Life › Seas, forests & the move to land
Carboniferous coal forests
c. 359–299 million years ago· Event· Plants & fungi
Well establishedStrong fossil, genetic or geological evidence.
Vast tropical swamp forests whose buried remains became much of the world's coal, from about 359 to 299 million years ago.
See it on the Life map →What happened
Carboniferous means 'coal-bearing'. Across what is now eastern North America and Europe, then near the equator, wetlands held scale trees (lycopsids) 30 m or more tall, giant horsetails, tree ferns and seed ferns. Fallen trunks piled up in waterlogged ground as peat, which burial and heat later turned into coal. Burying so much carbon raised atmospheric oxygen, perhaps to about 30% (estimates vary), and drew down carbon dioxide, helping bring on a long ice age.
Why it matters
These forests made the coal that fueled the Industrial Revolution, and their oxygen-rich air probably allowed giant insects. Burning that coal now returns ancient carbon to the air.
The evidence (fossils, genes, rocks)
Coal seams with fossil bark, leaves and roots; petrified peat ('coal balls') preserving plant cells; standing fossil stumps at Joggins, Nova Scotia; and chemistry-based oxygen models.
Still debated
Why so much coal formed then: fast-sinking wet tropical basins, or a lag before fungi could rot tough wood? See the wood-rot debate.
Inside this (1)
Connections (4)
▸First trees & forests led to Carboniferous coal forestsWell established
The connection
Devonian forests established trees, wood and deep roots. In the Carboniferous, tree groups such as giant scale trees and horsetails spread across vast tropical wetlands. Their buried trunks and peat became coal on an unprecedented scale.
When and how
From Devonian forests about 385 million years ago to Carboniferous coal swamps in what is now eastern North America and Europe, about 359–299 million years ago.
The evidence
A continuous sequence of fossil plants through dated rocks, with tree types first seen in Devonian forests becoming dominant in coal seams.
Debates and open questions
Why the Carboniferous produced far more coal than other forested periods: geography, climate, or a delay in wood-rotting fungi.
▸Carboniferous coal forests were home to Giant insects (Meganeura)Well established
The connection
The coal forests gave giant insects food, perches and warm, humid air. Vast plant growth and buried carbon are thought to have raised oxygen to perhaps 30% or more of the air, which may have let insects, which breathe through tiny tubes, grow far larger than any alive today.
When and how
Late Carboniferous swamp forests, about 305–299 million years ago; Meganeura comes from coal-measure rocks at Commentry in central France.
The evidence
Giant griffinfly wings occur in coal-forest deposits. The oxygen link rests on geochemical models and insect-rearing experiments: well supported, but not proven.
Debates and open questions
Whether high oxygen or the absence of flying predators mattered more is debated, and past oxygen estimates carry wide uncertainty.
▸Why so much coal? The wood-rot debate tries to explain the coal of Carboniferous coal forestsDebated
The connection
The Carboniferous laid down some of the world's richest coal beds, and the debate asks why. One side says dead wood piled up because fungi could not yet break down lignin. The other says the main coal trees were low in lignin anyway, and coal formed because wet tropical basins kept sinking and burying plant debris as Pangaea assembled.
When and how
The coal formed about 359–299 million years ago in Europe and North America; the fungal explanation was tested in Science (2012) and challenged in PNAS (2016).
How sure are we?
Coal balls, mineral-filled lumps of peat, show what the swamp plants were made of; basin geology shows where and why coal accumulated.
Debates and open questions
How much of the coal peak was biology and how much was geography and climate remains open.
▸Carboniferous coal forests made the coal that fueled Industrial Revolution & fossil fuelsWell established
The connection
Carboniferous swamp forests buried vast amounts of plant matter, which heat and pressure turned into coal seams. Britain sat on rich, easily mined seams from this age. Burning that coal ran the steam engines, iron furnaces and railways of the Industrial Revolution, releasing carbon stored roughly 300 million years ago.
When and how
Coal formed about 359–299 million years ago in what are now Britain, Europe and eastern North America; British mining, already centuries old, boomed from the 1700s.
The evidence
Fossil plants such as scale-tree bark and roots occur in and around coal seams; geologists date these Coal Measures to the Carboniferous by fossils and radiometric ages.
Debates and open questions
Why so much coal formed then: an older idea says fungi could not yet rot wood; newer work says wet tropical basins and shifting plates explain most of it.