by Ted Nield
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.)
One geologist of whom we shall hear more later has already suggested, on the strength of this paper, that we should name the Supercontinent Cycle the ‘Sutton Cycle’. He is Professor Mark McMenamin, of Mount Holyoke College, Massachusetts, who like Sutton forms one half (with Dianna McMenamin) of a connubial scientific partnership. But this coincidence serves to remind us of a problem peculiar to the older team, which might scupper the chances of the term ‘Sutton Cycle’ gaining widespread acceptance.
Just as it is hard to se
e the join between the work of unrelated scientists in the collective activity of science, it is nigh impossible to separate the work of John Sutton from that of his brilliant wife, Janet Watson. Just about everything they did, whether acknowledged in authorship or not, was done together. John’s energy, ambition and drive, and Janet’s daunting clarity of thought, complemented each other perfectly. Neither would have done the work they did without the other, and there is undoubtedly as much of Janet as of John in the great 1962 paper.
Despite the fact that Janet not only joined her husband as a Fellow of the Royal Society but also became President of the Geological Society of London (something John never achieved), there is a fairly widespread belief that Janet Watson still languishes unjustifiably in the shadow of her powerful husband. Today the gender politics surrounding the scientific legacy of Sutton and Watson is every bit as delicate as that of the Cold War. Naming the Supercontinent Cycle for Sutton alone would not be popular in many quarters; but leaving that aside, the case is a strong one. But there may be an even stronger one.
Like Wegener, Sutton had observed a pattern that cried out for a mechanism. Finding out what happened in Earth history is step one in geology; the next step is a search for the reason it happened. Why should mountain building be cyclic? Radioactivity, the discovery that gave Earth science both a clock to measure the Earth’s age and a mechanism to explain why the planet did not just cool down and die, provided Sutton with the mechanism.
But that idea was neither his, nor Janet Watson’s. Moreover, it was the oldest of all, having first appeared in the literature in 1924, waiting for its moment to join, in the right way, with another set of ideas, and finally make new sense.
Trinity – the third man
Trinity College, Dublin, founded by Queen Elizabeth I in 1592, has a tradition of supporting individualistic thinkers. Within its grey granite walls three things came together: long-standing interest in the age of the Earth, the new discovery of radioactivity and John Joly.
One of Trinity’s very first graduates was James Ussher (1581–1656), Archbishop of Armagh and Primate of Ireland. Ussher was a versatile scholar, who set himself the task of analysing astronomical cycles, historical accounts and several sources of biblical chronology, to determine the precise date on which his God had created the Earth. His timetable of creation, Annales Veteris Testamenti, was first published in 1650; but in 1701 it was incorporated into the authorized Bible, and from that time the Archbishop’s calculations came to be seen by believers in much the same dim, religious light.
Although today most people who have heard of Ussher know only about his dating of the Creation to the evening preceding Sunday 23 October 4004 BC, Ussher’s project did not rest on the seventh day. After succeeding in his main task the indefatigable Archbishop went on to date other biblical events as well. Adam and Eve, he decided, were driven from Paradise on Monday 10 November that same year, and Noah’s ark alighted on the summit of Mount Ararat on 5 May 1491 BC (a Wednesday, apparently).
It is far too easy to laugh at the good Archbishop and his pedantic prose today, not to mention the full English title of his work (1658), which reads: The Annals of the World Deduced from the Origin of Time, and continued to the beginning of the Emperour Vespasians Reign, and the totall Destruction and Abolition of the Temple and Common-wealth of the Jews. Containing the Historie of the Old and New Testament, with that of the Macchabees. Also the most Memorable Affairs of Asia and Egypt, and the Rise of the Empire of the Roman Caesars, under C. Julius, and Octavianus. Collected from all History, as well Sacred, as Prophane, and Methodically digested, by the most Reverend James Ussher, Archbishop of Armagh, and Primate of Ireland.
We do not need, after reading that, to go into the details of Ussher’s calculations. Clearly this was a serious scholarly attempt, according to the ruling beliefs of his time, to consult the records of many cultures and answer a nagging question that has only been finally determined by science in the past sixty years.
