IELTS Reading 146: The Forests That Became Coal

Adapted from Bynumpedia / Nelsen et al. 2016 palaeobotany review. IELTS Reading Section 3 difficulty, approximately 1,050 words. Source: https://bynumpedia.com/paleontology/carboniferous-coal-forests

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Reading Passage

A. Every lump of coal is, in a sense, a fossilised forest — and the coal that powered the Industrial Revolution, and still supplies a large share of the world's energy, grew mostly in one extraordinary chapter of Earth's history. The Carboniferous period, lasting from roughly 359 to 299 million years ago, saw the most extensive tropical wetlands the planet has ever known. These were not the familiar rainforests of the modern Amazon, draped in flowering trees. Their canopy was formed by plants with no living equivalent: giant relatives of the tiny club-mosses found on today's forest floors, alongside tree-sized horsetails and towering seed ferns. Buried and compressed over hundreds of millions of years, the dead vegetation of these swamps became the thick seams that miners still quarry today. Understanding why so much wood accumulated in one narrow window of geological time has occupied scientists for more than a century, and the accepted explanation has itself changed in recent years. The story matters beyond geology, because the carbon locked away in those seams, when burned today, returns to the atmosphere that the plants once worked so hard to empty.

B. The trees that built the seams were as strange as their times. The dominant canopy plants were arborescent lycopsids, giants such as Lepidodendron and Sigillaria that climbed to thirty or forty metres yet bore little real wood. A modern tree supports itself with dense timber added year by year; these plants instead leaned on a massively thickened outer bark that could make up as much as ninety percent of the trunk's cross-section, leaving only a slender core of pith and a narrow ring of wood. The strange architecture made them remarkably lightweight for their height. Nor did they live for centuries like modern giants. Their growth was a single, rapid, preordained trajectory: a sapling shot up like a pole, branched once into maturity, reproduced by shedding spores from cones, and died within one or two decades. When such a tree finally toppled, it fell not onto dry soil but into swamp water. For a modern forester, a forty-metre tree that lives barely twenty years sounds improbable; to the swamp, such short, quick generations were ideal, because they supplied a steady rain of fallen trunks.

C. That waterlogged fate is the first half of the coal story. The swamp floor was perpetually saturated, acidic and starved of oxygen — conditions in which the microbes that ordinarily rot fallen trees cannot easily operate. As branches and trunks piled up faster than they could decompose, they turned into peat, a spongy, carbon-rich mat preserved rather than recycled. For peat to thicken into a future seam, however, two further conditions had to hold together. The basin floor had to sink slowly and continuously, making room for ever more accumulation; and the buried layer eventually had to be sealed beneath sediment. Heat and pressure over millions of years then squeezed out water and gas, raising the proportion of carbon and turning peat, first into soft brown coal and ultimately into hard bituminous coal or even glossy anthracite. Geologists estimate that roughly ninety percent of the world's economically important coal formed within a single thirty-million-year stretch, between about 323 and 290 million years ago. No earlier episode locked away carbon on anything like that scale, and none since has come close.

D. For decades, a neat theory explained that extraordinary burst. It was said that woody plants had recently evolved lignin — the rigid polymer that makes timber tough — before the fungi capable of digesting it had caught up. During this "lignin gap", so the story went, dead trees simply could not be broken down, so they piled up unrotted for tens of millions of years. The tale is attractive because it names a single missing consumer. Yet a comprehensive reassessment published in 2016, drawing together evolutionary, chemical and geological evidence, rejected it. The wood-rotting fungi, the researchers showed, were probably already present before and during the Carboniferous. More importantly, the bark that made up most of the lycopsid trunk was built not from lignin but from a different, waxy material that resists decay regardless of whether fungi are there to eat it. The collapse in coal formation later, they added, tracked changes in climate and tectonics rather than the moment fungi finally learned to digest wood. The appeal of the old tale lay in its neatness; the new one, messier but better supported, insists that no single missing consumer can explain a continent of buried forest.

E. The better explanation, on this revised view, is a rare meeting of geography, climate and biology. The slow assembly of a supercontinent had left broad, low-lying equatorial basins that sank at just the right rate to accept peat. To the south, a vast ice sheet waxed and waned, tugging global sea level up and down by tens of metres; when the sea retreated, swamps spread and peat grew, and when it flooded back, marine mud sealed the accumulating layer for the next cycle. The strange lycopsids were simply well suited to it. The consequences reached into the atmosphere itself. Locking away so much carbon drew down carbon dioxide and pushed oxygen to about thirty-five percent of the air, far above today's twenty-one — a world so rich in oxygen that even damp trees burned readily, and charcoal is found throughout the seams. When the climate dried out around 305 million years ago, the great swamps fragmented and most of their species vanished, and no later period has ever matched the coal forests' remarkable burial of carbon. In that sense, the seams burned in factories today are a kind of debt that a vanished landscape once agreed, for reasons of climate and chance, to accumulate on our behalf.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. The strange architecture of the coal-swamp trees ii. The old explanation — and why it was overturned iii. How a fallen tree becomes a seam iv. Why dinosaurs dominated the Carboniferous v. The combined geography, climate and biology behind the record vi. The invention of modern coal mining vii. Why flowering trees disappeared

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. What made lycopsid trunks unusual? A. They were made almost entirely of dense annual wood. B. They relied mainly on a thick outer bark rather than wood. C. They took centuries to reach full height. D. They reproduced by seeds like modern pines.

6. Why did fallen trees accumulate rather than rot? A. The swamp water was acidic and low in oxygen. B. The trees were too heavy to fall. C. The climate was too cold for fungi. D. Insects ate all the decomposers.

7. What did the 2016 reassessment conclude? A. Lignin-rotting fungi evolved long after the Carboniferous. B. The "lignin gap" explanation was rejected on combined evidence. C. Coal formed mainly from flowering trees. D. Sea level had no effect on coal.

8. Why is charcoal commonly found in the seams? A. Miners set fires underground. B. Oxygen was so high that even damp vegetation burned. C. The peat was heated by volcanoes. D. Charcoal is a modern contaminant.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. The Carboniferous period lasted from about 359 to 299 million years ago.
10. Lycopsids typically lived for several centuries like modern giant trees.
11. The bark of lycopsids was composed mainly of lignin.
12. Atmospheric oxygen peaked at roughly thirty-five percent during the late Carboniferous.
13. The largest known Carboniferous insect was discovered in Australia.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

In the swamp, fallen trees could not decay because the water was acidic and (14) _____________, so they accumulated as peat. Over millions of years, heat and pressure raised the proportion of (15) _____________ as peat turned into coal.

Answers & Explanations

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