Supercontinent: Ten Billion Years in the Life of Our Planet

by Ted Nield

  Many scientists say it is dangerous to assume that all these soft-bodied forms share any common kinship, even among themselves. In fact, so curious are they that some scientists, among them the formidable German geologist Adolf Seilacher of the University of Tübingen, have put forward the view that they represent a completely unrelated evolutionary group that flourished and then vanished leaving no descendants. Professor McMenamin has taken this view further with his highly controversial theory that Ediacarans represent a unique evolutionary creation: in some ways like animals, but also able to grow like plants by absorbing energy from sunlight.

  The McMenamin theory suggests that in this early shallow-sea environment surrounding the fragmenting supercontinent Rodinia, these unique life-forms lived, happily sunbathing, fixed to the layer of algal slime and lime mud that then coated practically every square metre of the Earth’s shallow sea floor. The McMenamins have called this gentle Eden the Garden of Ediacara: a garden from which these peaceable inhabitants were driven to extinction by two (for them) highly unfortunate biological events. One was the evolution of burrowing (which broke up and destroyed the algal mats on which they sat) and the other was predation. Against these two nemeses the poor Ediacarans had no defence: they were undermined and grazed out of existence.

  When McMenamin got back from his 1995 field season, he enlisted the help of the Mount Holyoke media-relations man Kevin McCaffrey and announced the oldest Ediacaran fossil to the world. To release information in this way is guaranteed to annoy many scientists, who prefer their colleagues to publish their findings in the scientific literature before talking to the media. And sure enough, McMenamin took his fair share of criticism, especially when the story received huge coverage.

  But just as his new fossil’s fifteen minutes of fame were passing, in October that year McMenamin received a communication from James (‘JJ’) Johnson, a central figure in the Urantia movement. In the course of his many interviews with the media, McMenamin had described the supercontinent Rodinia as the cradle of complex life; and the unfolding story had begun to ring a bell with Mr Johnson. For there was, he said, a passage in Section 8, paper 57 of The Urantia Book that read: ‘1,000,000,000 years ago is the date of the actual beginning of Urantia history. The planet had attained approximately its present size … 800,000,000 years ago witnessed the inauguration of the first great land epoch, the age of increased continental emergence … By the end of this period almost one third of the earth’s surface consisted of land, all in one continental body.’

  These quotations are selective, of course, which is always the key to making the prophecies of mystics look ‘uncanny’. If you look at other parts of the same passage from which those quotations come, you can find a rich and colourful mixture of half-correct ideas and plain nonsense. For example: ‘850,000,000 years ago the first real epoch of the stabilization of the earth’s crust began. Most of the heavier metals had settled down toward the center of the globe …’

  Not bad: the separation of iron and nickel to the Earth’s core was indeed an event that took place in the early evolution of our planet, but it happened a lot longer ago than 850 million years. To counterbalance this, as an example of the nonsense among which these little nuggets of correctness lie thinly distributed, we find: ‘Meteors falling into the sea accumulated on the ocean bottom … Thus the ocean bottom grew increasingly heavy, and added to this was the weight of a body of water at some places ten miles deep …’

  But the trick of a successful prophet is to say enough things, and to phrase them sufficiently elliptically, so that the occasional correct hits within the general rambling leap out at the prepared mind – just like cloud patterns, or the face of the Man in the Moon. If you are looking for something, in other words, you will tend to find it, which is the very reason why early-twentieth-century American scientists so mistrusted what they saw as the ‘selective search after facts’ in Wegener’s deductive treatise on continental drift. What this story also reveals is that, unlike any other supercontinent that really existed, Rodinia was not envisaged by scientists and later colonized by mystics (like the zoogeographers’ idea of Lemuria) but apparently independently ‘discovered’ by both groups – and it was the mystics who sleepwalked there first.

  What happened subsequently to Mr Thompson’s communication with Mark McMenamin was ‘business as usual’. The devotee was latching on to science because its current conclusions seemed to offer confirmation of a revealed myth. McMenamin, unsurprisingly, fought shy of Mr Johnson’s invitation to attend a conference for followers of The Urantia Book; but he plainly found the experience thought-provoking, even going so far as to suggest in his book The Garden of Ediacara that it might repay scientists’ effort to trawl through other mystical maunderings, just in case. It is not possible to be entirely certain how serious he is about this idea. I suspect it might fail simply for lack of volunteers.

  What particularly struck McMenamin about the prophecy was that during the mid-1930s – a time when such ideas were distinctly out of fashion – the Urantians had hit upon the existence of a supercontinent dating from one billion years ago (correct), surrounded by a global ocean (obvious, but also correct), at a time when the continents emerged from the ocean more strongly (correct; see Chapter 10) and which subsequently split up about 650 million years ago (about 100 million years out, but still in the right ball park) to form widening ocean basins that became the crucible for the evolution of early complex marine life (also correct). It is also true that until Eldridge Moores and distinguished palaeontologist Jim Valentine wrote their joint paper proposing one in 1970, no legitimate Earth scientist had ever considered the existence of a supercontinent older than Wegener’s Pangaea.