This question of the age of the Earth was next taken up at Trinity by Samuel Haughton (1821–97), Professor of Geology from 1851, who tried to estimate the Earth’s age by adding up thicknesses of sedimentary strata in the belief that their maximum observed thicknesses would turn out to be proportional to the time it took to deposit them. His immensely laborious arithmetic came out with an Earth age of 200 million years, a figure that then seemed so large he scarcely believed it himself. However, his method depended on so many assumptions about rates of deposition in different kinds of rock that you could, by tweaking the sums a bit, obtain almost any answer you wanted. This did not discourage scientists from trying, and Haughton’s work was continued by another Trinity professor, William J. Sollas (the eccentric father of Hertha Sollas, who translated Eduard Suess’s great book into English). However, it fell to Sollas’s successor, the great Irish geophysicist John Joly (1857–1933), at last to make progress in tackling Kelvin on his own terms.
Birth-time of the world
John Joly claimed descent from a line of King’s counsellors at the French court dating from as far back as the fifteenth century. His mother went by the title of Julia Anna Maria Georgiana, Comtesse de Lussi. But Joly’s father, who died not long after his youngest son was born, lived modestly as a simple country vicar in County Offaly.
Contemporary cartoon of John Joly.
Joly was a remarkable all-round intellectual who made important scientific contributions in geology and physics. But, along the way, he also found the time to take first-class honours in modern literature, to invent the first single-plate colour photographic process, to pioneer the use of radium in cancer treatment, devise new navigational techniques and to write poetry, including sonnets on scientific themes, many of which are much better than merely competent. Like du Toit, he rode a motorcycle and also sailed. Joly was a popular man, with his pince-nez, swept-back hair, walrus moustache and rolled r’s (an affectation he thought helped to disguise a slight lisp; though many wrongly imagined it was a French accent) and he cut a tall, dapper, even roguish figure among the Trinity dons. He was not without his eccentricities either, notable among which was his habit of wearing a radioactive hat to see if he could detect the effect of gamma rays on his memory.
Joly’s was a restless and wide-ranging mind. Like many a don before and since, and despite developing a taste for world travel, he gave Trinity College his life; never marrying, but maintaining an intense long-term friendship with his opposite number in the Department of Botany, Professor Henry Horatio Dixon. Joly worked with the younger scientist on botanical problems, and they are now for ever coupled in the annals of botany for being the first to work out, in 1895, how sap rises. The two men lived close to each other in suburban Dublin, and are today even united in death. Ignoring his friend’s wish to be buried in his native Offaly, Dixon had Joly buried in Mount Jerome Cemetery, Dublin, not far from Trinity.
Though a brilliant technical scientist, at his best when solving problems by devising cunning pieces of equipment, Joly seems to have been a little naive. Like many patriotic men of science with some knowledge of the sea, he wrote letters to the Admiralty on the outbreak of war, one of his ideas being to reduce submarine attacks on British merchant ships by building all British ships in the shape of German submarines. However, like Eduard Suess, Joly was no armchair general and was not above taking to the barricades.
On Easter Sunday 1916 Joly, armed with a Lee Enfield rifle, helped to secure his beloved College against the Uprising that was then raging through the city outside. It was a tense time. By Monday, 2000 Nationalists had taken up strategic positions and their leaders had proclaimed an Irish Republic; but the Uprising lasted only a few days before its leaders surrendered. Fifteen of them were executed and up to 3000 more were interned.
Joly put away his rifle, though his Loyalist sympathies remained with him and deceived him badly. As a futurologist he proved no more successful than Lord Kelvin had been, when he predicted that the Nationalists woul
d never succeed in gaining independence from Britain. Only five years later the Irish Free State was established.
Although Trinity first employed Joly as an assistant to the professor of engineering, and then to the professor of natural philosophy, Joly turned increasingly towards geology and used his own colour photographic process (which he patented in 1894) to produce the first colour pictures of minerals in thin section under the microscope. It was following this work, in 1897, that he bid successfully for the vacant Chair of Geology and Mineralogy. He held the job for the rest of his days, and healing the divide between his two main loves – geology and physics – became a lifelong mission. The age of the Earth was too large a question, and too wrapped in Trinity’s academic tradition, for him to ignore.
Not thy stars
In 1840 mysterious markings had been discovered on some rocks at Bray Head in County Wicklow: marks evidently made by the feeding activity of some long-vanished organism. The trace fossil was called Oldhamia, after the very same Thomas Oldham we have met before in India, but who took up that colonial post with the Indian Survey after serving as Professor of Geology at Trinity.
Joly wrote a sonnet to this humble trace, and Dr Patrick Wyse Jackson, who is today curator of Trinity’s Geological Museum and an expert on Joly’s life and work, believes that it betrays Joly’s special feeling for the immensity of geological time.
Is nothing left? Have all things passed thee by?
The stars are not thy stars. The aged hills