We used coal long before the Industrial Revolution. When Marco Polo travelled along the Silk Road to China in the late thirteenth century, he described how the Chinese had the odd practice of burning pieces of black stone for fuel.7 And even in Britain, by the end of the second century AD, the Romans were mining many of the main coalfields in England and Wales for use in metalworking or their underfloor heating systems.8
It was textile manufacturing that set in motion the process we call the Industrial Revolution. A series of inventions transformed this cottage industry in the second half of the eighteenth century, with machines now able to spin cotton and wool fibres into thread, and then weave these threads into fabrics. The availability of cheap cotton from Britain’s colonies in America and India – we’ve explored these international trade networks in the last chapter – supplied this growing demand as the mills rapidly increased their capacity, and at first waterwheels provided the motive power. But the force that really drove the progression of the Industrial Revolution was the virtuous circle that existed between coal, iron production and the steam engine.
The Industrial Revolution started gathering momentum with the introduction of coke to fuel blast furnaces. Coal dug from the ground is not pure carbon fuel, but contains impurities like volatile organic compounds, sulphur and moisture. Coking is the process by which coal is first heated without allowing it to ignite and burn – in much the same way as charcoal is produced from wood – to drive out these impurities to create a hotter-burning fuel, and in particular to remove the sulphur which can taint the iron and make it brittle. Coke-fired blast furnaces made the production of iron much cheaper, providing the material for construction projects and increasingly sophisticated machine tools.
The exploitation of huge underground coal reservoirs, and the coke produced from it, released early industrialising Britain from the limitations of coppiced forests and provided an enormous supply of energy for creating the products needed by society. But it was the steam engine that marked a truly monumental advance, providing force and movement without the need for animal muscles. Fundamentally, the steam engine is a converter, able to turn thermal energy into kinetic energy: it transforms heat into motion. The first steam engines were employed at coal mines to pump out groundwater so that ever deeper seams could be dug. Given that they were sited at collieries, it didn’t matter that the earliest, primitive designs were enormously hungry for fuel. But a string of innovations and improvements made steam engines increasingly energy-efficient and powerful.
The steam engine became a general-purpose power plant. It served as the ‘prime mover’ in factories, where a single engine could drive a whole workshop floor of machine tools via a system of overhead belts and chains. More compact and fuel-efficient, high-pressure steam engines were developed for transport, their considerable weight spread out across the surface by the laying of railway tracks; or they were mounted in a ship, supported by the buoyancy of the hull. Steam soon came to haul freight and passengers around the world. By 1900, steam engines provided about two-thirds of the power needed in Britain, carried 90 per cent of all land transport along railways and bore 80 per cent of cargo across the seas.9
This was the essence of the three-way process that drove the accelerating pace of industrialisation. Steam allowed us to mine ever greater amounts of coal, coal-fired smelters and foundries produced more and more iron, and both coal and iron were used to construct and run more steam engines to mine coal, produce iron, and build yet more machinery at ever increasing rates. In this way, coal, iron and the steam engine formed a virtuous triangle.
The reason why this industrial transition is so important in our history is that it released us from the previous energy limitations on human civilisation. Coal provided prodigious amounts of thermal energy without the need for coppicing, and the steam engine removed the reliance on animal and human muscles. Without huge reserves of buried fuel it is unlikely that civilisation would ever have progressed beyond an essentially agrarian state. So how has the Earth provided this ready-to-go energy resource waiting for us?
FOSSILISED SUNSHINE
You will no doubt know that coal was created by the burial of ancient trees. And as we have seen repeatedly throughout this book, once again there was something quirky about the geological period which saw the most productive and widespread era of coal formation. These prevailing conditions had profound ramifications for life on Earth.
Although plants first colonised the land about 470 million years ago, evolving from branching green algae growing in lakes,10 it took a long time for plant cover to build up sufficiently to produce the earliest, and still very minimal, coal deposits. Within the almost 400 million years when substantial forested areas covered our planet, by far the most massive and widespread coal deposits were created in the Carboniferous, a 60-million-year period ending about 300 million years ago. Indeed, it is coal formation that gave this geological age its name. There have been other, later periods of coal formation in our planet’s history, but the Carboniferous dominates by the sheer quantity of coal deposited and its widespread extent. Around 90 per cent of the coal we’ve used since the Industrial Revolution dates to this short period of geological history.
