And it wasn’t just the physical building materials that were exported around the globe. The characteristic architectural components of antiquity – from columns to caryatids, from pediments to pilasters – were emulated in Europe for centuries, from the Renaissance through the baroque to the neoclassical of the mid eighteenth century, and on. It was adopted with particular enthusiasm by the nascent United States of America. After winning independence from Britain, this new nation forged its own governmental system, a federal republic, by drawing on some of the political structures developed by the most powerful republic in Western history, that of ancient Rome. At the same time, the architecture of many major public and municipal buildings in America emulated the styles of antiquity. They were constructed not of limestone and marble originating from the ancient Tethys Sea, but replicated in the same imposing style and purity of hue with rock quarried in the young American nation.fn2
CHALK AND FLINT
Chalk is another form of limestone, although at first glance its properties couldn’t be more different. Chalk deposits can be found on almost every continent and are the distinctive signature of the Cretaceous Period of Earth’s geological history.13 In fact, the very name given to this chapter in our planet’s story comes from the Latin word for chalk, creta.
A thick layer of chalk underlies much of southern England (see here). It shows as outcrops along the backbone of the Isle of Wight, continues east as the hilly ridges of the North and South Downs, and sits beneath London, where it forms a bowl that holds overlying layers of clay. The flat chalklands of Salisbury Plain are home to one of the most impressive monuments of early human habitation of Northern Europe, Stonehenge, begun around 3,000 BC. Although the enormous sarsen blocks making up the main ring consist of sandstone, it seems that the builders were attracted to this area by the flint that could be dug from the chalk landscape for tools like knives and arrowheads. Other, less arduously constructed, but equally eye-catching, monuments have been created in this geological band. Humanity has been exploring the artistic potential of this landscape for millennia, scraping away the thin layer of turf overlaying the porous chalk to reveal the bright white rock beneath, or cutting trenches into the ground and filling them with chalk rubble. Chalk figures crafted on hillsides, visible from miles away, include the stylised outline of the Uffington White Horse in Oxfordshire, created in the Bronze Age,14 and the proud salute of the Cerne Abbas Giant in Dorset which probably dates to the first century AD.15
The chalk layer is most clearly visible on the South Coast, where it forms the eye-catching White Cliffs of Dover. It continues under the English Channel to France, where it has produced mirror-image white cliffs and provided the terroir of the great French wine regions of Champagne, Chablis and Sancerre. The Channel Tunnel carrying high-speed trains between Folkestone and Calais was burrowed 50 kilometres through a chalk marl layer, a muddy chalk deposit that is soft but impermeable. And as we saw in Chapter 2, the chalk bridge that used to physically connect Britain to mainland Europe was scoured away in a cataclysmic flood event.
Some rocks contain beautifully preserved fossils. Along the Jurassic Coast in South West England, for example, where 190-million-year-old mudstones are rapidly worn back by the sea, a pleasant day can be spent strolling along the eroding cliff face hunting for spiral ammonites, bullet-shaped belemnites or brittle star fossils. However, the great chalk layers don’t so much contain fossils: rather, they are fossils. The White Cliffs of Dover are a 100-metre-high exposed slab of biological rock.
The larger fossils that are visible if you look at a lump of chalk under the microscope, about a millimetre across, are the multi-chambered shells of forams – the same kind of single-celled marine organism that formed the giant nummulitic fossils in the limestone used to build the Great Pyramid. But the bulk of chalk is made up of what looks like a very fine white dust. Zoom in on these powdery particles with a high-powered electron microscope, however, and you’ll see that even these have the unmistakable intricate detail of biological shells. These particles come in a variety of forms, but perhaps the most distinctive are the fragments of tiny spheres that look like overlapping ribbed dinner plates. They are the minute armour casings of coccolithophores – tiny single-celled algae found among the floating plankton of sunlit surface waters.
