Ever since the late 1990s, there's been one dominant theory for how Earth's enormous coal reserves were created: Coal is the fossilized carbon remains of plants that died hundreds of millions of years ago.
The conventional explanation for why so much coal dates from a particular era in history (the Carboniferous period) is that the plants from this period had just evolved lignin. Lignin is a critical support structure in wood and bark, it makes cell walls rigid, and it allows for stems and branches. Because lignin was new, there were no bacteria or fungi available to break it down. Over time, this thick layer of non-decomposing plant matter was compressed, baked, and transformed into coal deposits.
That's been the theory, at least -- but a new Stanford paper(Opens in a new window) argues that it's not true. Geobiologist Kevin Boyce, an associate professor of geological sciences at Stanford School of Earth, Energy & Environmental Sciences, argues that the evolutionary lag hypothesis doesn't fit the rest of the geologic data. Many of the plants that grew during the Carboniferous period had low levels of lignin, and there's no clear link between lignin levels and the size or quality of coal deposits within the relevant geologic layers. Furthermore, given the estimated volume of plants growing on Earth at the time, we would've laid down Earth's entire estimated coal reserves within a millennium -- not the millions of years scientists assumed it took for lignin-eating microbes to evolve.
If coal formation wasn't linked to this particular plant structure, why was most of our coal laid down at the same time (geologically speaking)? According to Boyce and his co-authors, the answer lies in the unique climate conditions and geological processes that existed at the time.
In the United States, we call our chunk of the Central Pangean Mountains the Appalachian Mountains.
During the Carboniferous, the supercontinent Pangea was coming together. Shallow wetlands were common during this area, and the geological fault lines between the various continents created both mountain ranges and deep basins as the continental plates ground against each other. Remnants of the Central Pangean Mountains, shown above, still exist in the Appalachian Mountains in the United States, the Little Atlas of Morocco, and the Scottish Highlands.
"If you want to generate coal, you need a productive environment where you're making lots of plant matter and you also need some way to prevent that plant matter from decaying," Boyce said. "That happens in wet environments."
Because these basins and mountain ranges were created over millions of years, there was more than enough time for plants to fill the area, die, and then be replaced. This created huge deposits of organic material that was eventually transformed by heat and pressure into coal. The authors note that the Appalachian mountains (known to be rich in coal) may have been created by precisely this process, as tropical wetlands sank into the buckled bedrock.
This theory could explain why the Rockies are another coal-rich area. The coal beds in the western United States were laid down at a time when the climate in that region was wet and tropical. The same plate tectonics that created the Rockies also formed neighboring basins, possibly trapping organic material in the same fashion.
