This re-melting and chemical processing of the crust also means that granite is less dense than basalt. So in the recurring clashes of plate tectonics the granitic rocks ride over the heavier oceanic basalt and don’t become subducted – they survive and accrete together as the basement layer of the continental crust. Thus granite forms the very foundations of the continents, lying beneath the veneer of sedimentary deposits, and only becoming exposed on the surface as austere outcrops when the softer landscape has been eroded around it.
As we have seen throughout this book, no sooner have they been thrust up into the skies than mountain chains experience the punishing forces of the planet that work to rub them away again. The expansion and cracking of freeze–thaw cycles cleaves and pulverises their rocks; rivers coursing down their flanks gouge out great valley networks; and driving glaciers scour the very summit, picking up and rasping fragments of the mountain’s own substance to further grind it down. But as mountains are eroded away, the weight pushing their thick crustal roots into the dense mantle is reduced and so they buoy back upwards a little more. The diminishing peaks are therefore relentlessly raised back into the grinding maw of erosion, like a block of wood that a carpenter smoothly pushes into a spinning sanding disk to wear it down. In the end, even the mightiest mountain chain is disassembled grain by grain over the expansive gulf of time of our planet’s history. Eventually, the mountains will be worn down to the merest stump, exposing their heart of hard granite.
So when you stand on a pillar of granite, you are stepping on the very core of an ancient mountain range. During its formation, this granite would have had at least 10 kilometres of rock piled on top of it, now worn away over 100 million years or more of erosion. The tors of Dartmoor, El Capitan in Yosemite National Park, Rio de Janeiro’s Sugarloaf Mountain and the Towers of Paine in Chile were all created and then revealed in this way.25
Granite is hard and durable, with a coarse-grained texture from the large crystals that had time to grow and develop as the melt cooled slowly deep underground. Since granite epitomises solidity and permanence, we have used it to build impressive monuments throughout history. Perhaps the most famous granite feature in the world is Mount Rushmore, in South Dakota. This granitic mass formed 1.6 billion years ago, and in the 1930s the faces of four US presidents – Washington, Jefferson, (Theodore) Roosevelt and Lincoln – were sculpted into its south-east side, so as to catch most sunlight. (The project as originally conceived was to carve the presidents’ forms down to the waist, but funding ran out.) The granite of this sculpture is exceedingly hard-wearing, and erodes at the rate of only about 2.5 mm a millennium – it will remain a symbol of American ideals for a very long time. In fact, the designer of the monument took this into account and had the presidents’ features carved several inches thicker, so that they will have worn back to their intended shapes 30,000 years in the future.26
In the ancient world, the Egyptians were the masters of granite working, sourcing the material from the upriver Nile Valley in quarries in Nubia, in what is today northern Sudan.27 This they carved into their most enduring columns, sarcophagi and obelisks, such as the ‘Cleopatra’s Needles’ that now stand in London, Paris and New York (although a misnomer, as they were made over 1,000 years before the rule of Cleopatra).fn8 It was the rediscovery of the ancient Egyptian monuments and their display in the British Museum that inspired European stonemasons in the early 1800s to attempt to emulate their works and carve granite, only becoming successful with the development of steam-powered machinery for cutting and dressing granite in Aberdeen.28 Much of the granite used in Britain is sourced from Aberdeen, where it had been formed beneath the great Grampian mountain range 470 million years ago – long enough ago for erosion to rub away the kilometres of overlying rock to reveal the granitic core.29
Even the durable resilience of granite is not impervious to the merciless action of the elements, though. As it reacts slowly with water, granite is chemically rotted and undergoes an almost magical transition. The quartz crystals tumble away as grains of sand, and another mineral component of the original granite, called feldspar, is chemically converted into kaolin, a type of clay. The water leaches out other impurities from the decomposing granite to leave behind only the fine, flakey particles of this purest of clays, snow-white in appearance. This can happen when the deep granite has been slowly exhumed and exposed to the elements, or while it is still underground and its own heat drives hydrothermal systems in the subterranean cracks and fissures.fn9
Not only is the kaolin a pure, snow-white colour, but its powdery, plate-like particles make it wonderfully soft and malleable. This clay can be fired at high temperatures, creating pottery that is particularly strong and also translucent. Kaolin is therefore the raw material for the very finest of ceramics – porcelain.
Porcelain was first developed by the Chinese around 1,500 years ago, and reached the Islamic world in the ninth century AD. The trade of porcelain into Europe gave it its name in English: fine china. The firing of porcelain vases, jugs, bowls and tea sets at high temperature makes them strong even when very thin to give them a refined delicacy and an almost ethereal translucency. It’s this that made porcelain so highly prized in comparison to other clay ceramics – earthenware or stoneware retain their opaque muddy colour even when colourfully glazed.
