed coal, and unfortunately makes up a quite small amount of the total remaining coal in the world. Finally, the coal is converted to graphite, which makes a poor fuel, being too dense to ignite well. All of these types are found in thick, flat lying layers- just like any other sedimentary rock. This has major consequences for the mining of coal.

Anthracite, the highest grade of coal. [Image source]

Next time: The Environmental and Human Costs of Coal

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Bibliography:
The Emerald Planet: How Plants Changed Earth's History, by David Beerling
Squid Empire, by Dana Staaf
Life: A Natural History of the First Four Billion Years of Life on Earth, by Richard Fortey
http://www.devoniantimes.org/who/pages/lycopsid.html
https://en.wikipedia.org/wiki/Coal
https://en.wikipedia.org/wiki/Anthracite
https://en.wikipedia.org/wiki/Carboniferous
https://en.wikipedia.org/wiki/Carboniferous_rainforest_collapse
https://en.wikipedia.org/wiki/Bog
https://www.eia.gov/tools/faqs/faq.php?id=73&t=11