Coal and fossil forests

I spent my lunch hour at an OLLI lunchtime lecture, learning about why we have coal in Illinois, and why sometimes coal formations have fossilized forests on top. The talk by Scott Elrick (of the Illinois State Geological Survey) was absolutely fascinating.

The first part of the talk looked at the history of continental drift, looking at where the land mass that eventually became North America (and the piece of it that became Illinois) was over the last few hundred million years. During the Pennsylvanian period, Illinois was roughly on the equator, which turns out to be important.

To get coal, you need to have lots of plant matter, but very little sediment. If you don’t have the plant material, you’ve got nothing to turn into coal. But even if you have the plant material, if you have any significant amount of sediment—inorganic material washed in by water and deposited on the ground—you don’t end up with coal, you just end up with shale.

There’s an area along the equator called the “convergence zone” where the weather of the northern hemisphere meets the weather of the southern hemisphere. Most of the time, this zone shifts north and south over the course of a year, meaning the tropics experience wet seasons and dry seasons. However, during the period in question there was extensive glaciation, meaning lower sea levels, which turns out to mean much less shifting of the convergence zone. Which means that, for a geologically long period of time, it rained a lot, all year.

That’s the circumstance that lets you get coal. To be more specific, that’s the circumstance that gets you peat.

Lots of plant matter, but very little sediment (because those plants had lots of roots to stabilize the ground, and they never had to die back, because there were no seasons). The plants grow, the plants die, the dead plants end up on the wet ground, they get covered with water, which limits the oxygen that gets to the plant, meaning that more plants can grow on top of them before they decay. Result: peat.

To get coal takes one more thing: Your peat has to get buried. If it gets buried well enough that air never gets in there, and if it ends up buried deep enough that there’s some serious pressure and heat, and stays there for long enough, all the volatile (i.e. non-carbon) elements in the peat get cooked off. Result: coal.

So, in the Pennsylvanian, we had this long period of nothing but rainy season, allowing layers of peat to build up. But eventually the glacial period ended.

It turns out that glacial periods can end really fast. They start slow, with ice building up gradually over decades and centuries. But they can end very quickly, with centuries of ice melting in a matter of years.

The ice melts, the sea levels rise, and the convergence zone starts showing seasonality, moving north and south over the course of the year. Forests full of plants that expected rain every day suddenly had to adapt to tolerate dry seasons.

This produced a lot of changes, of course. The plant species show dramatic shifts. Crucially, they die back during the dry season—meaning that you start to see a lot more sediment.

In the fossil record, you see this as a thick vein of coal with a thick vein of shale on top.

And right here in east-central Illinois, something very interesting happened. Along a fault line, a series of earthquakes caused the ground on one side to sink. In that sunken area the sediment built up even more quickly—quickly enough to cover whole plants. Fallen trees were covered up faster than they could rot away. Branches with leaves were covered before the leaves could fall off.

The result is a thick vein of coal, with a fossil forest on top of it.

Is that cool or what?

This particular forest, near Danville, Illinois, was the first one discovered that was big enough that paleobotanists could study the forest at the level of the forest community. As opposed to just seeing what plants grew near a few other plants, they could see how the plants that grew near one another changed as you moved from one part of the forest to another.

Painting by J. Vriesen and K. Johnson via nature.com

Scott Elrick showed us all kinds of cool stuff. One thing was this artist’s rendition of the forest, showing large, tall trees growing very close to one another, something that would be rare in forest today. Turns out that these trees—Lycopods—had photosynthetic bark, and didn’t grow leaves until they reached their full height. So they didn’t shade out their neighbors the way modern trees do. They also had very long roots that extended many meters from the trunk, but the root systems were quite shallow, going just a few meters down.

He also had pictures taken from within the coal mine, showing the fossils of these trees—trunk and roots—growing right up out of the coal seam: Trees that had been alive when the weather changed and that ended up with a meter or two of sediment covering the bottom of the trunk fast enough that the tree never fell down. It just fossilized in place.

It was a great talk at which I learned all sorts of things about geology and paleobotany. I’m going to have to follow this guy’s work in the future.