Our current age of cheap, mass-produced steel began in the 1850s with the development of a simple way to remove carbon from pig iron. The Bessemer Process involves holding the molten pig iron in a tall cauldron and then blowing air up through the liquid metal. This burns off the carbon and removes other impurities, essentially creating a blank slate of pure iron, so that measured amounts of carbon can be mixed back in to produce whatever grade of steel you require. This innovation slashed the time to process 5 tonnes of steel from one day to about quarter of an hour,36 prompting an explosion in steel output and dramatically dropping its cost. Thus the late Industrial Revolution transformed society into a far more metallic world. Today, steel is ubiquitous in household utensils and appliances, tools, machinery, railway tracks, ships and cars. We’ve also come to use it as the structural skeleton of our buildings, to provide embedded rebars to reinforce concrete and the framework of skyscrapers.
So if the Iron Age revolutionised human settlements, agriculture and warfare, our modern world is built with its alloy, steel. But where did this iron come from?
THE IRON HEART OF STARS
Ultimately, all the iron on Earth – from that in crustal rocks to the red-coloured haemoglobin carrying oxygen around in your veins – comes from the nuclear fusion reactions in the cores of stars. The universe created by the Big Bang contained mainly the simplest element, hydrogen, with some helium and a tiny amount of lithium thrown in. All the other elements in our periodic table were made by nuclear fusion in stars – cooked within their cores as they burned, or created when massive stars exploded at the end of their lifetime.
Iron is the star-killer element. When enough helium ‘ash’ produced by hydrogen fusion has built up in the core of massive stars, this then reacts to create heavier elements like carbon, oxygen, sulphur, silicon, and finally nickel and iron. Iron is the stablest element, and no more energy can be released by fusing it. As the giant star can no longer produce enough energy to hold up its outer layers, it collapses in on its own core, before exploding in an extremely powerful event known as a supernova. This final burst of fusion creates many of the heavier elements of the periodic table, and scatters all these atoms out into the cosmos. Several other key elements are produced by the violent collision of neutron stars, such as gold in a wedding ring, rare earth metals in a smartphone, lead on a church roof and uranium in a nuclear power station.37 In this way, not only our planet but also the molecules of our bodies are made of stardust.38
The Earth formed out of a disc of dust and gas swirling around the proto-Sun around 4.5 billion years ago. Dust motes stuck together to build grains, which coalesced into larger and larger lumps of rock, and these accreted with gravity to form our planet. The heat of all these impacts melted the primordial Earth, and most of the dense iron sank to the very core, leaving a thick layer of silicate-rich mantle, which slowly cooled and solidified on top to form a thin crust. Many other metals dissolve readily in iron – they are known as siderophile (‘iron-loving’) – and so were also scrubbed out of the Earth’s mantle and dragged down to the core as the iron sank. Consequently, siderophile elements like gold, silver, nickel and tungsten, as well as the platinum group metals we’ll come to shortly, are depleted in the rocks of the Earth’s crust. The precious gold that we’ve coveted through history was delivered to the Earth’s surface by asteroid impacts after the planet had differentiated into its iron core and silicate mantle.39 fn3
The iron heart of our world also serves to generate the Earth’s magnetic field. Churning currents of molten iron in the outer core of our planet generate this field just like a dynamo. This has been hugely important since the eleventh century for the navigation compass used first by Chinese and then Islamic and European sailors (and for the migrating animals that were able to sense Earth’s magnetic field long before us). But even more fundamentally, this magnetic cocoon has acted like a deflector shield to fend off the stream of particles gusting from the sun – called the solar wind – and so protect the Earth’s atmosphere from being blown away into space. Thus the existence of complex life on Earth is itself dependent on this core of hot iron: the iron in your blood not only links you to the ancient stars that created it in their nuclear forge but also to the magnetic shield around our world that protects life on Earth.
