Geoscientist Online

This month sees the 100th birthday of the science that put an end to uncertainty over the age of the Earth. Cherry Lewis* celebrates.

Geoscientist 21.05 June 2011

I was recently asked what I thought was the most important 20th Century development in geology. I replied, without hesitation, that it was the discovery of isotopes in 1913 by Frederick Soddy (1877-1956). Without them, we might still be in the dark ages, geologically speaking.

Today some 40 different isotopic techniques help us not only to determine the age of rocks, but also to understand the geological and environmental processes of our planet and beyond. They help us reconstruct past climate so that we can predict the climate of the future; they explain the formation of the chemical elements and are now widely used in many other sciences such as archaeology and in medicine. Geochronology, however, is where they first came into their own.

Today we take isotopic dating very much for granted, so it is all the more remarkable to realise that Arthur Holmes (1890-1965) dated his first rock two years before the discovery of isotopes. It is the publication of this first date and geological timescale, exactly 100 years ago this month, that we are celebrating in this issue of Geoscientist and which the GSA will commemorate by holding the Pardee Symposium in Holmes’s honour this October.

In March 1911, after three months’ laboratory research at Imperial College (where he was studying for a BSc in geology) Holmes wrote up the results in his first-ever paper, The Association of Lead with Uranium in Rock-Minerals, and its Application to the Measurement of Geological Time. His diary records that he wrote this seminal work in just a single morning - on March 9. A few days later, on the eve of his departure for Mozambique to prospect for minerals on behalf of the Memba Minerals Company, he went to say goodbye to his supervisor, physicist Robert Strutt (1875-1947), who “was pleased with my paper and offered a few suggestions which I adopted”. Strutt submitted it to the Royal Society on March 20 and, in Holmes’s absence, read it there on April 6. Published in the Proceedings on June 9 and on August 3, Holmes received a dozen copies from Strutt while still in deepest Mozambique.

In the introduction to his paper Holmes explained why he was analysing minerals that contained uranium and lead: “Such minerals may be regarded as storehouses of the various series of genetically connected radioactive elements. In them the parent element slowly disintegrates, while the ultimate products of the transformation gradually accumulate. The analysis of these minerals ought, then, in the first place, to disclose the nature of the ultimate product of each series; secondly a knowledge of the rate of formation of this product, and of the total quantity accumulated, gives the requisite data for calculation of the age of the mineral.”

RADIOACTIVE DECAY

Not only did he want to date the rock, he also wanted further support for the theory that lead was the ultimate decay product of uranium. We should not forget that only 15 years had passed since the discovery of radioactivity, and it was a mere nine years since Ernest Rutherford (1871-1937) had recognised the phenomenon of radioactive decay Thus, the mechanism by which decay happened, and the products that resulted, were still poorly understood.

Strutt claimed that as soon as he read of Rutherford’s discovery that helium was a product of radioactive decay he realised at once that it could be used to measure geological time, but Rutherford beat him into print by publishing the very first radiometric age in 1904. Strutt, however, quickly recognised that helium, being a gas, was escaping from the minerals that contained uranium and that the dates obtained could therefore only be minimum ages. Holmes was set the task of finding a better method with which to date rock minerals, and suggested he try to improve on the 1907 work of Bertram Boltwood (1870-1927), which indicated that lead was uranium’s ultimate decay product.

So, back in January of that year, Holmes had cut short his Christmas holiday in Gateshead to return to Imperial College and start his research. Although the physics laboratory was new at the time of Professor Strutt’s appointment –1907, the same year that Holmes joined Imperial – there was still a serious shortage of apparatus. By 1908 this had become acute. Strutt wrote to the college authorities: “[We] are at present largely subsisting on loaned apparatus, some of which belongs to other public bodies, such as the Royal Observatory, the Royal Society, etc., while some has been borrowed of private friends. I need hardly say that it seems rather below the dignity of an institution like the Imperial College that its teachers should have to beg apparatus of their personal friends for the purpose of teaching the students.”

This appeal to their dignity had the desired effect and elicited a grant of £700 for special equipment, plus £800 for annually recurring expenditure. By 1911 Holmes had access to some of the best equipment available at the time, carefully wrapping it up again each time he used it.

Holmes set out to determine the age of a thorite-bearing nepheline-syenite from Norway, believed to be Lower Devonian in age, in which he considered there were no less than 17 different minerals bearing uranium and lead. From Strutt’s work on helium, and assuming that the rate of decay was constant over time (which was still not accepted by some), Holmes calculated that “a gramme-molecule of lead would take the place of a gramme-molecule of uranium in 8200 million years”, and that the age of a mineral could be given as: “Pb/U.8200 x 106 years, where Pb and U represent the respective percentages of these elements at the present day”. Another assumption he had to make was that all lead originated from the decay of uranium – which of course we now know is not so.

