Most coal on Earth is the same age. Not the same kind. The same age. The seams under Pennsylvania, under Yorkshire, under the Ruhr, all came out of one window of time. And then the world almost stopped making it.
Not every lump. There is coal under Wyoming that is only about sixty million years old. But roughly half the coal on Earth comes out of one stretch of about sixty million years, three hundred million years ago. We named the whole period after it: Carboniferous. It means coal-bearing.
So what is a lump of coal? It is a forest. The actual plants, pressed until they stopped being plants. The tree that made most of it stood as tall as a nine-story building. It was a giant relative of a club moss, and it had very little wood in it. When one fell, it went under water. Waterlogged ground has no oxygen in it, and without oxygen nothing can finish rotting. So the forest floor piles up as peat, and peat squeezes down to roughly a tenth of its thickness. A seam you can stand up in is tens of thousands of years of forest, pressed to your own height.
You may have heard that trees had just invented a stiffening called lignin, that no fungus could break it down, and so the dead wood simply stacked up. It is a wonderful story. The coal itself disagrees. A bog is forming right now, in a world full of fungi that can rot wood, and the dead plants still pile up, meters deep. They pile up because they are under water. And most of a seam from those forests is bark, a tissue that had no lignin in it at all. It was never that nothing could rot the forest. It is that nothing could reach it.
So why then, and only then? Two continents were closing. As the mountains went up, the crust beside them bent down, and kept bending. Peat only becomes a seam where the ground sinks at about the same rate the peat grows. Too slow and it rots. Too fast and it drowns. And the hollow sat on the equator, in a belt that was wet all year. Far away, ice came and went, and the sea came in and out with it. Every time, the forest drowned and then came back. That is why coal comes in stripes.
Three things had to line up: a wet equator, ground sinking as fast as the peat rose, and a whole continent's worth of room to do it in. In four hundred million years, that has happened exactly once. Burying that much carbon left more oxygen in the air than at any other time we know of, more than a quarter of every breath. On the Clock, making the coal took two days. We are burning it in under two seconds.