These vast deposits of chalk formed during the late Cretaceous Period, roughly between 100 and 66 million years ago. This was a time of exceedingly high sea levels around the world which were around 300 metres above where they are now. As much as half of the continental land area that is dry today would have been submerged back then. The Tethys Sea rose up to inundate much of Europe and South West Asia, extending its wide arms as great seaways through the centre of North America and into North Africa.
The reasons for these high sea levels weren’t just the sweltering conditions of the late Cretaceous that stopped ice caps from forming at the poles – this has been the case for much of Earth’s history. They were the result of the frantic activity of the continental break-up at the time. During the late Permian Period, 200 million years earlier when the world’s large landmasses had congregated into the huge supercontinent of Pangea, global sea levels were at one of their lowest marks of the past half-billion years. The huge mountain ranges thrown up as the continents crashed and fused together meant that more continental mass was lifted up out of the oceans. But with the subsequent disassembly of Pangea, rifts tore up the supercontinent. First, Pangea ripped apart roughly along its middle as Laurasia moved north away from Gondwana. Later the South and then North Atlantic oceans formed as new spreading rifts tore apart Africa and South America, and North America and Eurasia, respectively. The new, hot oceanic crust formed at these long rifts buoyantly rose up in vast submarine mountain ranges, displacing the surrounding seawater – just like when you lower yourself into a bath. It was this planetary process that caused ocean levels to peak during the late Cretaceous.16 Warm seas covered wide areas of the continental land, providing boom conditions for the growth of forams and coccolithophores, their tiny shells building up in thick deposits of calcareous sediment on the sea floor; and these became chalk.
Unlike limestone, the soft and crumbly chalk does not generally offer a great building material itself. But it does lend itself to being crushed and spread on agricultural fields to lower the soil acidity, in the production of quicklime for cement and in a whole range of chemical processes. Bricks can be baked out of moulded blocks of clay, but to build a sturdy wall they need to be stuck securely together. It is limestone and chalk that we have learned to use for this construction alchemy. These calcium carbonate rocks are crushed and roasted in a kiln so that they chemically break down (releasing carbon dioxide in the process) before being mixed with water to make a soft putty. In this way, limestone not only provides us with building blocks, but also with the glue for sticking other materials together. Mortar, cement and concrete are essentially artificial rock that can be spread or poured into any desired form and when set becomes hard as stone.
Chalk also contains beds of flint nodules. Unlike the soft, bright-white, almost chemically pure calcium carbonate of chalk, the flints are hard and dark-coloured lumps of silica. While forams and coccolithophores build their casings out of calcium carbonate, other single-celled plankton like diatoms and radiolarians form their hard parts out of silica. When these organisms die, their siliceous carapaces drift to the seafloor and dissolve. This produces a siliceous ooze on the sea bed that then forms into flint nodules within the chalky sediment.
As the soft chalk is weathered, the durable flint nodules are eroded out and remain scattered across the landscape. Flint was incredibly important in Stone Age toolmaking. Like volcanic obsidian that, as we saw in Chapter 1, was used for many of the earliest implements in humanity’s cradle within the Rift Valley, flint can be knapped to create a very sharp edge or point, perfect for butchering a kill, skinning and scraping animal hides to prepare into clothing, shaping wood, or creating knives,
spear points and arrowheads. And flint has remained important ever since. Glass-making requires high-purity silica, and flint offers one such source. For example, flints from South East England were used by George Ravenscroft in 1674 for his lead crystal glassware.fn3 This lustrous glass was produced to rival that of Venice, where craftsmen obtained their silica by roasting white quartz pebbles picked from the bed of the river Ticino, flowing down from the Swiss Alps.17
FIRE AND LIMESTONE
We’ve explored so far how rocks like limestone and chalk have defined landscapes, and provided the raw materials for construction in the form of masonry blocks and as ingredients for mortar, cement and concrete. We build with these materials to protect us from the elements, but the very creation of this biological rock may also have helped to protect life on Earth from the threat of cataclysmic mass extinctions.
