Understanding Coal


From the current issue of Economist, a bit of readable geological science to help make sense of the splashier headlines:

The origin of coal

The rock that rocked the world

More than any other substance, coal created modern society. But what created coal?

FOR 60m years of Earth’s history, a period known to geologists as the Carboniferous, dead plants seemed unwilling to rot. When trees expired and fell to the ground, much of which was swampy in those days, instead of being consumed by agents of decay they remained more or less intact. In due course, more trees fell on them. And more, and yet more. The buried wood, pressed by layers of overburden and heated from below by the Earth’s interior, gradually lost its volatile components and was transformed into a substance closer and closer to pure carbon.

The result was the coal that fuelled the Industrial Revolution, providing power for factories and railways, gas for lighting, a reducing agent for turning ore into iron and steel, the raw ingredients for drugs, dyes and other chemicals, and the energy that has generated most of the world’s electricity. Yet the abundance of Carboniferous coal is a puzzle. Forests began in the Devonian, the period before the Carboniferous, and have existed ever since. Not all coal is Carboniferous but, as the chart shows, the spike in coal accumulation then was far higher than anything which happened subsequently. Indeed, the very name Carboniferous alludes to this fact.

So why, the curious ask, was it then in particular that so much coal was created? The swamps certainly helped. Lacking oxygen, they would have slowed the activities of wood-destroying micro-organisms. But swamps are not uniquely Carboniferous. To explain the special boost coal got in this period, it has been suggested that the micro-organisms around at the time were not up to the job of rotting wood. Changes in plant chemistry which let trees grow tall, this hypothesis goes, stymied these micro-organisms, making much plant material indestructible. It is an intriguing idea. But a paper just published in the Proceedings of the National Academy of Sciences, by Kevin Boyce of Stanford University and his colleagues, takes issue with it. Instead, Dr Boyce thinks abundant Carboniferous coal, swamps and all, is an accident caused by the movement of the continents.

Reach for the skies

The idea that Carboniferous micro-organisms could not properly digest wood depends on a hypothetical evolutionary time lag. The first vascular plants (those with internal channels to move water around) evolved in the Silurian, the period before the Devonian. Vascularisation meant a plant could suck water up its stem, and thus grow tall. This led to a race, conducted throughout the Devonian, to be tallest and thus able to capture light without being overshadowed. The consequence was trees—and therefore forests…

Read the whole article here.

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