Staff Writer| The Columbus Dispatch
Coal. It powered the industrial revolution and fueled the early development of the science of geology.
When the geologic time periods were being named, mainly in Great Britain, the time during which coal was being deposited was christened the Carboniferous Period. Modern dating based on radioactive elements shows the Carboniferous lasted from 359 million to 299 million years ago.
Geologic exploration showed that the Carboniferous was a great period of coal production not only in Great Britain but also in many other parts of the world. Here in the United States, the rock record is so extensive that the period is divided into two smaller periods: the earlier Mississippian Period (named for rocks in the upper Mississippi River Valley) 359 million to 323 million years ago, and the later Pennsylvanian Period (named for rocks in that state) 323 million to 299 million years ago.
Coal forms when woody plants are buried and compacted. When you burn coal, you are burning prehistoric firewood. Burning is rapid oxidation, so to produce coal, wood must be buried before it oxidizes. That is why burial in swamps is usually required — swamp water often is devoid of oxygen.
Coal was formed at other times in Earth's history, but nowhere near on such a scale as during the Carboniferous. One explanation for that is that the fungus that can break down lignin, a major structural component of wood, had not appeared by the Carboniferous. It is an interesting idea, but a recent article in the journal Proceedings of the National Academy of Sciences rejects that idea for two main reasons.
First, not all wood is the same. Most of the trees that eventually produced coal in the Carboniferous belonged to a group called lycopsids, which were related to small living plants called ground pines. Their trunks were composed largely of material similar to the bark of modern trees — material that does not contain lignin. Strike one against the delayed fungus theory.
Second, evidence of fungal decay, if not fossils of fungus itself, has been found in fossil wood from the Devonian Period that is much older than the Carboniferous.
So why was so much coal formed during the Carboniferous? The authors state that it was “a unique confluence of climate and tectonics.”
During the Carboniferous, the supercontinent of Pangea was in the final stages of its assembly, as separate pieces of Rodinia, the previous supercontinent, came together again.
Those continental collisions wrinkled the crust, forming the basins, or downwarps, in which large amounts of organic matter could accumulate in water-logged conditions. Many of those basins, including those in North America and Great Britain, were located near the equator, where tropical conditions were favorable for the luxuriant growth of plants.
Furthermore, the repeated growth and melting of glaciers in Africa (which at that time was at the South Pole) and the other southern continents caused sea levels to drastically fluctuate. Each fall of sea level exposed huge amounts of land that plants rapidly colonized. In turn, each rise caused the burial of the accumulated plant debris.
So, to get the electricity that powers the lights that help you read these words took continents coming together, glaciers and wood — lots of wood.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.
gnidovec.1@osu.edu
