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ARMAGEDDON strikes. Perhaps it comes in the shape of a nuclear war, or a new virus 鈥� deadlier than AIDS and more infectious than the common cold 鈥� or a collision with a huge meteorite. Whatever the cause, imagine that Homo sapiens suddenly passes into history. It鈥檚 a shocking scenario, but perfectly plausible 鈥� witness the demise of the dinosaurs 65 million years ago. What kind of legacy would we leave behind? Today, we are rulers of the planet. A hundred million years from now, will we just be history, or geology and palaeontology too?

The dinosaurs, masters of the Earth in their day, certainly left impressive remains for palaeontologists to pick over. But we are only one species, whereas they were many. Also, they were around for about a hundred million years, while we have managed less than half a million. And it is only in the past 250 years, since the Industrial Revolution, that we have had a truly global impact.

What鈥檚 more, although dinosaur remains are impressive, they are strikingly rare: around the world only a few thousand skeletons have been found that are anywhere near complete, together with scattered footprints and occasional eggs. This is partly because dinosaurs were near the top of the food chain and so there were relatively few of them, but also because they lived mainly on land. When they died, their bodies were exposed to the elements, and scattered and recycled by the myriad agents of scavenging and decay. Only those few that were rapidly buried by floods or sandstorms were destined to be preserved for posterity.

How would our legacy compare? Fossil human skeletons might turn up here and there. After all, we鈥檙e very numerous and, by burying our dead, we certainly give our remains a head start over those of the dinosaurs. There may even be a few examples of soft tissues being preserved (see 鈥淔lesh and blood鈥�). But it鈥檚 not just bodies that can survive the ravages of geological time. Trace fossils such as burrows, trails and footprints leave their own more oblique signature. Dinosaur footprints and even nests have been found.

Our own trace fossil systems are a lot more robust than those of the dinosaurs. They include roads, houses and foundations. On average, over a lifetime we each account for some 500 tonnes of sand, gravel, limestone and clay, from which the hard-wearing artificial rocks known as concrete and brick are made. Then there鈥檚 iron, steel, copper and plastic. In urban areas the accumulated rubble of centuries of building form a significant geological deposit. Let鈥檚 call it the urban stratum. Unlike the dinosaurs, we鈥檙e good burrowers, and the urban stratum is shot through with a complex skein of pipes, tunnels, cables and pilings which can go deep underground. The steel and concrete pilings that prevent the skyscrapers of New Orleans from sinking into the soft muds of the Mississippi Delta, for instance, extend to depths of nearly a hundred metres. We have transformed substantial parts of the planet鈥檚 surface, and done so amazingly quickly.

Safely buried

That鈥檚 quite a start. But ultimately all would erode away if the burial did not become permanent. Once the urban stratum is underground, out of reach of the wind and rain, eroding rivers and scavenging animals, the process of fossilisation can begin. Such burials happen surprisingly often. The keys are location and tectonics, aided or hindered by the rise and fall of global sea level.

Let鈥檚 start with tectonics. Much of the Earth鈥檚 crust is moving up, down or sideways in response to the movement of the tectonic plates from which it is constructed. These plates, crashing together and grinding past each other, can throw up mountain ranges and cause earthquakes. Less conspicuous, but more widespread, are the wrinkles that extend hundreds of kilometres from the immediate regions where plates touch, and around the stretched area of crust where plates are pulling apart. These wrinkles make sections of the crust seesaw, placing them on a tectonic escalator that takes tens or hundreds of millions of years to ascend and descend. Britain, for example, has been nudged by the opening of the Atlantic Ocean on one side, and the building of the high Alps on the other.

We have a pretty good idea how today鈥檚 tectonic escalators are behaving. Los Angeles, for example, is on an upward trajectory, pushed by pressure from the adjacent San Andreas Fault system, and is doomed to be eroded away entirely. But areas that are steadily going down, and where sediment has been piling up layer by layer over tens of millions of years, are more promising sites for preservation. The more sediment piles up, the more the crust is compressed by the sheer weight of deltas and silting-up coastal plains.

So a descending tectonic escalator is given a powerful additional push, and the stage is set to produce ideal pickling jars for cities. The urban strata of Amsterdam, New Orleans, Cairo and Venice could be buried wholesale 鈥� providing, that is, they can get over one more hurdle: the destructive power of the sea.

Sea-level changes play a central role in preserving the sedimentary record, particularly in the low-lying, subsiding coastal plains and deltas around the world on which much of our urban stratum is being built. A drop in sea level would tilt the balance towards erosion and destruction. Conversely, a sea-level rise could favour preservation.

The speed of sea-level rise is critical. If it is slow, then there will be plenty of time for the sea鈥檚 destructive power to remove large swathes of land. But if sea level rises rapidly, these low-lying landscapes will be drowned and preserved. It is increasingly clear that there were very rapid changes in the recent geological past, caused by the tendency of the world鈥檚 great ice sheets to collapse suddenly. If the delicately poised West Antarctic Ice Sheet were to slide suddenly into the sea, as the ice sheets of North America did 10 000 years ago, many coastal cities would be plunged underwater, and in a geological instant would be carried into the realm where fossilisation begins.