When trying to emulate porcelain, English potters added the ground ash of bones from their abattoirs, but although reproducing the white colour this bone china was still inferior to porcelain. They eventually discovered the secret ingredient of kaolin clay, and the first commercially successful production in England was achieved in Stoke-on-Trent in the final years of the eighteenth century. The area has abundant coal for firing pottery kilns, and the Staffordshire potteries originally made use of the clay deposits found between the local coal seams, firing them into building bricks, floor tiles or huge pots for transporting butter down to London by packhorse.31 But with the development of techniques for manufacturing fine bone china, Stoke-on-Trent became the leading production centre in Europe for this rival to porcelain. Yet although the Stoke potteries had abundant coal for their kilns nearby, and came to use it for steam engines for crushing and mixing the raw materials and driving the potters’ wheels, they needed to import the crucial kaolin from Cornwall. Like Aberdeen, Cornwall has exposed granite formations, but here the rock has been hydrothermally processed into the soft white kaolin clay. And the demand for sending Cornish kaolin to the potteries of Stoke, as well as for transporting the delicate, finished china around Britain, was one of the major drivers behind the digging of the network of long canals in the early stages of the Industrial Revolution.fn10
In this way granite, formed as the slow-cooling sweat of the crushing pressures and heat of plate tectonics, both gives monuments their enduring solidity and is converted into one of the most delicate and fragile of substances – porcelain china.
THE GROUND BENEATH OUR FEET
We saw earlier in the chapter how the ancient Egyptians and Mesopotamians constructed their civilisations with the local endowment of building materials provided by the underlying Earth. This is as true through modern history as it was for the earliest civilisations. Let’s explore how the normally unseen, subterranean world is reflected in the appearance of buildings across Britain, the place where the first nationwide geological map was drawn.fn11
The geology of Britain is particularly diverse, displaying outcrops of rock from almost all ages of Earth’s history over the past three billion years. Tectonic shifts and erosion have, over time, re-exposed these different strata in complex swirling stripes across the country. In age they trend roughly north to south, from the most ancient rocks in the Scottish highlands to the youngest formations, created over the last 65 million years, in the South East. It is fascinating to see how throughout history the characteristics of buildings across Britain generally reflected the local geology: we recognise the dark granite of city buildings in Aberdeen and of farmhouses around
Dartmoor, the buff-coloured Carboniferous sandstone of Edinburgh and Yorkshire, the golden Jurassic limestones of the Cotswolds villages, and the warm brown colour of clay used for bricks and roofing tiles in and around London. We have pulled the geology from beneath our feet and piled it into walls, and just by looking at a photo of a traditional building a geologist would have a good idea in which part of Britain it was taken.
Geological map of the British Isles.
Places with no suitable local stone had to make do as best they could. Chalk does not make a great building material – it’s a soft and crumbly rock, and doesn’t fare well in the face of weathering. It has, however, occasionally been used as a material called clunch, in the form of irregular lumps of rubble or cut into blocks and laid in courses – in East Anglia as well as in Normandy, for example. But generally across the Cretaceous landscape alternatives had to be found. Many cottages in the chalklands of Suffolk and Norfolk were constructed from timber frames, which were filled in with wattle and daub – a lattice of twigs covered with wet soil and straw – and then whitewashed with a solution made with chalk. These timber frames are sturdy, and if properly protected from damp, are durable enough to survive for centuries. As the chalklands also offered little material for roofing tiles, buildings in this geological area were traditionally thatched with reeds or else the long straw left over after the wheat harvest. So, while such timber-framed, thatch-roofed buildings have come to epitomise the quintessentially English countryside, it reflects in reality the paucity of the local geology for suitable building stone.34
These idiosyncratic building styles became much more homogenised with the Industrial Revolution. Bricks were mass produced for constructing mills, factories and workers’ housing in the growing cities, and transported much greater distances along the canals and then railways. Slate, which has long been mined from the half-billion-year-old Cambrian rocks around Snowdonia in North Wales, began to be used as a roofing material across the country. Slate is a fine-grained rock that began as seafloor mudstone before it was squeezed and metamorphosed in the vice of plate tectonics. This forced all its particles to lie along a particular plane, so that a skilful tap with a chisel can split it into thin, perfectly flat slices: it is therefore ideal for making roofing tiles. Welsh slate supplied the expanding industrial cities throughout the nineteenth century and to this day these thin wafers of the Cambrian Period cap buildings across Britain.35
The rocks of different regions around the world have been important not just for providing the raw materials for our construction projects throughout history: the underlying geology has also determined how our modern cities have developed.
If you can remember a trip to Manhattan, or visit it now with Google Earth, you’ll see that there are two main areas of towering skyscrapers: the dense cluster of the downtown financial district on the southern tip of the island; and Midtown, sporting the Chrysler Building, Empire State Building and the Rockefeller Center. Between these two nodes of ultra-highrise edifices lies a spread of lower buildings. It was first argued by a geologist in the late 1960s that this distribution of buildings echoes the invisible strata beneath the streets.36
Lumps of a dark, hard metamorphic rock known as schist – originally mud or clay transformed in the crushing heat deep in Earth’s interior – outcrop all over the city; New Yorkers on their lunch break might sit on a slab of it in Central Park while they munch their sandwiches. New York’s schist was baked beneath a huge chain of mountains running along the eastern United States from the Labrador coast down to Texas, as well as to eastern Mexico and Scotland (before the North Atlantic Ocean rifted open). This Grenville mountain range ran down the middle of a supercontinent even more ancient than Pangea, called Rodinia. Around 1 billion years ago the continent of Laurentia collided with two other continental plates, fusing them together and crumpling up the Grenvilles. Over aeons of time since, whilst the continents have split and recombined in different configurations, the slow but persistent erosion has whittled down this mountain range so that only its base remains today.
