Supercontinent: Ten Billion Years in the Life of Our Planet (21 page)

If this sum is done carefully, accounting for all the reverse
mechanisms
that may return salt to the continents (such as the creation of shallow evaporating seas like the Zechstein) what it actually measures is the average residence time of a sodium atom in the ocean. That is neither an uninteresting nor trivial fact, and the correct average figure is probably around 250 million years, or about the same time that has elapsed since Pangaea began to break up. Unfortunately, it has
nothing
whatever to do with the age of the Earth. Halley’s and Joly’s initial assumptions were just as wrong as Kelvin’s.

However, Joly did not know this; and when he performed the
calculation
he came up with an estimate of eighty-nine million years, which was near enough ninety million, which seemed near enough to the 100 million years that geologists had got used to before Kelvin reduced his estimate. When Joly published this research with the Royal Dublin Society in 1899, it was hailed immediately. What was more, geologists (who had chafed under Kelvin’s yoke for long enough by this time) at last saw the good Lord being tackled on his own, quantitative, terms – and found it good.

Kelvin’s reign was not to last; though, instead of succumbing to attack from without, his chronology collapsed from within. The dawn of the new century brought interesting times for physics, when, with the discovery of radioactivity, subatomic particles and relativity, physicists suddenly realized they actually knew a lot
less
about the
world than they had thought. Radioactive decay not only provided the tools to solve the age of the Earth problem once and for all but gave the planet the independent internal source of heat that fatally wounded Kelvin’s method and made his hitherto infallible
conclusions
seem as nonsensical as Archbishop Ussher’s. Joly, who corresponded with all the great physicists of his time, was well up to speed with the new thinking. He quickly saw that the Earth could at last be very old indeed. You can sense the excitement in his writing: ‘No! The slow exhaustion of primitive heat has not been the history of our planet. Our world is not decrepit by reason of advancing years. Rather we should consider it as rejoicing in the gift of perpetual youth …’

And he went on: ‘Endlessly rejuvenated, its history begins afresh with each great revolution.’

Joly’s first great insight into the uses of radioactivity concerned
something
he and other geologists had seen under the microscope, in rocks cut in thin section so that light could pass through and allow all the mineral crystals within to be identified.

For some time it had been observed that crystals of a kind of mica called biotite, which has a beige-brown colour in transmitted light, sometimes seemed to exhibit a rash of dark, circular spots that looked rather like some virus infection on the leaf of a plant. Minerals under the microscope can change their colour when the viewing stage is rotated in polarized light, a phenomenon known as pleochroism. The puzzling spots therefore received the lovely name of pleochroic haloes.

Before the discovery of radioactivity, it was thought that the haloes must result from chemical diffusion of some sort, from whatever lay at the halo’s centre, rather like an ink drop on blotting paper. Looking
carefully at the haloes, however, Joly realized that this could not be. The haloes were not just fuzzy diffuse blots but were made up of many concentric rings, more like the rings of Saturn. In three
dimensions
, what presented to the microscopist as two-dimensional rings were, in fact, spheres; and Joly was the first to realize that they formed because a radioactive source at their centre had been sending out high-velocity particles into the surrounding crystal. The colour change was an optical distortion caused by the microscopic damage inflicted by these emanations. What was more, the concentric rings represented different travel times within the surrounding lattice, which meant that different kinds of radiation were being emitted, each with different powers of penetration. From this Joly deduced that it should be possible to work out what the radioactive element was that had given rise to the haloes, since each radioactive element has a
distinctive
radiation signature.

And so it was that Ireland nearly entered the Periodic Table of the elements, because much of Joly’s data defied ready analysis and at one stage he mistakenly thought that he had detected a new element with a hitherto unseen radiation signature. He proposed the patriotic name of Hibernium for this new element; but alas, it turned out that the
element
in question was already known.

Most pleochroic haloes in biotite are caused by tiny crystals of the mineral zircon enclosed within the mica. Zircon, often used today in jewellery as a substitute for diamond, is chemically zirconium silicate, but within its crystal lattice it is quite common for some zirconium atoms to be replaced by atoms of the naturally occurring radioactive elements uranium and thorium.

