1. What Is Coal Made Of?
Coal is a combustible rock formed from ancient plants piled underground that changed over long years after being buried. It is explained that long-term exposure to heat and pressure underground is one condition for its formation [1]. Although coal comes from living things, it is generally not classified as a fossil because it does not retain the original shape or structure [11]. It is less like the forest itself and more like a "mass of fuel" created from the forest.
Here is a sticking point. Fallen leaves and dead wood, if left alone, are decomposed by microorganisms and disappear. To become coal, they must be buried before they decompose. What kind of place allows this to happen?
2. How Dead Plants Pile Up as Peat in Wetlands and Swamps
The first stage of coal is peat. In places submerged in water, like wetlands or swamps, dead plants do not fully decompose and pile up little by little [2]. The Kushiro Coalfield is described as having formed from rich peatlands around Kushiro about 38 million years ago, according to the Ministry of Land, Infrastructure, Transport and Tourism [2].
This explanation mentions a cold environment, but the Carboniferous forests discussed later were in warm, humid environments. It can be read that what is important for peat formation is not the temperature, but that plants are submerged in water and easily buried. When soil and sand pile up on top of the peat, the weight and geothermal heat gradually turn the peat into hard, black coal. This is the first half of the "formation process." The second half looks at what changes happen underground.
3. How Burial Turns Peat into Young Coal and Advanced Coal
Coal has stages of maturity. Young coal from newer eras is called lignite, and it is said to contain more than half water when mined [6]. However, water content figures vary by source, so it is safe to understand it as "about half or more."
One of the more advanced types is bituminous coal. It is intermediate between lignite and the most advanced anthracite, with a carbon ratio of about 80 to 90 percent. When heated, it softens and melts, and when cooled, it solidifies into coke. Those with strong stickiness are used as raw material for coke in steelmaking [5]. So, what kind of forest provided such a huge amount of material for coal?
4. How the Giant Forests of the Carboniferous Period Differed from Today's Trees
The Carboniferous period is the era from about 360 million to 300 million years ago. Under a warm and humid climate, large forests of fern relatives spread over wide lowlands [3]. The main formation period for the world's coal is from the Carboniferous to the subsequent Permian period [1].
A representative tree is Lepidodendron. It reached heights of about 30 meters (about 100 feet) and had scale-like patterns on its trunk surface. It is close to clubmosses and belongs to a different group from the "trees" we imagine today. It flourished in the Carboniferous and went extinct in the Permian [4].
This period was special in Earth's history. One paper states that during the past 500 million years, carbon in organic matter was buried underground fastest between about 330 million and 260 million years ago [9]. Why did so many forests remain only during this time?
5. The Leading Theory: Few Organisms Could Decompose Lignin
For a long time, the leading theory was that few organisms could decompose lignin. Carboniferous trees contained a lot of lignin, a component that hardens wood, but few organisms could decompose it, so dead wood did not rot and was buried [7].
A 2012 study supported this idea. By comparing the genomes of 31 types of fungi and focusing on white-rot fungi, which have enzymes that efficiently decompose lignin, the origin of these enzymes was estimated to be around the end of the Carboniferous period [7][8]. Researchers suggested that the origin of lignin decomposition may have coincided with a sharp drop in the rate at which organic carbon was buried underground [8].
The evolution of fungi might have closed the curtain on the age of coal. It is an appealing theory that is easy to explain. However, in 2016, a paper appeared that challenged this.
6. The 2016 Rebuttal: Giant Trees Were Supported by Bark, Not Lignin-Rich Wood
In 2016, a rebuttal paper was published in the Proceedings of the National Academy of Sciences (PNAS) in the United States. It examined the structure of giant trees that became coal material. It stated that giant trees of the lycopsid group maintained their height not with lignin-rich wood, but with periderm (outer bark) that has almost no lignin [9].
Furthermore, it stated that even when the types of plants making up the forest changed significantly, there was no major change in the rate of coal accumulation [9]. If "coal formed because fungi appeared late," the rate of organic carbon burial should change when fungi appear. The researchers argued that this theory is rejected [9].
The alternative explanation is a combination of climate and terrain. To form coal, you need a "wet tropical climate" and a "hole that can bury organic matter for a long time." It is explained that when continents collide to form mountain ranges, the adjacent crust sinks to form large basins, where plants accumulated [10]. However, this is just one strong rebuttal, and it is not yet decided which theory is correct.
7. Why Japan's Coal Comes from a Different Era Than Much of the World's Coal
While much of the world's coal comes from Carboniferous to Permian forests, Japan's coal is mainly from the Paleogene period [1]. The Kushiro Coalfield formed from peatlands around Kushiro about 38 million years ago [2].
Comparing the numbers, 38 million years ago is about one-eighth as old as 300 million years ago (38 million ÷ 300 million = about 0.13). Even though it is the same "coal," Japan's coal is a legacy of forests from a much newer era than the world's main coal.
Thinking about this, the debate between the lignin theory and the basin theory is about the world's Carboniferous to Permian periods. How Japan's coal formed seems to require considering a different path.
8. Activity: Compare Where Fallen Leaves Remain in a Dry Place and a Wet Place
Try finding two places in a nearby park or garden where fallen leaves are piled up. Look for a sunny, dry place and a place that is always wet. Pick a few leaves from the same tree and place them in each location. After a few weeks, compare how much of the shape remains. Do not go near the edges of ponds or rivers. Do this with an adult, checking the safety of your footing.
Which place has leaves that retain their shape, and which has leaves that are falling apart? This observation is a way to test the content of materials explaining peat conditions in a nearby place. Whether it actually becomes coal takes tens of millions of years underground, so you cannot check that.
If you want to see real coal, try checking if there are coal mines or museums dealing with coal nearby. Exhibits may have explanations of coal types like lignite and bituminous coal mentioned in this article.
Safety note: Do not go near the edges of ponds or rivers. Always do this activity with a parent or guardian.