Supercontinent: 10 Billion Years In The Life Of Our Planet Read online

  You learn as much as you can about this precious place from orbit, but the next thing on your mission directive is to check if any among those living things down there is sentient. But you already know the answer to this question. Sentient life becomes civilization in a geological instant, and the chances of finding the first living planet at just that tiny moment between the evolution of an intelligent being and its ability to build cities and get power from atoms are too small to imagine. There are no satellites orbiting. There are no transmissions. This planet hangs in space like a great unseeing eye. There is no civilization down there. The creatures that may swim in its seas, or fly through its air, wander those forests or cling to its fertile coasts are dreaming their innocent world, unaware of anything beyond it, or that over them all, your shadow has now fallen. It is a kind of paradise. You envy it.

  But hunches are not enough. Rules are rules and the manual says you have to check, run tests, write reports. From your vantage point, with your instruments, you can now scan the surface of this planet’s landmass in precise detail, centimetre by centimetre. If there is (or was) a civilization down there, you will find it. Even if some extinct creature had built something, or carved the sacred images of its great leaders into some granite mountain, you will see it.

  You begin the scans, which eventually will be assembled in a computer that will remove all the obscuring clouds; but you know this is hopeless. If the absence of transmissions tells you there’s nothing intelligent there now, the planet’s reflected light tells you there was probably never anything there. Sentient life quickly learns the secrets of matter and makes power from atoms. That process creates forms of matter that never exist in nature and which take millennia to decay away to nothing. Even on an active planet like this one, with weathering and erosion and deposition and a seething crust that renews and recycles itself, these substances endure. They are the only truly lasting products of civilization.

  You scan the arid surface of the supercontinent for radioactive isotopes of the most insoluble elements with the longest half-lives. You find some: Thorium 232, and Uranium 235 and 238. But these all occur naturally. There is nothing more. If there ever had been an advanced civilization on this planet it must have vanished more than 100,000 years ago, though this does not yet depress your archaeologist, because the surface scans are not finished.

  Much of the land surface is dry. Physical traces of civilization might have survived for more than 100,000 years, a great city perhaps, or some massive monument hewn from living rock, whose outline would still be visible. But even after you have assembled in your memory banks a complete inventory of every valley, mountain and hill on this supercontinent, the archaeologist finally admits defeat. Now only the geologist is interested in the possibility. But looking for fossils is something you cannot do from a spaceship.

  As the weeks go by, you turn (with little real enthusiasm but because the mission directive says you must) to the planet’s moon. You have seen so many other bodies like it, a dull, cratered space rock, dry, dead, circling for ever, its inert surface open to space, almost unchanging from its earliest violent days of heavy meteorite bombardment.

  Yet here you are in for a surprise. Almost immediately your preliminary scans of the surface turn up real and unequivocal evidence of advanced civilization. At six separate sites you identify the remains of landing craft, transportation vehicles, transmitters, a seismic array. Another enormously significant revelation for your mission. Not only is your home world no longer the only living planet in the universe; you know now there have been other space explorers; and what is more, they have passed this way.

  Subsequent archaeological research on the lunar artefacts reveals the trail to be a little cold, however. As the moon is clearly not itself a living planet, you can check out one of the alien spacecraft. Landing some way off and approaching with care, you see it and, most exciting of all, the footprints around it. They still look as though they arrived yesterday: unequivocal evidence of alien beings with spaceflight capability. Your archaeologists take samples of the spider-like landers, from each of which an escape module once blasted into space. Clearly the six visitations to this moon were made by the same beings, at more or less the same time. But when?

  The answer is not long in coming; microscopic examination of the metal surfaces reveals that they are pitted with billions of tiny micrometeorite impacts. From their density, and what you know about the rate of influx of such tiny objects, you work out that these visitors – whoever they were – must have left for the last time 200 million years ago. But there’s a nagging question. Why would these travellers come to this moon, and not to the much more interesting planet?

  Perhaps, despite the lack of any trace of them there now, these beings had come from the planet?

  You ask your geologist, who has been studying the planet’s single continent and its volcanic mountain chain. From what she now knows of the planet’s crust and how it moves, even if some civilization of 200 million years ago had completely covered that same planet in cities and then wiped itself out in some gigantic global nuclear holocaust, nothing – not even the faintest trace of some unnatural radioisotope – would now remain on the surface. What is more, if those vanished beings were to be brought back today, they wouldn’t recognize the world below as theirs. At the speeds at which the planet’s crustal plates move, even with all the land locked together in a great supercontinent, she can be certain that 200 million years ago the planet looked nothing like this. Perhaps then there were once many smaller, separate continents, all scattered about like islands in the ocean.

  The geologists begin to write a research proposal to break the mission directive and visit a living world for the first time, spurred on now by the possibility of finding fossils of a vanished sentient race that may have developed space flight before vanishing completely. Now the only trace of them and their culture could be six short visits they had made in their heyday to their dead, unchanging moon, lasting in all not much more than 300 hours.