  Palimpsest

  Now that geologists know the age of almost every part of the ocean floor, and can colour it accordingly on ocean-floor maps, it is relatively easy to see how Pangaea fragmented. The ocean floors of the modern Earth are a road map that leads us to Pangaea, by showing us how the modern continents should be put back together. No such map exists for any older supercontinent because the oceans that once opened within them have now all been destroyed, eaten up by subduction and recycled. All that is left of those lost worlds are the broken fragments of ancient continental rock, heavily deformed, embedded within younger rocks, in the shield areas of the world, the ancient hearts of our continents. As the Norwegian geologist Trond Torsvik has written, attempts to reassemble these pre-Pangaean supercontinents ‘resemble a jigsaw puzzle, where we must contend with missing and faulty pieces and have misplaced the picture on the box’.

  Imagine yourself sailing out of a frozen Baltic port in winter, your ferry butting a channel of black water through the thin ice. As you look over the side at the jagged, jostling floes, you can see a mixture of old and young. Young ice, formed since the last boat passed that way and cleared a lane through the chaos, has been broken for the first time. But that previous boat had itself broken through fresh ice. Pieces dating from that event are still floating about, but are now embedded in floes that tell of two phases of fracturing. Still other floes contain ice fragments of three or more distinct ages, having been through the same process several times, on each occasion the freshly re-broken floes becoming re-frozen into new ice awaiting the passage of yet another ship.

  In a similar way the Earth’s shields – the ancient hearts of every continent – bear the remaining traces of all the cycles of supercontinent break-up and coalescence since plate tectonics began. During subsequent history many of the pieces may have been destroyed by erosion (Torsvik’s missing pieces); but, using the evidence that is left to them, somehow geologists must try to work out which parts of each shield were once fused together in a supercontinent at a given time, and how they fitted together when they are no longer the same shape that they later became. It is one of the most intractable problems in science.

  Studying Earth history through interpreting those rocks that have survived is an activity that has a lot in common wi
th the study of ancient texts. Scholars estimate that only 1 per cent of the wisdom of the ancients has found its way to modern times, and the great works of classical antiquity that we have, come to us in the form of documents that were copied, scribbled over and even partially destroyed. Most copies were preserved by pure chance just because of the preciousness, not of the words, but of the material on which they were written.

  Take the example of Archimedes and cast your mind back to the principle of isostasy. Although isostasy applies to the way rocks of different density ‘float’ high or low on the Earth’s solid mantle and thus give rise to either ocean or continent, it is really no more than an extension of Archimedes’ Principle, which states that any floating body displaces its own mass of the substance in which it is immersed.

  Every half-educated person in the world today knows that Archimedes (287–212 BC) shouted ‘Eureka!’ and leapt out of his bath. But what they should also know is that the story began with a problem put to the great thinker by his patron, King Hiero II. The king was worried that a goldsmith whom he had engaged to make a new crown had adulterated the royal bullion with silver, keeping the remainder for himself. How could Hiero be sure?

  Archimedes is reputed to have seen the answer as he lowered himself into his bath, when it dawned on him that every substance has a distinctive density. If you compare the mass of any material with the volume of water it displaces, you have a powerful means of testing its purity, for in the case of gold, any added metal will reduce its density. Having solved the king’s problem, Archimedes developed the idea further in one of his greatest works, the Treatise on Floating Bodies. However, the only copy of that book to survive to our own day in the original Greek is a rather small, unprepossessing manuscript damaged by mould, fire, and twelfth-century religious zealots. It is called the Archimedes Palimpsest, and this precious document came up for auction at Christie’s in New York in 1998.

  Almost inevitably, there was a legal dispute over its ownership (the Greek Orthodox Patriarchate of Jerusalem contending that it had been stolen from one of its monasteries in the 1920s) but the judge in the case decided against the Patriarchate on ‘laches grounds’ (that is, because they had left it too long before asserting a legal right). The palimpsest was eventually sold for two million dollars in October 1998 to ‘an anonymous buyer from the IT industry’. It is now held by the Walters Art Gallery in Baltimore.

  When Archimedes lived and wrote, there were no books like the one you are holding. Archimedes would have copied his theorems and diagrams on to papyrus scrolls, leaving it to succeeding generations to preserve his work by recopying. By the tenth century AD, when what became the palimpsest was originally made in Constantinople, scrolls had given way to more recognizable books composed of leaves of parchment (the preserved skins of sheep, goats and cows) bound between wooden boards. The emperor-scholar Constantine VII Porphyrogenitos and his successors put many scribes to this kind of work, thus rescuing for future generations the rarefied intellectual works of antiquity.

  The project was not, however, fully effective, because 200 years later Archimedes’ great book was cut up and reused. Autres temps, autres mœurs; the great barbarian invasion that was the fourth crusade had sacked Constantinople. In one of the worst disasters ever to befall European culture, many manuscripts were destroyed and the Archimedes Palimpsest only survived by chance. The new priority of the age had become the saving of souls; and so Archimedes’ text became a Euchologion, a prayer book. The new writer took Archimedes’ treatise to pieces; scraped off the writing (‘palimpsest’ is Greek for ‘scraped again’), cut out the pages, folded them to half the size, wrote over the original text at right angles and then reassembled the book in its new form. Although this act now seems like desecration, it probably saved the original, since the palimpsest eventually found a home in the Convent of the Holy Grave in modern Istanbul, where it was rediscovered in 1907.