In New York the schist rocks exist in a syncline, a trough-like dip underground that brings the schist layer close to the surface at the southern tip of Manhattan, and again in Midtown. This hard metamorphic bedrock provides the perfect foundation for bearing the immense weight of soaring skyscrapers. In between, held in the cup of the schist syncline, lies softer rock less supportive of massive buildings. Socioeconomic factors also played a role in the pattern of skyscrapers, with development occurring in already established commercial centres, for example, but on the whole the skyline of Manhattan follows the underlying geology: the areas with the tallest buildings are supported by the hard schist. The invisible subterranean world – the worn-down stumps of a truly ancient mountain range – is reflected above ground in the towering skyscrapers in the commercial districts: monuments not to the gods but to capitalism.37
London is in some respects the opposite of Manhattan. Rather than an island bounded by two rivers, it is a city built around a river. But it is located in a similar geological setting. The wedge-shaped London Basin sits at the bottom of a syncline, where the rock layers have been buckled into a trough – in this case by the tectonic forces that also crumpled up the Alps. Indeed, the London Basin is part of the same rippling of surface rocks as the bulge of the Weald–Artois anticline that once formed the land bridge between Dover and Calais that we explored in Chapter 2. While the syncline in Manhattan brings hard, metamorphic schist rocks close to the surface in Downtown and Midtown, London and the whole of the lower Thames Valley runs along the floor of the syncline trough. This became filled with a layer of clay when a warm shallow sea lapped into the tapering depression around 55 million years ago.
This London clay is decidedly unaccommodating for constructing the tallest buildings of the modern age. The reason why London, in contrast to New York, has so few skyscrapers is this thick layer of soft, putty-like clay beneath the city. Towers like The Shard or One Canada Square in Canary Wharf had to be built with very deep-piled foundations to support their weight. The thick clay layer is, however, ideal for digging tunnels: it is soft to bore through, but forms a stable and water-impermeable blanket for the tunnel.
London built the world’s first underground metro line in 1863, and today, the Tube has been developed into a network of over 400 kilometres of lines, serving 270 stations (although not all underground). The underlying geography also explains why North London is so well served by the Tube network, but the south has far fewer lines. South of the Thames the clay layer dips to beneath the depth of the network, and tunnels must instead be bored through much trickier strata of sand and gravel. The London clay is also the reason why the Tube has become so uncomfortably hot. Underground caves are normally refreshingly cool, so this would appear as something of a paradox. In fact, when the tunnels were first dug the temperature of the clay was around 14 °C – indeed, in the early days the Tube was advertised as a place to keep cool on a hot summer’s day. But after more than a century the heat released by the trains’ motors and brakes – as well as by the millions of passengers – has been absorbed into the tunnel walls. And as the dense clay is a remarkably good thermal insulator this heat has found nowhere else to go.38
So while the first true cities in the world, in the muddy plains of Mesopotamia, were constructed of sun-dried adobe bricks, underlying clay continues to direct how our modern metropolises develop: the extensive underground Tube network of London contrasting with the towering skyscrapers of New York.
Let’s turn now from how the geology beneath our feet has offered us the natural fabric for building our civilisations and cities, to how humanity learned to extract from the rocks the materials for the tools and technology with which we transformed our world.
Chapter 6
Our Metallic World
We’ve seen how humanity’s earliest tools were constructed from stone – by knapping lumps of chert, obsidian or flint – or made of wood,
bone, leather and plant fibres. As we progressed through the Palaeolithic, Mesolithic and Neolithic periods (the Old, Middle, and New Stone Ages) we refined these technologies, moving from making chunky hand cutters and scrapers to small sharpened stone flakes suitable for spearpoints and arrowheads. Yet the beginning of the Bronze Age marked a deep transition in the human story: rather than making tools by simply reshaping what could be picked up from the natural world around us, we learned how to purposefully transform raw materials, extracting shining metals out of their rocky ores, forging and casting them, and perfecting alloy mixtures. And the rate of technological innovation accelerated over time. It took 3 million years from hominins making chipped stone tools for humans to smelt the first copper; yet we progressed from the Iron Age to space flight in just 3,000.
Metals have been so revolutionary in human history because they offer a range of properties that no other materials provide. They can be extremely hard and strong, but unlike brittle ceramics or glass they are also flexible and shatter-resistant. For our more recent technologies, they are able to conduct electricity and resist the searing temperatures to which high-performance machinery is exposed. And over the past few decades we’ve come to exploit a staggering diversity of metals for our latest technology, especially in modern electronic devices.
Origins Page 14