  And who knows what they would find if they got permission? Maybe those alien explorers are in for yet another shock. Perhaps those fossils that they discover of a small, forked creature would look very familiar – just as the footprints on the moon had done. Perhaps the space visitors from a small planet in the vicinity of Betelgeuse would find themselves meeting their ancestors. Perhaps they would discover themselves to be one lost tribe within a galactic diaspora that had saved the human species from inevitable extinction on a home world to which it had now, for the first time, returned.

  Future worlds

  Scientists are already trying to predict what this supercontinent of the distant future will look like, and the version I have just described is based on the work of a British scientist who divides his time between the chilly waters of the South Atlantic and the British Antarctic Survey’s headquarters in Cambridge. Roy Livermore is a marine geophysicist. He is interested in computer-modelling the way the plates of the Earth’s crust move, and his main research area is the stretch of ocean floor between the tip of South America and the Antarctic Peninsula. As Roy pointed out to me these two eastward-sweeping points of land, Graham Land and Tierra del Fuego, they reminded me of a section through a piece of armour plating pierced by a high-velocity round. The hole through which the bullet appears to have passed is known as Drake Passage.

  Drake Passage, between the southern tip of South America and the Antarctic Peninsula.

  Knowledge about how the Earth’s tectonic plates have moved since Drake Passage opened up thirty million years ago has many uses. Oil companies are interested in how continents break up and move because fertile environments for oil formation are created at the continental margins. Climate modellers are interested in the Drake Passage because its opening contributed to a climate switch some thirty million years ago from the warm ‘greenhouse’ Earth to the cool ‘icehouse’world we live in today. But Roy’s theoretical interest in computer-modelling plate motions has recentl
y enjoyed a more unusual application. For Roy Livermore created the future world of Novopangaea (the name he chose for it reflects the return of a condition that prevailed 250 million years ago), when all the present continents were last assembled into a single landmass, known to geologists as ‘Pangaea’ (‘All land’).

  His vision of the deep future was the setting for a TV documentary series about the likely course of animal and plant evolution. Its producer, John Adams, wanted to show what the Earth might be like five, 100 and 200 million years from now, and particularly how the animals and plants of that world might look. The focus of the series was principally the animated living forms; but on what kind of a world would these CGI creations disport themselves?

  Adams began by asking geologists what the world would look like in the future. Livermore had to create a set of credible plate-tectonic forward projections or ‘preconstructions’ of our future Earth. In other words, he had to put the continents into the positions they would occupy in five, 100 and 200 million years from now. Climate experts then took his maps and predicted how the atmosphere and oceans would behave given those arrangements of land and sea, and so deduce what conditions could be expected. The resulting future world could then be populated with appropriate fauna (including ‘flish’, flying fish, and a rather attractive hopping snail) dreamt up by evolutionary biologists.

  ‘It really grew out of work I did with Professor Alan Smith in the Department of Earth Sciences here in Cambridge, back in the eighties. I have been interested in preconstruction and thinking about the future for maybe fifteen years,’ he told me, producing three Lambert projections of the globe, the land in green, ocean in light blue and shelf seas in a darker shade.

  So how did he arrive at Novopangaea? Is it simply a matter of knowing how the continents are moving now and winding the clock forward on a computer? ‘Well, first of all this exercise isn’t driven simply by scientific curiosity,’ he explains. ‘There were other considerations like, from my point of view, the desire to illustrate a range of geological processes.’ In other words, in making his preconstruction, scientific constraints were mingled with the need to arrive at an interesting outcome for the programme makers. This means there has to be a point when forward projection ceases to be mechanistic and objective; when the experimenter must intervene. (As we shall see later, the imaginative ambitions of all those who have ever dreamt about supercontinents, past or future, have rarely been unmixed.)

  Livermore took me through the process. ‘We start with the present day, when we know how the plates are moving, and extrapolate a few million years into the future. The five-million-year projection is quite tightly constrained. It’s what you get if you just wind everything forward a bit.’ Because of that, the outcome is not terribly exciting; nothing seems to have changed very much. ‘No, nothing terribly exciting happens until we get to 100 million years. But whenever you go that far, there inevitably comes a point where you have to make a decision.’ This is where the model operator plays God. The planet is not a simple perpetual-motion machine that cycles for ever in the same old way. From time to time unique events happen that alter the outcome of the process, like a massive meteorite strike, or super-volcanic eruption. These events are often unpredictable, but the Earth carries their consequences for all time. Understanding this has been a major breakthrough in our thinking about the Earth in the past 200 years.

  ‘The biggest “decision” I made,’ Livermore told me, ‘was that the Atlantic Ocean would continue to open and the Pacific would continue to shrink.’

  All land

  The continents of today’s Earth are the wreckage of that single supercontinent, Pangaea, which began to break up about 250 million years ago. The name was given to the last supercontinent to have formed on Earth by German geophysicist Alfred Lothar Wegener. It first occurs in the third edition of his great book The Origin of Continents and Oceans. When it first appeared, in 1915, it was the first serious attempt by a modern Earth scientist to convince the world that continents drift.

