Supercontinent: Ten Billion Years in the Life of Our Planet (19 page)

These two ancient realms were found to broadly parallel the shores of the modern Atlantic Ocean and were described by Charles Doolittle Walcott (1850–1927). Walcott, who had received little formal education, rose to become Director of the US Geological Survey in 1894 and was perhaps one of the most industrious people ever to do and administer science in the United States. He named these assemblages the ‘Pacific’ and ‘Atlantic’ provinces; rocks in North America containing the Pacific assemblage, and rocks of the same age in Europe containing the Atlantic.

Had this split been perfect it would have raised no eyebrows among continental fixists because the division would have been easily explained by the present arrangement of continents and oceans. Unfortunately there were some distinctly awkward exceptions to the
rule. In some places in Europe, such as the north of Scotland,
geologists
found rocks with typical ‘American’ fossils in them, while in some places in North America rocks turned up containing typical European species. In an echo of one of the two scenarios that puzzled Victorian biogeographers, things were being found close together that should, by their differences, have been far apart; but with the added twist that, by and large, they usually
were
far apart.

This conundrum could be explained, Wilson reasoned, if the
present
Atlantic Ocean was not the first to have separated its opposing shores: if there had been an older Atlantic, which had closed and then reopened to form the modern one. According to his idea, the old Caledonian–Appalachian mountain chain had formed as the vice shut for the first time, eliminating a now long-vanished ocean that Wilson called the ‘proto-Atlantic’. But when this suture had reopened, more or less (but not perfectly) along the same line, some of the rocks squeezed between the forelands had stuck to the
opposite
jaw of the vice, stranding some American fossils on the European side and vice versa. The fossil distributions were saying that there had been continental drift
before
Pangaea. Moreover, if this particular example could be extended into a general rule, mountain building itself was inherently cyclic. This process, involving the repeated
opening
and closing of oceans along ancient lines of suture, has since come to be known as the Wilson Cycle, a term first used in print in 1974 in a paper by Kevin Burke and the British geologist John Dewey.

Wilson did not address another interesting problem, which was the question of exactly
where
on the Earth all this pre-Pangaean action had played out. From the geological evidence it was clear which
continental
blocks had done the colliding: which had acted as the jaws of the vice. But where had they been on the globe at that time?

Wilson did not address this issue because (as van der Gracht might
have said) the relevant facts were too little known. They were not long in coming. It soon turned out that Wilson’s ‘proto-Atlantic’ had in fact been sitting right at the bottom of the world. Before ‘our’ Atlantic had opened, the two jaws of the vice (now represented by North America and Eurasia) had not only opened and closed (and thus helped build Pangaea) but had since migrated north together as far as the Tropic of Cancer before deciding to reopen hundreds of millions of years later, in the great Pangaean split-up.

Wilson’s name for this ancient vanished ocean, the ‘proto-Atlantic’, soon came to seem inappropriate, particularly since the same name was coming to be used for the early stages of the formation of the
modern
Atlantic. Wilson’s ocean had been squeezed out of existence by about 400 million years ago: 200 million years before the present Atlantic had even begun to form within Pangaea; so it was no true ‘proto-Atlantic’ in any real sense. Therefore, in 1972, Wilson’s Ocean was renamed Iapetus, which maintains a shadow of the Atlantic link, since in Greek myth Iapetus, son of Earth (Ge) and Heaven (Uranos), was brother to Tethys and Okeanos, and father of the Titan, Atlas.

However, Wilson’s great idea was a crucial step forward. It reopened the whole question of ‘what happened before Pangaea?’ By suggesting that his ‘proto-Atlantic’ had opened within an earlier supercontinent (just as the modern Atlantic did within Pangaea) he also linked his process to a grander cycle leading from one
supercontinent
Earth to another.

Wilson’s originality consisted chiefly of being among the first to consider pre-Pangaean plate tectonics; but it would be stretching things a little to name the whole Supercontinent Cycle after him because his model refers only to ‘introversion’: the opening and
closing
of an ‘interior ocean’, one which opens within a fragmenting supercontinent. And that, as we have seen, is only one way a
supercontinent
can re-form.