Not all the Earth’s iron sank to the core, though: it is still the fourth most abundant element in the crust, making up 5 per cent of the weight of all rocks on average. But to be useful for humanity, iron must have become concentrated into rich ores that can be mined and smelted. And this takes us to a very particular moment in our planet’s history.
WHEN THE WORLD RUSTED
Virtually all the iron ore mined around the world throughout history comes from a kind of rock that formed during a singular period in the Earth’s development.
Banded Iron Formations or BIFs (and the deposits eroded from them) make up by far the largest share of the iron ore we use. Each formation can be hundreds of kilometres long and several hundred metres thick, and the best ores contain more than 65 per cent iron.41 As their name implies, they have a distinctive stripy appearance, each band being between a millimetre and several centimetres thick. The layers are made up of iron-oxide ores – haematite and magnetite – alternating with chert or shale rock.
And they are almost unimaginably old. The great majority of Banded Iron Formations were laid down in a relatively brief period of worldwide deposition 2.2–2.6 billion years ago,42 around the time the first continents were forming on our planet.fn4 The fact that iron ore all over the world dates to pretty much the same moment in the Earth’s history indicates that something truly profound was happening to the planet at that point. The BIFs were laid down on the bed of the ancient oceans, and their stripes reveal fluctuating conditions in the primordial waters: the ore sedimented as a gentle drizzle of grains of iron minerals settled out of the water onto the sea floor, alternating with periods of deposition of normal marine mud. But the curious thing is that today iron only dissolves in seawater in vanishingly tiny concentrations. So how was all this iron deposited from the seas in a prolific spell around 2.4 billion years ago? What was different back then?
If you were to travel back to this time of BIF, you’d encounter a truly alien world. The young Earth was still much hotter inside than today, and this would have driven rampant volcanism. The planet-spanning ocean was broken only by arcs of volcanic islands and tiny continents that had begun to emerge. Ultraviolet radiation from the sun blazed down onto the barren surface. The skies were probably permanently shrouded in sickly-yellow, hazy clouds and the air was full of nitrogen and carbon dioxide. And, crucially, there was no oxygen – you would have needed a spacesuit to walk around on your own homeworld.
Today, oxygen makes up a full fifth of every breath you take. But for the first half of the Earth’s lifetime, the world had essentially no oxygen gas in its atmosphere and oceans. The oxygen in our air, and that dissolved in the seawater, was put there by life. Some organisms are able to harvest the energy in sunlight to transform carbon dioxide into the organic molecules that make up cells, and in the process they split water, H2O, to release the oxygen as a waste gas. This biological alchemy is known as photosynthesis, and it empowers the cell to be incredibly self-sufficient and manufacture all it needs from only light, carbon dioxide and a few other dissolved nutrients.
The kind of cells that developed this ability to photosynthesise and release oxygen are known as cyanobacteria.44 All the more complex sunbathing life forms – diatoms, algae, seaweed as well as all plants and trees on land – inherited this capability from a crucial evolutionary event about a billion years ago when their single-celled ancestor took cyanobacteria inside itself. And it was these minuscule early cyanobacteria, swarming in the primordial seas and giving off oxygen exhaust fumes from their photosynthetic machinery, that eventually oxygenated the entire planet. Geologists studying the change in ancient rocks can see a sharp indicator of the first rise in oxygen level
s 2.42 billion years ago, known as the Great Oxidation Event (GOE). Although this only saw oxygen levels rise to perhaps a few per cent of today’s,45 still far too low for a breathing human, it had profound implications for the chemistry of the Earth and the development of life. In fact, the GOE is the most significant revolution in the history of the planet.46
Shortly after the Great Oxidation, around 2.2–2.3 billion years ago, the Earth appears to have descended into the longest, and probably the most severe, instance of glaciation in our planet’s history. Back then, the sun was about 25 per cent dimmer than it is today, and to remain warm enough for water on its surface to remain liquid the Earth would have needed a substantial greenhouse effect to insulate the world. The ancient atmosphere contained significant amounts of methane, which is a powerful greenhouse gas, but the increased oxygen would have reacted with the methane and removed it, effectively stripping the planet of its warming blanket. Temperatures plummeted and caused a global glaciation, dubbed Snowball Earth, with thick ice smothering almost the entire surface of our planet.47 It remained locked in this whitened state for 10 million years until the ongoing volcanic activity had built up enough carbon dioxide in the atmosphere for the big thaw to begin.48 Rescuing the planet from such deep glaciations is one of the major benefits of volcanism for life on Earth.fn5