One of the greatest spasms in the history of life on our planet occurred at the boundary between the Permian and Triassic periods, 252 million years ago. This end-Permian global extinction event happened when all the world’s landmasses were fused together into the single supercontinent Pangea, and it was by far the worst mass extinction to have occurred in the half-billion years of complex life on Earth. The fossil record reveals that around 70 per cent of all terrestrial and up to 96 per cent of marine species were wiped out in this apocalypse and it took the world’s biodiversity almost 10 million years to recover.18 This global wiping clean of the slate also marked a fundamental shift in the characteristic life forms on Earth: the era of ‘old life’ (the Palaeozoic) gave way to that of ‘middle life’ (the Mesozoic) – an age that came to be characterised by dinosaurs and gymnosperm conifer trees.fn4
The cause of the Permian Great Dying is thought to have been massive outpourings of lava. Several pulses of extensive volcanism disgorged a total volume of perhaps 5 million cubic kilometres of runny lava that flowed for hundreds of kilometres, covering huge areas of land with seas of the hot stuff which then cooled and set as large regions of basalt rock.19 fn5 As these regions were flooded again and again with lava, layer upon layer of basalt built up. They can be seen today as the extensive mountainous plateaus of the Siberian Traps: the hundreds of layers stacked on top of each other resemble a staircase and so were named after the Dutch word for stairs, ‘trap’.fn6
Such extensive volcanic eruptions would have released huge amounts of carbon dioxide into the atmosphere. Moreover, geologists reckon that the magma gushing out to form the Siberian Traps may have been supercharged with volcanic gases by two other factors. It is thought that as the mantle plume rose up from deep in the Earth’s interior beneath Siberia it melted some ancient oceanic crust that had been previously swallowed by subduction. This recycled crust was rich in volatile compounds and so released a large amount of gas when heated. It also appears that on their way up to the surface through the overlying crust these flood basalts encountered strata like coal seams, which the magma baked to high temperatures to release yet more gas.
It seems likely, therefore, that the onset of the outpouring of the Siberian Traps wasn’t like any volcanic eruption we’d be familiar with today, but began with colossal belches of gases from the belly of the Earth. The huge volumes of carbon dioxide released by these eruptions created a powerful greenhouse effect. The Earth’s surface temperature rose rapidly, and the deeper ocean waters became anoxic – lacking in oxygen – asphyxiating life on the sea floor. Other noxious volcanic gases, such as hydrogen chloride and sulphur dioxide, may also have been projected high into the stratosphere. The output of hydrogen chloride would have severely depleted the ozone layer, allowing harmful ultraviolet rays from the sun to reach our planet’s surface. And the sulphur dioxide would have acted to partially block the sunlight, hampering photosynthetic life and the other life forms supported by it, before precipitating back out of the atmosphere again as acid rain.
It’s this multi-whammy at the end of the Permian that rapidly collapsed ecosystems across our planet and triggered the largest mass extinction in the history of complex life on Earth. And the phenomenon wasn’t limited to the Permian: another flood basalt event around 200 million years ago, at the juncture between the Triassic and Jurassic periods, is believed to have caused the mass extinction that cleared the way for the dinosaurs to become the dominant land animals.
But then something curious happened. There have been a number of other large flood basalt eruptions since the Permian and Triassic events, yet none of them seems to have triggered a similar mass extinction. Something must have changed on our planet to have made the Earth much more resilient to the potentially cataclysmic effects of mega-eruptions.fn7
Two huge outpourings of lava about 60 million and 55 million years ago created the North Atlantic Igneous Province as North America rifted away from Eurasia, constituting the final cut in the break-up of Pangea. The basaltic rocks from this event – the distinctively geometric columns of Giant’s Causeway in Northern Ireland and corresponding features in eastern Greenland – have become separated by the opening of the North Atlantic Ocean. These outpourings of lava probably released even more molten rock than the Siberian Traps during the Permian extinction.21 And like the Permian flood basalt eruptions, the magma spewing out to form the North Atlantic Igneous Province also passed through volatile sedimentary rocks near the surface which would have released vast amounts of carbon dioxide as they were baked, in addition to that given off by the volcanic lava itself.22
But these events triggered no mass extinction. There certainly was a shock to the Earth’s climate, and the second phase 55 million years ago coincided with the Palaeocene–Eocene Thermal Maximum we looked at in Chapter 3. Yet although a few deep-sea species did perish during this temperature spike, these events seem instead to have stimulated the rapid evolution of the three major orders of mammals that dominate the land today: the artiodactyls, perissodactyls and primates.
