So the problem isn’t so much their overall abundance in the crust, but the difficulty in extracting them. The fact that the rare earth metals are chemically similar and so occur in the same kinds of minerals means that they are also difficult to isolate from each other as pure metals. Even more troublesome is the maximum concentrations at which they occur within rocks. Many other metals become concentrated by particular geological processes into rich ores, such as the Banded Iron Formations or the thick seams of silver running through Cerro Rico that we’ll come to in Chapter 8. But the chemistry of the REMs means that they tend not to become enriched into high-grade ores, but instead are mostly thinly dispersed in low concentrations through rocks. On the whole, therefore, mining them specifically isn’t economically feasible – it costs more to extract them than they are worth. Thus the geographic availability of rare earth metals that can be mined profitably is limited around the world. They are extracted today in small amounts in India and South Africa, but since the 1990s the vast majority of global production has been in China.
The six platinum group metals – rhodium, ruthenium, palladium, osmium, iridium and platinum – are clustered in the middle of the periodic table, and like the REMs they are chemically similar, which again means that they tend to occur together in the same mineral deposits. But unlike their rare earth cousins, the platinum group genuinely are precious metals. They are among the rarest stable elements in the Earth’s crust – some are millions of times scarcer than copper. Platinum itself is one of the more common metals within this group, yet worldwide production is only a few hundred tonnes a year, compared to 58 million tonnes of aluminium or over 1 billion tonnes of pig iron. Iridium is particularly rare and present in the Earth’s crust at only about one part per billion: on average, 1,000 tonnes of crustal rock contain no more than 1 gramme of iridium. Like the other platinum group metals (and gold), iridium is a siderophilic element and consequently virtually all that was present on the primordial Earth was dragged deep into the interior as iron sank inwards to form our planet’s core.fn8
The PGMs are also known as noble metals, as they are resistant to chemical attack and corrosion, even at high temperatures. Being both rare and unreactive, platinum is an attractive material for jewellery, and about a third of the annual production of this precious metal goes to adorning our bodies.fn9 But unlike other precious metals like gold – which is primarily used today in jewellery or reserves of wealth, with only about 10 per cent going to industry, mainly as electrical contacts – the platinum group metals are employed in a huge range of practical applications: they are used in everything from turbine engines to spark plugs, from computer circuits and hard drives to contacts in heart pacemakers.57
The majority of platinum itself is used for catalytic converters on vehicle exhausts to reduce harmful emissions, and for catalysts for the chemical industry. These are employed for refining petroleum and creating pharmaceuticals, antibiotics and vitamins, as well as in the production of plastics and synthetic rubber. Perhaps the most significant use, however, is for agriculture. Here it serves as a catalyst in the chemical process that produces artificial fertiliser – an activity that effectively mines the atmosphere for nitrogen.58 It is estimated that today around half of the human population are fed with the help of this metal.59
The extreme rarity of platinum group metals means that they can only be mined from rocks where these elements appear in substantially higher concentration than their average in the Earth’s crust. They are therefore limited to sites that have undergone somewhat quirky geological processes. The platinum group can become enriched within certain ores of copper and nickel, and so some PGM extraction is achieved as a by-product of mining these industrially significant metals. Sources include mines near Norilsk in Russia, where deposits formed by the eruption of the Siberian Traps at the end of the Permian Period 250 million years ago (see here) are being dug,60 and the Sudbury Basin in Canada. The Sudbury Basin is one of the largest, and oldest, impact craters known on Earth. This crater was originally around 250 kilometres across, and formed 1.85 billion years ago when an asteroid over 10 kilometres in diameter slammed into the planet. This colossal hole in the ground filled with magma containing copper, nickel, gold and platinum group metals, which then crystallised into rich ores.61 But by far the greatest source of platinum group metals in the world is a single region of South Africa.62 Around 95 per cent of the global reserves of PGMs occur in what is known as the Bushveld Complex.63
The Bushveld Complex is one of the most metal-rich spots in the world. This is a vast saucer-shaped lump of igneous rock, about 450 by 350 kilometres in size and up to 9 kilometres thick in places. It formed about 2 billion years ago – not too long after the Banded Iron Formations were deposited in the oceans around the world – when a huge mass of magma intruded to within a few kilometres of the surface, and then cooled slowly underground. As the magma cooled, different minerals separated out and solidified, like a huge layer cake. One of these layers was enriched in platinum group metals to a level of about ten parts per million, substantially higher than most other rocks, but still offering only about 5 grammes of platinum and palladium for every tonne mined.64 It’s still not entirely clear what unusual geological conditions acted to concentrate such rare PGMs about a thousandfold, but 2 billion years later it is this thin layer that we now mine for the vast majority of the platinum group we use.65
Historically, metals have been used for their mechanical strength for tools and weapons. Today we still employ a wide range of them in construction, and high-performance alloys serve in power generation, transport and industry. But we’ve also come to use a staggering diversity of metals for their catalytic properties in accelerating chemical reactions – which, as we saw, includes helping feed the global population – or their electronic characteristics for modern devices. Compared to the metals of antiquity like copper or iron, many of these elements that run the modern world are very hard to find in appreciable ores around the world, and the Earth has only provided us with them in rare spots with unusual geological conditions. Indeed, several of the metals we’ve looked at in this section are now considered as ‘endangered elements’ of the periodic table.