Many microorganisms that were around at the time of the Great Oxidation Event could not cope with reactive oxygen gas and were wiped out by this toxic pollution – effectively an oxygen holocaust. In order to survive in the new world order, organisms had either to evolve to survive the presence of this toxic gas – by developing ways to exploit its reactivity to unleash greater amounts of energy from their metabolism, as our cellular ancestors did – or else become restricted to secluded habitats where oxygen doesn’t penetrate, like sea-floor mud or deep underground.fn6
But more complex multicellular life like animals and plants depends on oxygen to survive, as well as an ozone layer to shield the planet’s surface from destructive UV rays. And so although there were vast numbers of organisms poisoned by the reactive oxygen gas or banished to anoxic refuges, the Great Oxidation Event paved the way for all complex life on the planet. Atmospheric levels finally approached those of today, sufficient for the emergence of animal life, around 600 million years ago.
This brings us back to the creation of the Banded Iron Formations that we mine around the world. Oxidised iron is barely soluble in water – and this explains why in today’s well-oxygenated oceans iron is so scarce. But the reduced form of iron dissolves very well, and so on the primordial Earth before the Great Oxidation Event levels of this reduced, soluble form of iron were very high in the oceans, released from submarine volcanoes or washed in by rivers from the eroding landmasses. During the Great Oxidation, cyanobacteria proliferating in the oceans slowly but surely oxygenated the surface waters. The ocean depths, however, remained anoxic and so were rich in dissolved iron – around 2,000 times more than what we find in the seas today. But every time deep water was pushed up onto shallow marine shelves it mixed with oxygen, the iron was oxidised so that it could no longer remain dissolved, and it settled onto the sea floor, creating the Banded Iron Formations. And so the planet rusted.
Virtually all the iron ore mined today and throughout history was created within 200 million years of the Great Oxidation Event 2.42 billion years ago as BIFs. In this way, today’s blue skies, the lungfuls of life-giving air we inhale, and the iron that has provided the tools of our civilisations for millennia are all deeply linked. And oxygen has another benefit: it enables us to make use of fire.
For 90 per cent of the planet’s history there has been no fire on Earth. While there were volcanic eruptions, there was not enough oxygen in the atmosphere to sustain combustion.fn7 Thus the rise in oxygen not only allowed more complex life to evolve on Earth, but it gave humanity fire as a tool. We first used it for keeping the night’s cold and predators at bay, cooking food, and clearing land. Humanity then learned how to exploit the transformative heat of fire: to bake clay into hard ceramic pottery or building bricks, to make glass, or to smelt metals for tools. Today, we use fire for generating electricity and driving a huge range of industrial processes; and we harness tiny bursts of flame in the engine cylinders of our cars. We are as utterly reliant on fire today as were our Palaeolithic ancestors who huddled around a campfire; we’ve just hidden it behind the scenes of the modern world.
THE PERIODIC TABLE IN YOUR POCKET
In the ancient world, only a handful of different metals were used across society, including copper and zinc in bronze utensils, iron in steel tools and weapons, lead in plumbing, and precious metals like gold and silver in decoration, jewellery and currency. These metals remain important in the modern world and, indeed, we still very much live in the Iron Age. Iron, and especially that mixed into the alloy steel, accounts for around 95 per cent of all metal used by today’s industrialised civilisation. Other metals are still crucial, but the applications we put them to have shifted substantially. Copper, for example, was used first as a major alloy component for the tools and weapons of the Bronze Age, but it declined in significance and trading value with the development of iron smelting and the availability of this superior metal. But in the last two centuries copper has resurged in importance as a relatively abundant metal that conducts electric current well, providing the wiring of our modern electrified world. We are using the same Bronze Age metal, but reflecting technological change through history, we now exploit different properties.