Joly, always on the lookout for a new physical measure of the Earth’s age, also had the idea that he might be able to use the haloes to date the rocks that contained them. Reviewing various dating methods in 1914 he wrote:

Working with Ernest Rutherford, Joly published the results of this method in 1913, using uranium haloes in micas from County Carlow. The research, he wrote, suggested ages of ‘from 20 to 400 millions of years’; the halo method, also, was disappointingly vague. But it was, at least, pointing in the right direction. The rock in question was
actually
about 375 million years old.

But Joly’s inventive mind had spotted still more implications of radioactivity for geology. He had realized that more heat was being generated under the continents than was actually being caught in the act of escaping.

In his 1924 Edmond Halley lecture to Oxford University, ‘Radioactivity and the surface history of the Earth’, Joly told his audience how radiogenic heat had a tendency to build up beneath the insulating cover of the Earth’s continental crust. If that went on unchecked, something would have to give. After hundreds of millions of years, Joly believed, rocks deep under the continents would start to melt. The overlying crust would then break up and massive
outpourings of lavas – such as are seen in India’s Deccan Traps, or the older Siberian Traps, whose eruptions coincided with the
breakup
of Pangaea – would herald a period of great tectonic instability and fluidity, even rendering possible the outlandish idea of
continental
drift.

Two years later Joly sat in his study in Trinity College’s Museum, writing his contribution for van der Gracht’s symposium volume. Though his powers as a scientist were diminishing, he remained open to new ideas and wrote that his theory of recurring cycles of
sub-crustal
melting now threw the question of continental drift wide open. Offering van der Gracht what support he could, he wrote that the acknowledged fact of radiogenic heat meant that it was now at last ‘legitimate to enter upon the problems arising out of continental movement’.

Joly had realized that radiogenic heat, building up inexorably inside the Earth, governed the way the Earth’s great heat engine worked. Like all the best scientific ideas, it was incredibly simple. All it needed was the continuous generation of heat, and a thermal blanket to stop it getting out. Van der Gracht noted with satisfaction: ‘If radioactive heat does accumulate in the manner discussed, a periodic
displacement
of the blanketing Sial floats [continents] becomes a requisite …’

Today Joly’s mechanism is still the accepted basic explanation of why supercontinents break up. A supercontinent sits over the warm Earth like a fur cap sits on your head, holding in heat. However, unlike a fur cap, eventually the supercontinent must break up because the heat has nowhere else to go but up and out. Magmas generated at depth break through the crust and create massive outpourings;
convection
rising deep below the continent tears it into many smaller landmasses, and sends them off like flotsam in a stream. New oceans begin to open within the supercontinent, whose fragments then, at the speed of your growing toenails, either race all the way around the
globe to meet one another on the other side (extroversion), or stall and shrink back on themselves, consuming the young interior oceans in the process as Tuzo Wilson predicted (introversion).

Of this great cycle of the making and breaking of continents, Wilson glimpsed a part. Before him, John Sutton, with his clustered radiometric dates from the Precambrian, outlined the whole. But almost forty years before the plate-tectonic revolution, John Joly already had the explanation of
why
the Earth’s grandest pattern
operated
: by predicting it from basic physics.

The wrong-way telescope of time, which makes the distant seem more distant still, has rendered Joly a sadly diminished figure in our view of history. Yet, if Wegener had discovered a phenomenon
looking
for a mechanism, Joly discovered a mechanism in search of a process, conceiving an idea that would sleep in the literature until its time was right. Perhaps now that Joly, who also published on possible life on Mars, has had a crater there named for him, the time is right to add to this the honour of the earthly Supercontinent Cycle; for it was his vision that predicted it, and still drives it.

The poet in him certainly rejoiced that the dull fate of gradual
heat-death
did not, after all, await our beautiful planet. He wrote presciently: ‘Our geological age may have been preceded by other ages, every trace of which has perished in the regeneration which has heralded our own … a manifestation of the power of the infinitely little over the infinitely great – the unending flow of energy from unstable atoms wrecking the stability of the world.’

Our planet had, Joly saw, an inner life: a life whose warmth demanded a long-term cycle of tectonic activity. Like Halley’s comet, supercontinents would keep returning. Mother Earth had a pulse.

In 1934, four years after Wegener met his death on the Greenland icecap and drift theory was perhaps at its lowest ebb, cosmic forces were at work. They were busy causing a set of divinely inspired papers to be translated from the ineffable language of the Universal Father into English, by a complex series of intermediary processes
administered
by an ‘editorial staff of superhuman beings’.