So what is it about the Earth since the Jurassic that has made our planet so much more resilient against mass extinctions from large flood basalt events?
One important factor is – again – the break-up of Pangea. Supercontinents are on the whole less effective at removing carbon dioxide from the air. Large areas of interior land far away from the sea become very dry with low rates of rainfall. This means less CO2 is being scrubbed by the erosion of rocks, and fewer rivers carry sediment and nutrients into the ocean to fertilise plankton growth, thus also suppressing the biological mechanism for absorbing CO2. So in the last 60 million years, since the final break-up of Pangea, the world has been more effective at removing carbon dioxide released into the atmosphere by large outpourings of lava. But this can’t be the whole story. The geological mechanism for lowering carbon dioxide in the atmosphere – by the erosion of mountains – works very slowly. Thus the sudden leap in CO2 resulting from the eruption of a large igneous province would trigger a mass extinction long before rock erosion was able to bring the levels back down again. It seems that the important factor was a crucial biological transition.
During the early Cretaceous Period, around 130 million years ago, coccolithophores expanded out from the shallower waters of the continental shelves to live as plankton in the open ocean. Around the same time calcite-shelled forams also spread from their deep seafloor habitat to the surface waters of the seas. This meant that the vast open ocean itself, and not just the shallower waters around the continents, hosted plankton that produced calcite shells. When shells from dead coccolithophore and foram plankton rained down onto the seafloor they formed a new kind of sediment, creating limestone in the ocean deeps and not just on the continental shelves.23 Thus marine life was becoming much more adept at removing carbon dioxide from the atmosphere and locking it away in biological rocks on the deep sea floor. And since this time, the carbon dioxide levels on our planet have been steadily diminishing.
Now even with the sudden injection of huge amounts of carbon dioxide into the air from flood basalt events, the oceans’ limestone-forming plankto
n were able to scrub this gas out much more rapidly than any geological processes. Since the early Cretaceous, therefore, the Earth has developed a powerful compensation mechanism for rapidly removing sharp rises in volcanic carbon dioxide before it can trigger runaway warming and mass extinctions. So when 55 million years ago the Palaeocene–Eocene Thermal Maximum started pushing carbon dioxide levels and global temperatures towards catastrophe, plankton saved life on Earth.
Thus the biological rock of the White Cliffs of Dover and the limestone facade of the United Nations building can both serve as reminders of the deep connections within the Earth that across time have created the world we inhabit today.
TECTONIC SWEAT
Granite is the most common rock type of the continents. As we have seen, oceanic crust is formed of basaltic rocks that have solidified from fresh magma seeping out of rifts spreading on the sea floor. But granite is instead forged at convergent boundaries where tectonic plates are forced together.
As oceanic crust is subducted, the water-bearing rocks of this descending plate are melted by the considerable pressure and temperature at depths of between 50 and 100 kilometres, while also being heated by the grinding friction as they slide underneath. This molten magma rises up into the overlying crust and pools into huge subterranean chambers. Here it begins to cool, and as the first minerals crystallise and sink out of the mixture – those with the highest melting point – the chemical composition of the melt left behind in this deep cauldron slowly changes. The early-forming minerals are low in silica (silicon dioxide), which means that the remaining magma becomes more and more enriched in it. Granitic magma is also formed when continents collide and the crust is thickened beneath the great mountain range that is created, partially melting at the bottom and again rising up through the overlying crust. When this silica-rich magma cools and solidifies it forms great subterranean masses of granite rock, often within the core of the mountain range formed above it by the same convergent tectonics. Granite is the sweat of plate tectonics.24
Origins Page 13