ENDANGERED ELEMENTS
One of the biggest concerns for continuing to meet our industrialised world’s appetite for resources is the future availability of several of the most important technological metals. Endangered elements include some of the PGMs, several REMs, and lithium, the lightest metal, used in rechargeable batteries. Indium and gallium too are among those picked out as being under serious threat in the coming years.66 fn10
The problem isn’t that these elements will disappear altogether, but that the rising demand for technological applications could greatly outstrip their limited supply. Take the rare earth metals, for example. That the world has become so reliant on Chinese production of the REMs – currently around 95 per cent of the global total – causes a great deal of concern over ensuring that their supply continues to meet the growing demand. This is only heightened by the fact that in many cases there is no known alternative metal that performs just the right function. REM prices spiked in 2010 after China announced a 40 per cent cut in its export quota, citing its own domestic demand and environmental considerations. Although this has been relaxed again there is still great concern over the continued supply of these elements so critical to our technologies.67
As is normal when supply restrictions cause price rises, this created the economic incentive for other sources to be exploited, and new mines and refining facilities are opening up in Australia, Brazil and the United States. But even when these become fully operational China will still dominate the production of the heavy REMs, which are the scarcest and most valuable of the rare earths.68
But another, far more surprising solution is being considered. Some of the scarce metals used in modern electronics, such as the indium of your smartphone’s touchscreen, are used in vanishingly thin films or mixed in tiny quanti
ties with other metals, which makes them hard to recycle at the end of the device’s lifetime. Many others, however, can be recovered with a little effort. After decades of our simply discarding obsolescent gadgets, many landfills may now hold veritable mother lodes of these valuable metals. And this raises an intriguing possibility: landfill mining – picking back over our rubbish for the buried treasure it contains. A test site at a landfill 60 miles east of Brussels, for example, aims to recover building materials and convert waste into fuel, but also seeks to sort and recover valuable metals. And landfill mining could begin soon in Britain too: four sites that have been tested were found to hold significant amounts of aluminium, copper and lithium.69 The opportunities for prospectors are particularly good in the high-tech dumps of Japan, however. It has been calculated that its buried waste contains three times the global annual consumption of gold, silver and indium, and perhaps as much as six times that of platinum. In fact such artificial ores made up of reduced mobile phones can contain thirty times the concentration of gold as an actual goldmine.70 fn11
This chapter has brought us from the Bronze Age to the modern world of high-tech metals, and explored how particular geological conditions on our dynamic Earth provided us with raw materials for the tools of civilisations. But precious metals like gold and silver have also served through history as a medium of exchange – they were minted into coins to facilitate commerce and trade between disparate cultures. One of the earliest long-range overland trade networks stretched across Eurasia and connected China and the Mediterranean: the Silk Road.
Chapter 7
Silk Roads and Steppe Peoples
The continent of Eurasia, stretching 12,000 kilometres from the Atlantic to the Pacific oceans, contains over a third of the total land surface area of our planet, and has hosted many of the most sophisticated civilisations in history. It is in Eurasia that different cultures developed wheeled transport, iron-smelting, transoceanic trade links and industrialisation. Two aspects have defined the course of history across this sprawling land mass: long-distance trading routes over the great breadth of the continent, and nomadic peoples repeatedly spilling out of the continental interior to challenge the civilisations growing around its margins. It is the fundamental planetary characteristics of climate bands, and the environments within them, that have created these themes.
THE HIGHWAY ACROSS
Long-distance overland trade across central Eurasia was well established by the first millennium BC to satisfy the Chinese demand for jade from Central Asia and the Mesopotamian desire for lapis lazuli from Afghanistan.1 But this long-range commerce intensified dramatically from the first century AD. By that time two great powers had arisen on opposite sides of the wide Eurasian landmass: Han China to the east and the Roman Empire in the west.
In China, civilisation had begun along the riverbanks of the River Wei and lower Yellow River,2 before spreading further south to the Yangtze. It is this plain between the mighty Yellow and Yangtze rivers that forms the heartland of China.3 Wheat and millet were grown in the drier north, and rice in the wetter climatic zone of the south, where two crops could be harvested every year.4 Whereas the fields of Egypt were rejuvenated annually by the flooding of the Nile, Chinese farmers had received their endowment of fertile soils as a lump-sum deposit. Blankets of loess soil were formed over the last 2.6 million years of the recurring ice ages by windblown dust from retreating glaciers and desert regions.5 Accumulations of this fertile soil can be 100 metres thick in places, forming impressive plateaus, but it is also eroded and deposited by rivers in alluvial plains.6 Loess soil is mineral-rich, porous and has a distinctive buff colour – indeed, the Yellow River is named after the loess sediments that it carries.fn1
This agricultural core of modern China was unified in 221 BC, after 250 years of warfare, by the victorious Qin dynasty (which gave us the name China). Like Egypt, China was able to achieve such early and long-lasting political unification, and protection from external threats, because of its natural frontiers:8 the Pacific coastline to the east, the inhospitable highlands of the Tibetan Plateau and Himalayas to the west, and dense jungle to the south. The main weakness was the northern boundary, marked not by a distinct topographic feature like a mountain range but by a smooth ecological gradation from the fertile agricultural plains into the Gobi Desert and then the arid grasslands of Central Asia. By around AD 100, under the Han dynasty, the Chinese empire had expanded north to the Gobi Desert and Korean Peninsula. It also reached west in a long arm following the contours of the landscape through the Gansu Corridor, marked by a string of oases between the towering Tibetan Plateau and the Gobi Desert, and into the Tarim basin holding the Taklamakan desert to protect its trade routes across Central Asia.