Normally, when living organisms die, whether that’s an oak or an owl, they are decomposed to release the carbon in the organic molecules of their bodies back into carbon dioxide in the air, which is then captured again by photosynthesising plants. For such vast amounts of carbon to be turned into coal during the Carboniferous, something has to block that decomposition process, and it seems that in that period for some reason the Earth’s carbon recycling scheme broke down. Trees died but didn’t rot. Fallen vegetation accumulated on the ground, becoming peat, which was then buried deeper and deeper underground to be cooked into coal in the internal heat of the planet.
The key prerequisite for peat to accumulate is simply that the growth of vegetation has to be faster than the rate at which the dead material can be removed by decomposition or, on a longer timescale, the deposits can physically erode away. And it was lush, vigorously growing forests in a lowlying, subsiding swamp environment, where dead trees became buried without oxygen before they could decay completely, that seems to have tipped the balance.
In the Carboniferous Period, our world looked very different. This far back in time, the layout of the continents, constantly scuttling around the face of the Earth under the influence of plate tectonics, was in a completely different configuration. Throughout this period, the major landmasses were crunching together to be welded into a single whole, assembling the supercontinent Pangea.
Great, lowlying basins in what is now eastern North America and Western and Central Europe sat across the equator, forming tropical swamps where dense forests flourished. The trees that filled these swamp forests still reproduced with spores – as described in Chapter 3 – and would have looked unsettlingly alien to us. They were ancient relatives of the horsetails, clubmosses, quillworts and ferns that in today’s forests humbly occupy the shady understorey. Much of the coal that eventually formed was produced by lycopsids,11 trees related to today’s clubmosses. Their metre-thick trunks were very straight with few side-branches, curiously green-coloured, and textured with a regular dimple pattern where old leaves had fallen off – fossils of these trees look almost like tyre tracks. Growing over 30 metres high, they supported a compact crown of long, blade-like leaves.
These lush wetland ecosystems also teemed with grotesque animal life. The Carboniferous undergrowth trembled with giant cockroaches remarkably similar in appearance to those of today, spiders the size of horseshoe crabs (although not yet spinning webs), and 5-foot millipedes. Newt-like amphibians as large as horses lumbered through these swamps with a wide-limbed, sprawling gait.12 And giant predatory dragonflies, with wingspans up to 75 centimetres,13 soared through the hot and humid air. But if you could travel back in time to take a stroll through these lush forests you’d be struck by the notable absence of certain
sounds which would feel eerie once you noticed it: the complete lack of birdsong. In these ancient skies only insects had taken to the air – birds would not appear for another 200 million years. Many other creatures you might expect in this sort of environment were also yet to evolve – no mosquitoes whined among the tepid pools, but there were also no ants, beetles, flies or bumblebees.14
While the Carboniferous provided the ideal environmental conditions for lush tree growth, later eras were also warm and muggy, so these alone cannot account wholly for the huge coal deposits from this period. It is less their vigorous growth than the fact that the dead trees didn’t rot away and accumulated in thick layers of peat that requires explanation. The languid swamps around the Carboniferous equator would certainly have helped, the oxygen-poor soil in these fetid marshes slowing the activity of decomposition microbes. But swamps have existed throughout planetary history: they’re not a uniquely Carboniferous feature.
So what might have been special about the world around 325 million years ago? Why did fallen tree trunks seem so reluctant to rot away? Why did the carbon recycling fail so spectacularly during the Carboniferous, leading to the creation of much of the coal that was to fire the Industrial Revolution?
One explanation, which has become popular over recent years, is that in the Carboniferous fungus, which plays a central role in the decomposition process, simply wasn’t biochemically equipped to break down the fallen trees.