Considerations of how your meagre collection of bone and tissue, rings and garments may be preserved almost pale into insignificance compared with the several hundred tonnes of concrete, steel, glass and metal that each of us 鈥渃onsumes鈥� in a lifetime. As the manufacturers of these future trace fossils, we will punch mightily above our weight.

Once in the burial realm, the abandoned foundations, subways, roads and pipelines of our ever more extensive urban stratum will be hard to obliterate. They will be altered, to be sure, and it is fascinating to speculate about what will happen to our very own addition to nature鈥檚 store of rocks and minerals, given a hundred million years, a little heat, some pressure (the weight of a kilometre or two of overlying sediment) and the catalytic, corrosive effect of the underground fluids in which all of these structures will be bathed.

The structures of the urban stratum will be crushed to varying degrees. If they are buried in mud, which loses 90 per cent of its original volume as it is squashed down and buried deeper and deeper, the compression will be considerable. If they are buried in sand, which only loses around a quarter of its volume, the compression will be much less severe. But crushing needn鈥檛 obscure their true identity. Ammonite shells and trilobite carapaces, crushed flat within ancient mudrocks, are, after all, still perfectly recognisable as ammonites and trilobites. It will simply provide more of a challenge for whoever 鈥� or whatever 鈥� studies our remains in the far-distant future.

Chemical changes are more difficult to predict. Water 鈥� a highly efficient solvent 鈥� permeates the subterranean realm. Much of it starts out as primeval sea-water which drains into the spaces between the original sedimentary grains, and is then forced back towards the surface as those grains are squeezed closer and closer together by the pressure of the overlying strata. Forced along slow, tortuous paths through the ever-narrowing spaces between the grains, this water is continually dissolving and precipitating mineral matter as it migrates through the rock. This is why some fossil shells, originally calcium carbonate, are replaced by silica, or are simply left as empty (but still recognisable) spaces within the rock.

Artificial rock

Take bricks for example. These are simply masses of clay, shock-heated so that the minerals transmogrify and fuse. Mudstone, which is mud that has been baked by a sudden inrush of magma, is a natural analogue of brick and can survive for aeons underground. It is trickier to make precise predictions about what will happen to concrete after 100 million years underground. Concrete is more complicated, made up of limestone and silicates for cement, sand and gravel for body, mixed with water and fired in a kiln to set off fiendishly complicated hydration reactions.

This new, manufactured rock has never been tested for true longevity. Originally, it wasn鈥檛 even tested for short-term longevity 鈥� witness the expensive problem of 鈥渃oncrete cancer鈥�, a catastrophic weathering reaction that caused buildings and bridges to crumble in the 1960s and 1970s. Underground, however, concrete is unlikely to be entirely dissolved away; the fabric of this 鈥渞ock鈥� may actually get harder as natural minerals precipitate around its grains, cementing it more thoroughly.

And how about glass? Some changes are likely here. Natural glasses, such as volcanic lavas which have cooled so quickly that crystals haven鈥檛 had a chance to form, are unstable over time spans of millions of years. In very tiny fragments, for example splinters of erupted pumice, glasses are easily dissolved. But larger bodies, such as lava flows, are less likely to dissolve en masse. Typically they are transformed into a fine meshwork of very tiny crystals 鈥� the rock becomes opaque, but loses little of its original form. Perhaps future geologists will be scratching their heads over opaque, milky-white Coca-Cola bottles fossilised within the rocks of the urban stratum.

Polymer fossils

Plastics, which are made of long chains of subunits, might behave like some of the long-chain organic molecules in fossil plant twigs and branches, or the collagen in the fossilised skeletons of some marine invertebrates. These can be wonderfully well preserved, albeit blackened and carbonised as hydrogen, nitrogen and oxygen are driven off under the effect of subterranean heat and pressure.

The record of our activity would not end with urban areas entombed by tectonic disturbances. Elevated concentrations of copper, nickel, cadmium, lead, mercury and a host of other metals from our inefficient smelting processes are being spread across both land and sea. The hugely polluting heavy industries of post war Eastern Europe, for instance, left rivers so enriched with heavy metals that the river mud occasionally approaches ore grade. These muds are being washed into the Baltic Sea and the Black Sea, creating a metal-rich stratum. Such pollution isn鈥檛 confined to Eastern Europe. The signature we have left in sediments extends across large parts of the world, and is being carried into deeper seas.

So, with a favourable concatenation of tectonics and sea level, our species could leave behind in a geological instant a much more striking record than the dinosaurs left in a hundred million years. It is a prospect that speaks volumes about the way we have engineered the face of the planet over a few short centuries. The superintelligent, geologically aware rodents of the future, stumbling upon the newly uplifted substructure of, say, New Orleans or Delhi, would see evidence of aggressive colonisation unmatched anywhere in the geological record. Perhaps to obtain a more rounded picture of the human race, though, they might seek examples of exceptional preservation. What chance Billie Holliday CDs surviving into the next aeon?