Carboniferous Coal Forests: How the World's Coal Was Formed

The Carboniferous coal forests were vast tropical swamp forests that covered lowlands across what are now Europe and eastern North America between about 359 and 299 million years ago. Dominated by giant relatives of clubmosses, horsetails and ferns, they built up deep layers of peat that were buried and turned into coal. That coal later powered the Industrial Revolution, making these ancient forests one of the most consequential ecosystems in human history.

Background: the Carboniferous world

The Carboniferous Period followed the Devonian and lasted from about 358.9 to 298.9 million years ago. The name, meaning “coal-bearing”, was introduced in 1822 by the English geologists William Conybeare and William Phillips for the coal-rich rock sequences of Britain. In North America the period is usually split into two: the Mississippian (earlier) and the Pennsylvanian (later).

During this time the continents were moving together to form the supercontinent Pangaea. Much of Europe and eastern North America lay near the equator as part of a landmass called Euramerica. Collisions with the southern continents raised mountain chains and created broad, slowly sinking lowland basins ideal for swamps.

Far to the south, ice sheets grew over the polar parts of Gondwana. As they expanded and melted, sea levels rose and fell repeatedly. This left a characteristic repeating pattern of rocks in coal basins, with coal seams alternating with marine limestones and shales.

What the forests looked like

The coal forests would look strange to a modern visitor. Their tallest trees were lycopsids, the group that today includes only small clubmosses and quillworts. Species such as Lepidodendron and Sigillaria are estimated to have reached heights of roughly 30–40 metres. Their trunks were covered in a diamond-shaped pattern of leaf scars, and they were anchored by spreading root systems known from fossils called Stigmaria.

Alongside them grew Calamites, giant relatives of horsetails, tree ferns such as Psaronius, and seed ferns, an extinct group of seed-bearing plants with fern-like leaves. On drier ground grew Cordaites, early relatives of conifers. There were no flowering plants and no grass.

The forest floor was waterlogged. Insects, millipedes, scorpions and spiders lived among the plants, including giants such as the dragonfly-like Meganeura and the millipede Arthropleura. Amphibians were common, and the first reptiles appeared during this period.

How the coal formed

Coal begins as peat. In the swamp forests, fallen leaves, bark and trunks dropped into stagnant water with little oxygen. Bacteria and fungi could not break this material down completely, so it piled up layer upon layer.

When sea levels rose or the land sank, the peat was buried beneath sand and mud. Over millions of years, heat and pressure drove out water and gases and concentrated the carbon. Peat became lignite (brown coal), then bituminous coal and, under the strongest conditions, hard anthracite. Several metres of peat were needed to make one metre of coal, with commonly cited ratios of around 5 to 10 to 1.

Scientists have long debated why so much coal formed in this period. One idea, published in 2012, was that fungi able to digest lignin, the tough material in wood, had not yet evolved. A 2016 study argued against this, pointing out that the coal-forming plants contained relatively little lignin and that fungi capable of breaking it down probably already existed. Many researchers now stress the combination of a wet tropical climate and subsiding basins next to rising mountains.

Fossil forests and discoveries

Coal miners have found fossil plants for centuries, and these discoveries helped shape early geology. At the Joggins Fossil Cliffs in Nova Scotia, Canada, upright fossil trees are preserved in the cliffs. In 1852 the geologists Charles Lyell and John William Dawson found remains of small tetrapods inside hollow fossil stumps there, and Dawson later described one of the earliest known reptiles, Hylonomus. The cliffs became a UNESCO World Heritage Site in 2008.

In Glasgow, Scotland, the Fossil Grove in Victoria Park preserves the stumps of eleven lycopsid trees, uncovered in 1887. Fossil forests are also exposed in Spain, Germany, the Czech Republic and the United States, sometimes over large areas of mine roof.

Effects on the atmosphere

Burying so much plant material removed huge quantities of carbon from the air and oceans. Because that carbon was locked away rather than decaying, the oxygen released by photosynthesis was not used up again. Geochemical models suggest that atmospheric oxygen rose well above today’s level of about 21 per cent during the late Carboniferous and early Permian, with many estimates in the range of roughly 25–35 per cent.

At the same time, falling carbon dioxide levels probably helped to cool the planet and sustain the ice sheets in the southern hemisphere. The exact figures are uncertain and differ between models, but the link between the coal forests, high oxygen and a cool climate is widely accepted.

By the numbers

Rainforest collapse

The coal forests did not last throughout the period. Around 305 million years ago, during the later Pennsylvanian, the climate of Euramerica became drier and more seasonal. The great lycopsid forests broke up into isolated patches and many of their plants disappeared from the region, an episode known as the Carboniferous rainforest collapse.

A 2010 study by Sarda Sahney, Michael Benton and Howard Falcon-Lang argued that this fragmentation changed the course of animal evolution. Amphibians suffered, while early reptiles, better able to live in drier conditions, diversified. Coal continued to form in some regions, notably China, into the following Permian Period.

Coal and the human world

The coal measures laid down in the Carboniferous underlie many of the great industrial regions of Europe and North America, including South Wales, northern England and the Scottish Central Belt, the Ruhr in Germany, northern France and Belgium, Silesia, the Donets Basin and the Appalachians of Pennsylvania and West Virginia.

From the 18th century, coal from these seams fuelled steam engines, iron smelting and railways. Innovations such as Abraham Darby’s coke smelting depended on it. Burning this ancient carbon also returned to the atmosphere carbon dioxide that the forests had removed hundreds of millions of years earlier, linking the Carboniferous directly to modern climate change.

Timeline

• c. 358.9 million years ago: The Carboniferous Period begins after the Late Devonian extinctions.
• c. 323 million years ago: The Pennsylvanian subperiod begins; extensive coal swamps spread across Euramerica.
• c. 305 million years ago: Drying climate fragments the tropical coal forests in the Carboniferous rainforest collapse.
• c. 298.9 million years ago: The Carboniferous ends and the Permian Period begins.
• 1822: Conybeare and Phillips name the Carboniferous after Britain’s coal-bearing rocks.
• 1852: Lyell and Dawson find early tetrapod remains in fossil tree stumps at Joggins, Nova Scotia.
• 1887: The Fossil Grove of lycopsid stumps is uncovered in Glasgow.
• 2008: The Joggins Fossil Cliffs are inscribed as a UNESCO World Heritage Site.

Why it matters

The Carboniferous coal forests were among the most productive ecosystems in Earth’s history. They shaped the atmosphere, the evolution of land animals and, through coal, the economic history of the modern world. Their legacy is still felt today in former mining towns and in the carbon released by burning their remains.