To grow ever taller, the early trees needed to develop greater internal strength to support themselves. All plants contain cellulose, a molecule made up of long chains of sugar units that strengthens their cell walls – a linen jacket, a cotton shirt and the paper page you’re reading at the moment (unless you’re swiping through on an e-book device, in which case think of the cardboard box it came in) are all composed of cellulose. But what really gave these towering trunks their strength was the biological invention of a second molecule: lignin. It explains why the small moss-like plants of the early Devonian became the towering trees of the Carboniferous. And importantly, lignin is much harder to break down than cellulose.15
If you walk through a forest today, your nose filling with the heady aromas of the humus-rich soil and the foliage, you’ll notice that the dead wood of a log to the side of the path has become pale-coloured, and soft and spongy in texture. This is caused by white rot fungi, which decompose the dark-coloured lignin in wood. (Particularly tasty varieties include the oyster mushroom and shiitake.) But during the Carboniferous Period, so this theory goes, trees had newly formulated lignin to reinforce their wood, but fungi had not yet had time to develop the necessary enzyme toolkit to break it down. Much of the solid bulk of trees had become indigestible and for millions of years when they fell they simply piled up on the ground.
But while this is a satisfying hypothesis, it unfortunately doesn’t stand up to more recent evidence. For one, the most common kind of coal-forming trees in the Carboniferous swamps didn’t actually contain much lignin. And although North America and Europe didn’t form much coal in the geological period immediately following the Carboniferous – the Permian – some areas in China did, and this was after the supposed emergence of lignin-decomposing fungi.16 So if it wasn’t an evolutionary time lag between forests reinforcing themselves with lignin and fungi developing the capability to digest it, what was it about the Carboniferous that made it so very prolific at turning trees into coal?
It appears that the reason for the vast Carboniferous coal deposits is not primarily biological but geological.
While the tropical region around the equator remained warm, the late Carboniferous Period was actually a pretty cold time in the Earth’s history, with large ice sheets forming in the south of Gondwana – so despite its popular conception, the Carboniferous world was not all steamy jungles. These glacial conditions were caused by the configuration of the continents at the time. The congregating land masses stretched from the South Pole, across the equator and pretty much all the way to the North Pole. This blocked the circulation of warm tropical and cold polar ocean waters around the world – the conveyor belt we encountered in Chapter 8 – obstructing the transfer of heat from the equator to the poles. The fact that Gondwana sat across the South Pole also supported the accumulation of thick glacial ice in the region: as we saw, ice caps cannot grow as extensively over open ocean.
The vigorously growing Carboniferous forests were also partly responsible for triggering these glacial conditions.17 The trees had been sucking carbon dioxide out of the air for their photosynthesis, and when they died and much of their organic material became locked up as peat rather than rotting away, they didn’t release that carbon back into the air. As a result, the amount of carbon dioxide in the atmosphere dropped considerably, and the low levels of this greenhouse gas would also have contributed to global cooling. And as the decomposition of dead organisms consumes oxygen from the air to make that carbon dioxide, an increase in peat production also led to an increase in atmospheric oxygen levels, perhaps to as much as 35 per cent (today the concentration of this life-sustaining gas in our atmosphere is 20 per cent).18 These high oxygen levels are believed to have contributed to the evolution of the giant insects, such as the large-winged dragonflies, we encountered earlier.
From the mid Carboniferous, therefore, Earth was becoming an icehouse. Fluctuations in the global temperature and thus the amount of water locked up in the ice caps (governed by wobbles in the Earth’s orbit, as we saw in Chapter 2) caused cycles of rising and falling sea levels, just as during the ice ages of the past 2.5 million years. As the Carboniferous sea rose and fell it repeatedly advanced and then retreated over vast stretches of the lowlying swamps. In the process, vegetative matter got buried regularly under layers of marine sediment, to one day become coal seams. Indeed, if you look at the exposed layers of rocks in a coal measure you’ll see a vertical sequence of coal seam, marine sediments like mudstones, lagoon sediments such as shale, and then sandstones from a river delta on which a new soil layer has formed, followed by another coal seam. These stacked layers within the coal measure can be read as a geological manuscript telling the story of the repeated flooding of the swamp basins.