In 1919 the Sutton’s Seeds dynasty was blessed with a son. Unfortunately for this British business, which still flourishes today, it would have to do without the drive and determination of John Sutton (1919–92), who would instead devote his talents to the study of the oldest rocks on Earth. He would eventually join the long line of charismatic leaders of Imperial College London’s Royal School of Mines, including Thomas Henry Huxley and Sutton’s predecessor, Herbert H. Read. He would also marry his near contemporary, Janet Vida Watson (1923–85), to forge perhaps the most formidable
husband
-and-wife team in geological history.

It was not always easy to work with the great Professor, who
suffered
from sudden fits of incandescent rage. As his obituarist Professor Dick Selley recalls: ‘To be one of his students was like living on the slopes of a volcano. The soil was fertile, the view awe-
inspiring
, but long periods of productive calm could suddenly be punctuated by an eruption.’ Collaborating on almost everything, Watson and Sutton together pioneered the study of the ancient,
complex
rocks of the Precambrian, but their aim was clearly summed up in the title of a lecture Sutton gave in 1967, ‘The extension of the
geological
record into the Precambrian’. Their aim was to learn how to extend the familiar picture of vanished oceans and the mountain ranges that grew up in their place, back into that mysterious age.

As the great physicist Ernest Rutherford had realized, physics had presented geology with an infallible clock by which to settle the
long-standing
argument about the absolute age of the Earth. Radioactive elements decay at known rates to products that either themselves decay, or which are stable, in which case the cascade, or ‘decay series’, comes to an end. The rate of decay is measured in terms of how long it takes a given amount of the radioactive element to be reduced by half, that period being called the element’s ‘half-life’. Half-lives vary
widely in length. The longest-lived atoms of seaborgium, for instance, have a half-life of thirty seconds, while element 104, the one that now bears the name rutherfordium, has a half-life of 3.4 seconds. But these elements, which come into existence for seconds and then just as
rapidly
decay to something else, cannot exist in nature. Naturally occurring radioactive elements tend to have much longer half-lives, some very long indeed.

To date a piece of rock from its content of a radioactive element, you need to compare the amount of decay product with the amount of the preceding element in the decay series. Then, by knowing the rate of decay (the half-life), you can work out how much time must have elapsed since the rock reached its final form. You have to choose your radioactive element carefully, because, just like clockwork clocks, radioactive ones run down at different rates. You have to choose one that runs for the sort of timespan you wish to measure.

According to Rutherford and his contemporaries, atoms could be thought of as being made up of three basic particles. In their scheme a central nucleus contains positively charged protons with a mass of one and may also contain particles called neutrons with the same mass but no charge. Orbiting the nucleus are a number of negatively charged electrons, whose combined charge normally matches the combined positive charge of the protons. Unlike protons and
neutrons
, however, electrons have negligible mass.

The number of protons is constant for any element; but elements can contain different quotas of neutrons. As neutrons have mass but no charge, this means that, in nature, some atoms of some elements may differ slightly in weight from others. These forms with different atomic weight are called ‘isotopes’ of the element because, although different in mass, they all have more or less the same (‘iso’) chemical properties typical of that element; hence they all occupy the same place (‘topos’) in the Periodic Table. (Because their weights are
different, though, the physical properties of different isotopes are often different, which makes them very useful in geology.) Some
isotopes
of normally stable elements may also be radioactive: for example, carbon, the element of life.

Carbon exists naturally as three isotopes of differing atomic weight; carbon 12 (the most common), carbon 13 (1.11 per cent of all carbon) and carbon 14 (0.0000000001 per cent). Carbon 14 is
radioactive
and is continually being formed by cosmic rays bombarding the atmosphere. Neutrons streaming in from space sometimes hit atoms of another element, nitrogen 14, knocking out a proton in the process and creating an atom of carbon 14. As soon as you absorb this carbon 14 – say, when you eat a lettuce that has first absorbed it from the atmosphere – you make that carbon 14 your own. It then begins its slow decay back to stable nitrogen 14 inside your body; but your overall levels of carbon 14 do not change because you top up your levels every time you eat. All food will do this for you, because
everything
that lives absorbs carbon 14.