  Dr Reviel Netz, Professor of Ancient Science at Stanford University, is a world expert on the works of Archimedes. He has written of the palimpsest: ‘A manuscript is a window into the past. It allows us to get a view of a lost world. Some manuscripts provide us with an indirect view only, others with a better picture. What scholars do is to put together all the evidence available, to form a single picture of the past.’ Netz could just as well have been describing the work of geologists piecing together the pre-Pangaean supercontinents.

  The Earth presents us with the most complex palimpsest of all; it is a text that has been written over, erased, defaced, cooked and reheated; its binding has been broken, its pages lost and shuffled. Each overwriting further obscures everything that has gone before; so that what was originally written may never, indeed, be fully decipherable.

  Orogenous zones

  Radiometric dating provided geologists with the first clue about the existence of older supercontinents: the clustering of radiometric ages noted by John Sutton throughout the long ‘Precambrian’ period providing the first hint that mountain ranges, that may today be widely separated from one another, might once have been joined and shared a common origin.

  When supercontinents form, all the continental blocks of the Earth come together in a big crunch, eliminating the oceans between them and building mountains in their place as the jaws of the tectonic vice come together. Rocks are pressure-cooked in the roots of each new mountain chain, and radiometric clocks reset. This process is long and complex and does not all happen at once. On the modern Earth, for example, the next supercontinent has already begun to form, following India’s collision with Asia and Africa’s with Europe. Many other collisions, spread out over the next 250 million years, will take place before the point at which the supercontinent achieves what geologists call ‘maximum packing’. But the Earth has a lot of time on her hands. Even dates that fall within 100 or 200 million years of one another, will look clustered within a timespan of (by then) almost five billion years.

  Rodinia seems to have formed 1–1.3 billion years ago, as indicated by the clustering of dates. These dates are known as ‘stabilization ages’ because they mark the point in the mountain-building process when the radiometric clocks were reset. Those rocks that show the joins in this great global collision occur all over the world, but the event itself (called an ‘orogeny’ because it created mountain ranges) is named for the Grenville Belt of eastern North America. The Grenville Orogeny was what created the supercontinent of Rodinia.

  Rocks of this Grenvillian Orogeny are hidden under younger deposits across the eastern and central United States, but crop out in New England, the Blue Ridge Mountains and west Texas. They extend up the great peninsula of Norway and Sweden, as well as down through eastern Mexico, where, once again, they lie mostly hidden under younger rocks laid, like some subsequent historical text, on top. They then skirt the western edge of Amazonia, passing through Bolivia, before diving again beneath younger rocks to the south.

  Across today’s Atlantic they crop out in Mozambique and Natal, as well as South Africa. In India they are found in the Eastern Ghats. In south-west Australia they are seen in the Darling Belt (which skirts the Yilgarn Craton, the country’s richest mineral region, turns the corner at Perth and stretches up the western half of the Great Australian Bight as the Fraser and Albany belts). And because the Bight is the hole out of which East Antarctica was bitten when Gondwanaland broke up, parts of coastal East Antarctica also display rocks whose deformation histories match their Australian counterparts precisely.

  So, having found the scars where the supercontinent’s component cratons were sutured together into the Rodinian quilt, the next question to be faced is, do they join up, and if so, how? This is much warmer work, and to help them geologists must engage the help of the Earth’s changing magnetic field.

  Jigsaw

  One of the most important geophysical tools to emerge through the 1950s and 1960s involved the discovery that many rocks preserve a trace of the Earth’s magnetism as it prevailed when they formed (or beca
me ‘stabilized’, depending on what kind of rocks you are dealing with). Everyone knows how you can destroy a magnet by heating it, because magnetization depends on the alignment of atoms, which heating disrupts. Rocks are not normally thought of as magnetic, but don’t forget that the original ‘magnets’ that humans discovered were lodestones, natural pieces of the iron mineral magnetite. Many rocks are rich enough in magnetic minerals for them to become very weakly magnetized in the Earth’s field.

  Rocks that get very hot, such as lavas, or rocks that are cooked deep inside mountain ranges, have to cool down below a certain temperature (called their Curie Point, after Pierre Curie (1859–1906), who discovered the phenomenon) before they can become magnetized. Sediments can also display a weak magnetization, because grains of magnetic minerals will become aligned as they settle out, lending the whole rock a weak magnetic imprint.

  When geologists take carefully oriented samples from these rocks and put them into very sensitive magnetometers, they can work out the ancient latitude of the continent – hence where the North Pole was, relative to the continent at the time the rock formed – and the continent’s orientation. Because continents drift, their position relative to the magnetic poles changes constantly. Combining palaeomagnetic data with radiometric ages therefore allows scientists to track the movement of a continent over a given time, though by convention they actually do it by pretending that the continent had stayed still and the pole did the wandering.