We’ve also discovered and learned how to make use of new metals. One of the most prominent is aluminium. This is in fact the most abundant metal in the Earth’s crust (about 8 percent overall), but it is devilishly difficult to separate from its rocky ores. It wasn’t until the end of the nineteenth century that we learned how to mass-produce it cheaply, by passing electricity through its molten ore. It then became widely used as a building material and for food packaging. In particular, aluminium is very lightweight, and so it came into its own with the expansion of aviation from the First World War. But it is in recent decades that the number of metals we use in our technological society has really exploded.
How many different kinds of metal do you think you have on your person right now? A handful? A dozen? You may be astonished to hear that today over 60 different metals are employed in a single hand-held electronic device alone. These include base metals such as copper, nickel and tin; special-purpose metals like cobalt, indium and antimony; and the precious metals gold, silver and palladium.50 Each one is exploited for its particular electronic properties, or for the tiny, powerful magnets used in the speaker and vibration motor. A whole range of non-metal elements are included in your smartphone too, such as carbon, hydrogen and oxygen in the plastics, bromine as a flame retardant, and silicon for the microchip wafers. Of the 83 stable (non-radioactive) elements in existence, around 70 are used in making an everyday consumer device like a smartphone – which means you carry about 85 per cent of the entire available terrain of the periodic table in your pocket.51
It’s not just electronics that employ such a multitude of metals. The high-performance alloys used in the turbines of a power station or an aircraft jet engine mix more than a dozen, and the reaction-accelerating catalysts in the chemical industry – including those making modern medicinal drugs – employ more than 70 different metals. Yet most of us have never even heard of many of these critical metals – elements with exotic names like tantalum, yttrium or dysprosium.
This expansion in the diversity of metals we’ve come to exploit has been staggering. While microchips today contain around 60 different metals, as recently as the 1990s it was only about 20.52 Take indium, for example. This metal was discovered in 1863, and in the Second World War was used to coat bearings in aircraft engines to protect them against corrosion. But it wasn’t until the 1990s that indium came into widespread use when a thin film of indium-tin oxide was added to our screens, exploiting a rare com
bination of properties – the metal oxide is both transparent and electrically conductive. Today indium is used in everything from flat-screen TVs to laptops, and in particular the touch-sensitive screens of modern smartphones and tablets.53 Similarly, gallium was discovered within a few years of indium, but again didn’t find any widespread application until the electronic age: today it is used in integrated circuits, solar panels, blue LEDs and laser diodes for Blu-ray discs.
Most of these exotic-sounding metals belong to one of two groups: the rare earth metals (REMs) and the platinum group metals (PGMs). The metals in each of these two sets are chemically very similar, which means that they have become concentrated in the same minerals and are extracted at the same time by our separation processes. These roughly two dozen metals really do define our present technological age – over 80 per cent of their exploitation has happened just since 1980.54 And if they are the key ingredients of our current technological age, they will be even more crucial in the future as we transition away from the current carbon economy. They will give us the compact but powerful magnets needed in the generators of wind turbines and the motors of electric vehicles, as well as high-capacity rechargeable batteries.
The seventeen rare earth metals are made up of the ‘lanthanide’ series of elements in the sixth row of the periodic table, as well as the chemically similar elements scandium and yttrium. Their title is something of a misnomer, though, as they’re not actually all that rare in the planet’s rocks – apart from the radioactive promethium, of which there is no more than about half a kilogramme in the entire Earth’s crust.55 Lanthanum, for example, is almost as abundant as copper and nickel, and in fact three times more so than lead. And all the REMs are at least 200 times more common than gold.
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