The expanse of the Roman Empire too was defined by natural boundaries. By AD 117, the time of its greatest extent, Rome had expanded from a small town halfway up the Italian Peninsula to a vast empire encompassing around a fifth of the global population at the time. At this high-tide mark, the Roman Empire completely encircled the Mediterranean – or mare nostrum, ‘our sea’, as it was called – its frontiers following the features of the landscape. In the west, the empire stretched to the Atlantic coastline of the Iberian Peninsula and Gaul (France), and up through drizzle-swept Britain. Its northern limits rested along the banks of the Rhine and Danube rivers that snake through the European plains. The frontier followed the Carpathian Mountains to the shores of the Black Sea, and then along the line of the Caucasus. Reaching down through Mesopotamia and round the coastline of Palestine, the empire then extended along the Nile, and finally followed the North African coast until the land gave way to the inhospitable dust of the desert.fn2
The limits of the Roman (top) and Han Chinese (bottom) empires in the second century AD were defined by natural features.
At the beginning of the second century AD, the Roman and Han Chinese empires shared many features. Both had roughly the same population of about 50 million, and covered approximately the same territorial area – around 4–5 million square kilometres. The Roman Empire was based around the rim of its internal sea, the Mediterranean, which made for easy internal transport and trade, whereas the core of China spread across plains watered by the mighty Yellow and Yangtze rivers. Rome built roads for overland transport, China more canals, and both civilisations constructed fortified walls to keep the barbarians at bay.10
At this point of their greatest extent, the Roman and Han territories combined stretched to encompass a full three-quarters of the complete breadth of the Eurasian continent between the Atlantic and East China Sea. And they were brought together by the trade of one precious commodity – silk.
China had been using silk to pay off the aggressive Xiongnu tribe beyond its northern borders, or to buy their horses,11 and it already traded silk to Persia. But now it found an eager new market even further afield with Rome, where the elites valued this beautiful fabric from the East.12 Chinese silk first reached the eastern Mediterranean by overland caravans,13 but was also traded along the sea passage we explored in Chapter 4: by ship across the Indian Ocean, up the Red Sea, through the desert by camel to the Nile and then by boat to Alexandria.14 fn3
Trade along the Roman–Han axis reached its peak in the early second century AD, before the collapse of the Han dynasty in AD 220 and the slow decline of the Roman Empire. But commerce between East and West continued through the centuries. Today we know this long-range trade between the extremities of Eurasia as the Silk Road. But the term is a misnomer. There was never just one road, but rather an extensive network of routes linking cities, oasis towns and trading entrepôts – an entire web of transport and commerce draped across Central Asia. And although we usually imagine the Silk Road as a transcontinental link between its remote termini in China and the Mediterranean, trade in between these waypoints was just as crucial, with the routes extending into northern India and Arabia.
The main overland routes and entrêpots of the Silk Road across Eurasia.
The history of the Silk Road illustrates the extraordinary extent to which the terrain of our world has ordered and directed our movements, lifestyles and trade. From the northern plains of China the Silk Road passed along the Gansu Corridor, a 1,000-kilometre-long passage running between the towering Tibetan Plateau and the Gobi Desert. After passing the oasis city of Dunhuang and the Jade Gate of the Great Wall, the route raches the lip of the Tarim Basin and the punishing Taklamakan desert lying in this depression. One fork of the Silk Road now headed north along the feet of the Tian Shan mountain range, while the other passage hugged the southern edge of the desert that meets the Tibetan Plateau. Both routes converged again at Kashgar and the road then threaded through mountain passes across either the Tian Shan to the west or the Pamir mountains to the south. A further route crossed through Ürümqi and the northern Tian Shan, making use of the Dzungarian Gate valley to pass through the mountains.
After negotiating the Taklamakan desert and Tian Shan mountains, the Silk Road passed along valleys and then wove across the deserts of Central Asia – through modern-day Uzbekistan, Turkmenistan and Afghanistan – connecting oases and trading stops like Samarkand, Bukhara, Merv and Herat. A southerly branch of the caravan network bore south to Kabul, and from there threaded through the Khyber Pass over the Hindu Kush mountains of the Western Himalayas and down into the Indus valley.18 Continuing west, the Silk Road passed south of the Caspian Sea through Persia, linking large entrepôts like Baghdad and Isfahan, and then carried on to Damascus and the ports of the eastern Mediterranean; or it turned north to the Black Sea, from where the goods were carried to Europe by ship.
Origins Page 17