But when you die, all the carbon 14 in your remaining flesh and bones goes on radioactively decaying to nitrogen 14; so a test of the carbon 14 in your mortal remains will enable a scientist to determine how long it has been since you ate your last meal. The half-life of carbon 14 is about 5500 years, making it an ideal tool for
archaeologists
interested in dating once-living things, though these cannot be very much older than about 50,000 years (by which time there’s too little carbon 14 left for the technique to work).

Radiometric methods used by geologists to date rock samples are basically the same but depend upon the decay of long-lived elements and their isotopes: substances with decay rates measurable over
hundreds
of millions, billions or even tens of billions of years. As radiometric dating came to be applied to different rocks all over the world, the first and most dramatic conclusion was that the Earth was
definitely not tens of millions of years old, as Lord Kelvin had insisted. Nor indeed was it hundreds of millions of years old, as
geologists
had suspected. The Earth was
billions
of years old.

Geologists had been more than vindicated; in fact, having grown comfortable with their estimate of ‘hundreds of millions of years’, they were now presented with a positive embarrassment of time. What is more, nearly all of that embarrassment appeared to fit into the rocks that geologists had until then lumped together in a tiny section at the base of their stratigraphic tables and known (or rather dismissed) as ‘Precambrian’. Geologists recognized with some horror that the greater part of Earth history had in fact been
written
long before complex life had even evolved; that is, before the rocks of the past 542 million years were laid down; which was to say, before those rocks they knew most about even existed. This was a real shock.

The base of the Cambrian Period had been defined according to the earliest appearance of abundant fossils; an evolutionary event caused by the development of hard skeletons that can fossilize
readily
. Precambrian rocks seemed at that time to be unfossiliferous. There was no reliable way of dividing up these cryptic, complex rocks until radiometric dating came along. And as more Precambrian dates were added to the collection, geologists began to notice a pattern. The dates were not evenly spaced through the 4200-million-year time span. They were clustered.

The older a rock is, the more likely it is to have been buried, cooked up under conditions of extreme heat, pressure or both, partially or completely melted, folded and stretched, and mixed up in the tectonic storm that is mountain building. Every such event will reset the atomic clocks, ticking away within the rock, to zero; so the primary radiometric dates obtained from rocks of this great age do not record the date of their
original
creation, but the date at which they became
stable in their present form. In other words, these rocks’ radiometric ages refer to the episodes of mountain building in which they have been caught up. The apparent clustering of ages from ancient rocks all over the world, and the broad agreement of these date clusters between different modern continents, soon began to look meaningful. Mountain building, which today we would think of in terms of the collision of tectonic plates, was episodic. And if that periodicity turned out to be regular, which it apparently did, for ‘episodic’ you could read ‘cyclic’.

And so it was, three years
before
Tuzo Wilson published his groundbreaking
Nature
paper on the ‘proto-Atlantic’, John Sutton published in the same journal a four-page paper titled ‘Long-term cycles in the evolution of continents’. In this visionary extrapolation from the global radiometric clustering pattern, Sutton suggested that there was a grand periodicity in mountain-building activity of
perhaps
750–1250 million years. His data suggested that ‘a structural rhythm of longer duration than the orogenic cycle’ might have repeated itself at least four times since the Earth formed.

Sutton termed this the Chelogenic Cycle, because it had been detected in the rocks making up what geologists call the ‘shield areas’ of the Earth, the ancient kernels of the modern continents. (
Chelos
is ancient Greek for shield, by analogy with the carapace of the
tortoises
(
Chelonians
to zoologists) and the defensive posture that Greek soldiers adopted sheltering underneath many shields when under fire.)