  As we shall see, for any imaginary supercontinent to catch the imagination of scientists and public, its most important asset is its name. However brilliant, instinctive and insightful Wegener was as a geophysicist, he understood and cared little for public relations. This was a shame, because had he understood and cared about PR a little more, his theory might well have fared a lot better than it did, especially in America.

  Today, when Pangaea ranks as high on the romance scale as such exotic names as Ushuaia or Zanzibar, it is amazing that Wegener should have introduced it so casually in the final chapter of his third edition. While writing about the single landmass he believed brought all today’s continents together, he simply says, ‘This Pangaea …’ inserting it as a synonym for the long-winded explanation in the sentence before – a synonym he clearly expects his readers to have enough Greek to understand.

  The third edition of Wegener’s book was the first to find an English publisher (Methuen & Co. of London). They engaged one John George Anthony Skerl as its translator, and he changed the Germanic spelling Pangäa to Pangæa, so must therefore be credited with bringing the word into the English language, in 1924. English palates soon found it easier to pronounce it ‘panjeea’, because the same Greek root – Ge, meaning ‘earth’ or ‘land’ – also gives us ‘geology’. Finally the Americans, dispensing with the archaic ligature æ, decided to spell it ‘Pangea’.

  Clinching evidence that Wegener cared next to nothing about his new term is provided in the fourth (and last) edition of his magnum opus, published in 1929. This edition had to wait thirty-seven years before being translated into English and published in the USA. In this edition, however, there is no ‘Pangaea’ in the index; nor in the speculative chapter on the forces that might cause continents to drift; nor anywhere. Wegener just left it out.

  Pangaea consisted of two smaller supercontinents joined at the hip in the region of the Equator: Laurasia in the Northern Hemisphere (North America, Greenland, Europe and much of what is now Asia) and Gondwanaland in the Southern Hemisphere, comprising South America, Africa, India, Australia and Antarctica. The world we see today is no more than Pangaea’s smashed remains, the fragments of the dinner plate that dropped on the floor.

  The main event in this slowest of all unfolding dramas was the opening of the Atlantic Ocean, which split North America from Asia and South America from Africa; though there were many other splits too. India emerged like a slice of pie from where it had lain wedged between Africa, Antarctica and Australia for the best part of 500 million years, drifted northwards across the Southern Ocean and smashed into Asia to make the Himalayas. Australia rifted from Antarctica, taking the Great Australian Bight out of its southern coast, and headed off for South-East Asia. Africa moved north and collided with Europe, a continuing process that will one day close up the Mediterranean.

  These created a whole set of young oceans whose floors are nowhere older than about 250 million years: the date when Pangaea’s rifting began. These ocean floors are forming along spreading centres like the Mid-Atlantic Ridge, scars that mark the original junction between rifted continental fragments, either side of which the ocean floor spreads away, carrying the increasingly distant landmasses with it at about the same speed your fingernails grow. Because the Earth is not getting bigger, one ocean expanding means another is shrinking. This is why, all around the Pacific, the ocean floor is being sucked back down into the planet in a process called subduction.

  That, in essence, is plate tectonics. Because we now know the age of nearly every bit of ocean floor all over the world, it is relatively easy to see how the split took place. The ocean floor is, in effect, a road map showing how the continents have moved into their present positions, like the concentric ridges on the growing plates of a turtle shell.

  If you feed all this information on continental trajectory and speeds of motion and rotation into a computer program like Atlas, the package Roy Livermore helped develop with Alan Smith and o
thers at Cambridge University, you can animate the whole process and watch it unfold before your eyes. It is then relatively easy to run the program forward a little. That is why Livermore can be fairly certain about the way the Earth will look in five million years, the near future to a geologist. It is as objective as a computer model can be. The Atlantic will be a little wider, and Africa will be closer to Europe and Japan to North America.

  But in order to look further and see how the drifting continents, riding the backs of convection currents flowing in the hot rocks of the Earth’s mantle beneath them, will one day recombine, this is not enough. Sooner or later even plate-tectonic motions, so seemingly inexorable, must take a step change. The geologist has to intervene in the model with some educated guesses about these crucial turning points.

  Roy’s decision that the Americas will go on heading west, eating up the Pacific as they go, and crash into the amalgam of Asia and Australia, follows one interpretation of how supercontinents can form. Because it involves all the fragments of a previous supercontinent flying away from one another until they meet again on the other side of the globe (turning the previous supercontinent inside out), this process is called extroversion.

  On the other hand, another process might apply. The old scars of the Earth’s continents are lines of weakness, and for that reason history along them tends to repeat itself. The Atlantic is not the first ocean to have opened between the old continental kernels at the heart of Europe and North America. Several hundred million years ago North America was separated from northern Europe by a wide ocean, much as it is now. This seaway eventually closed to form mountains that were probably as tall as the Himalayas of today. On this side of the present Atlantic we see their eroded remains in Wales, Scotland and Scandinavia. Take away the present Atlantic and this old chain marries up to another old mountain chain in the eastern US. It was split in two when the Atlantic opened through it, roughly (not perfectly) along the same line. It was such geological evidence as this that helped the early proponents of continental drift demonstrate that their impossible-sounding theory might have some truth in it.