In places like South Wales or the Midlands of England, where coal is found next to ironstone, both fuel and ore for smelting can be sourced in the same place – it’s like the Earth is offering us a 2-for-1 discount. Sometimes we even get a 3-for-1 deal. Lying just beneath the coal measures and so often found exposed on the surface in the surrounding landscape, are limestones formed during the early Carboniferous when global ocean levels were high and flooded lowlying land in shallow, warm seas. As we saw in Chapter 6, limestone is used as a flux in iron smelting to help the metal melt and remove impurities. Furthermore, the ‘seatearth’ layers just beneath each coal seam, which often preserve the fossilised roots of the swamp trees, are frequently rich in hydrous aluminium silicates. Such minerals make this clay layer exceedingly refractory and able to withstand high temperatures of 1,500 °C or more, so Nature also provides us with the perfect fireclay building material for lining the furnaces or crucibles for pouring molten metal.19 Thus the changing conditions through the Carboniferous sometimes provided in the same area in successive strata the raw materials for the Industrial Revolution.
It was this periodic flooding and burial of the lowlying swamps that preserved the peat and compressed it under successive layers of sediment to form coal. And the swings between glacial low and interglacial high sea levels, reflecting the icehouse conditions at the time, were a direct consequence of plate tectonics and the configuration of the word’s continents. But there was a second unusual feature of our planet during the Carboniferous which contributed to coal formation: the landmasses weren’t just arranged in a clump between the North and South poles but were still actively crashing together.
Major coal-forming basins during the construction of the supercontinent Pangea.
The Carboniferous Period saw the ongoing construct
ion of the supercontinent Pangea, as the great northern continent, Laurasia (containing North America and parts of northern and western Eurasia), collided with Gondwana (South America, Africa, India, Antarctica and Australia) along the equator. This slow crunching event is known as the Variscan Orogenyfn3 and it created a thick belt of mountain ranges, including the Appalachians along what is now the Eastern Seaboard of the US and Canada, the Lesser Altas Mountains in Morocco – which would have continued from the Appalachians before this huge mountain range was separated by the opening of the Atlantic Ocean – and many other ranges across Europe such as the Pyrenees between modern-day France and Spain.fn4 Then, in the Late Carboniferous, Siberia slid from the north-east into this continental pile-up, welding onto Eastern Europe and producing the Ural Mountains at the join.
As we have seen, the collision of continents not only creases up chains of high mountains, but creates lowlying, subsiding basins alongside as the crust flexes downwards. A good example is the Ganges Basin sitting along the foot of the Himalayas, formed by the Indian-Eurasian plate collision, which holds the Indus and Ganges rivers as they flow down from the mountains to the sea.
Such down-warping foreland basins were also created by the tectonic clashes of the Carboniferous and provided the setting for those huge areas of lowlying swamps that were prone to periodic flooding and so smothered and preserved the peat. But for coal deposits to build and not be eroded away through exposure as the cycle of deposition continues, you need a basin that is also continually subsiding. And this is what’s so important about the ongoing formation of Pangea during the Carboniferous: continental collisions kept the basins warping down at roughly the same rate that the coal built up, allowing enormously thick successions of coal seams to accumulate.
It is this chance coincidence of several factors all acting at the same time and place that made the Carboniferous such a unique period in Earth’s history for creating the massive deposits of coal that we came to rely upon. The Pangea supercontinent was still being actively constructed with clashing frontiers that just so happened to be situated around the tropics, creating foreland basins for lowlying wetlands in a warm, humid climate perfect for tree growth. These swamps were repeatedly flooded with sudden sea-level rises during a rare period of oscillating glacial and interglacial ages, burying and preserving the peat; and they were continually subsiding so the strata weren’t simply eroded away again. The process of plate tectonics was the ultimate force behind all this. There were to be later periods of coal formation around the world, but none would be as productive as during the Carboniferous assembly of Pangea.20
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