At least, that is the view according to followers of the resulting tome, known as
The Urantia Book.
Like other new religions, its followers make the claim that the teachings contained in its 196 papers are
literally
true. As Harry McMullan III writes in his
introduction
to
The Urantia Book
, it ‘claims to describe reality as it actually is’.

Describing reality as it actually is is, of course, what scientists think they are attempting, though as a rule, if the results involve
superhuman
beings at all, they tend not to set much store by them. Rather, scientists rely on thinking things out for themselves, producing original ideas that explain, as closely as they can manage, the
phenomena they observe. As the motto of the world’s oldest scientific society, the Royal Society of London, has it, ‘
Nullius in verba
’, or, loosely translated, ‘Take nobody’s word for it’ – and by nobody they really do mean nobody.

It is quite the reverse of the revelatory approach, where in the beginning there always tends to be somebody’s Word, which tends always to be with the writer, celestial or otherwise, and with which there can therefore be no argument.

So how spooky must it have been for geology professor Mark McMenamin of Mount Holyoke College, Massachusetts, to discover in 1995 that much of his work to date had apparently been predicted by a sleep-talking mystic from Illinois claiming to be in contact with a Universal Father and his superhuman editorial department?

Mark McMenamin researches rocks from another time, long before the time of Wegener’s Pangaea, when all (or most) of the
continents
were fused into one giant mass. It was also McMenamin who, in notes from 1987, first hit upon a name for it, calling it Rodinia, which he published in a book written in 1990 with his wife Dianna called
The Emergence of Animals – the Cambrian Breakthrough
. By the mid-1990s the name had stuck, particularly (one suspects) because so many of the scientists who work on rocks of this age are Russian. For it derives from the Russian noun
rod
, meaning family or kin; hence the Russian verb
roditz
, which means to give birth to, which in turn gives rise to the noun
rodina
, meaning birthplace, or native land. The McMenamins’ Rodinia was the supercontinent around whose shores, and during whose fragmentation, complex life first evolved towards the end of the Precambrian.

Native lands are important to us all, whether we happen to be a French Huguenot living in South Africa or the USA, or a Jewish Viennese born in London. Ultimately the concept is meaningless because, somewhere along the great chain of being,
everyone
has come from
somewhere else. But we are all products of the evolution of complex life. Rodinia is the oldest known supercontinent upon whose former existence scientists more or less agree, and so Rodinia can indeed be said to be the birthplace of us all – and of every moving creature upon the Earth.

In 1995 Mark McMenamin made an extraordinary fossil find while doing fieldwork in Sonora, Mexico. It turned out to be the oldest known example of a group of enigmatic, long-extinct fossil creatures, which existed before the major divisions of the Animal Kingdom, as we know them today, came into being. He had found the world’s oldest Ediacaran fossil.

Nobody really knows what the Ediacarans were, so opinions on the subject among palaeontologists are strong and divided. When I was a student in the 1970s they were known from just a few places
worldwide
, including Charnwood Forest in the UK and the Ediacara Hills in the Flinders Range of South Australia, after which they were named. But these rare discoveries had occurred in the 1940s and 1950s. Ediacarans were rare, perplexing and, above all, famous. New finds were like hen’s teeth.

So there was great excitement in 1977 at Swansea University when one of my lecturers Dr (now Professor) John Cope – a man whose fossil-finding talents are almost supernatural – discovered some new examples of these creatures right on our doorstep just a few miles from the sleepy market town of Carmarthen. The find was also completely unexpected, as it came from rocks that the Geological Survey had long previously mapped as Ordovician, and was made as part of a mapping project that Cope had begun with a group of
amateurs
. Needless to say, as soon as the find’s full significance was realized a mechanical excavator was brought in. The whole lot was
shipped back to the university for painstaking professional research and a preliminary note of the find to be made in
Nature
.

Ediacaran forms – some palaeontologists feel unable to say for
certain
whether many of them were animals at all, in the modern sense – display a variety of body plans. To be sure, some may have been the ancestors of later animal groups such as the trilobites; but others seem to show no obvious affinity with any other animal, living or fossil. This was a moment in Earth history when many different forms of life evolved, some highly peculiar when seen alongside modern life, and seemingly showing little or no kinship with anything we would feel comfortable calling either animal or plant.

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.

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.