Holmes spent many hours painstakingly separating these minerals from the crushed rock before performing exquisitely delicate chemical preparations to isolate the elements required for measurement. First, the amount of uranium present was measured - not directly, but by gauging the amount of radium emanation (radon) being emitted (the known constancy of its ratio to uranium being used to give the amount of uranium present). Having crushed up the mineral extracted from the rock, first in an iron mortar and then more finely in an agate one, the resulting powder was “fused with borax in a platinum crucible, and the resultant glass dissolved in dilute hydrochloric acid. After boiling and standing for several days in a corked flask, the radium emanation was boiled out, collected in a gas-holder, and ultimately transferred to an electroscope” to measure the amount of radon present.

FIRST DATE

While waiting for the radon to accumulate, the amount of lead was measured using either the gravimetric or colorimetric method. These chemical techniques required extraordinary dexterity and were incredibly time-consuming. Not only that, but in order to verify the results, analysis of each mineral was repeated between two and five times, depending on how much material was available. At one point Strutt made Holmes discard all the data and start again, because radon was found to have been leaking into the room, contaminating everything and giving spurious results. Holmes had to go cap in hand to Dr Prior at the Natural History Museum and ask for more sample. Eventually, however, he calculated the average U/Pb ratio from these minerals to be 0.045; the first rock Holmes ever dated was determined to be 370 million years old.

In his 1907 paper, Boltwood had collated all the published analysis of uranium-bearing minerals which also included a determination of lead – all eight of them – and this had allowed him to calculate the uranium-lead ratios and ages of eight rocks but, as Holmes pointed out in his 1911 paper, “Unfortunately, he omitted to give [their] geological ages”. Boltwood was a chemist and not really interested in the geological significance of his work; Holmes, on the other hand, realised that each radiometric date became a control point in the geological time scale that he envisioned creating, making it essential to also know the geological age of each rock. He therefore went to great lengths to assign a geological age to each of Boltwood’s rocks and then recalculated their radiometric ages using more recent techniques. The results from his own analyses fitted well with Boltwood’s data, enabling him to conclude that lead was indeed the final decay product of uranium and thus that the technique could be used for dating rocks.

 

SHOCKING

His results were truly shocking and not accepted by the majority of geologists, who still preferred to believe that the Earth was only about 100 million years old. For Holmes, however, an idea of the vast aeons represented by the Precambrian began to emerge. He wrote excitedly how, with this new dating technique, it should be possible to impose order on the hitherto “almost hopeless task” of sorting out the Precambrian. “Indeed”, he enthused, “it may confidently be hoped that this very method may in turn be applied to help the geologist in his most difficult task, that of unravelling the mystery of the oldest rocks of the Earth's crust; and, further, it is to be hoped that by the careful study of igneous complexes, data will be collected from which it will be possible to graduate the geological column with an ever increasing accurate time scale”. Through this paper, young Arthur Holmes – aged only 21 – demonstrated his vision for revealing the Earth’s geological history and was starting to build order out of chaos.

On the 14 September 1911, after six months in Mozambique fruitlessly prospecting for minerals, Holmes wrote to his best friend, Bob Lawson: “I’m really excited tonight and you will be too when I startle you by the unexpected news that as you read this I should be on the ocean blue sailing homewards”. He anticipated arriving back in Gateshead in early November and then, following a week “glorifying at home”, returning to Imperial College so he could continue his research. His letter goes on:

“I'm awfully keen to be back to work as I have heaps to do. … I intend writing all over the world to surveys and societies for material of known geological age to analyse for U and Pb. I am in hopes of gradually building up a geological time scale and hope it might do for a DSc!!! There’s conceit if you like! Still, I may as well confess to you that a DSc is my present aim and object, and with other published work I think it ought not now to be far away – if only I can avoid having to pass the honours BSc.”

While he already had a BSc in physics, he never did get one in geology, and it was another six years before he got his DSc.

BLACK WATER FEVER

Back at Imperial College, after a terrible attack of black-water fever that detained him in Mozambique for several weeks and from which he almost died, Holmes was given a demonstrator post at £100 a year and immediately started to write his booklet on The Age of the Earth. It opened with the now famous lines: “It is perhaps a little indelicate to ask of our Mother Earth her age, but Science acknowledges no shame and from time to time has boldly attempted to wrest from her a secret which is proverbially well guarded.”

With frustration mounting at the entrenched attitudes of established geologists who adhered to the old methods of measuring the Earth’s age, he wanted to explain to them, and to tell the world at large, about radioactivity and his vision for developing a geological time scale:

“As yet it is a meagre record, but, nevertheless, a record brimful of promise. Radioactive minerals, for the geologist, are clocks wound up at the time of their origin [and]…we are now confident that the means of reading these time-keepers is in our possession.”

But it was to be at least another 10 years before attitudes began to change. At a meeting of the British Association for the Advancement of Science in 1921, an impressive array of geologists, physicists and astronomers participated in a discussion that attempted to bring into harmony the wide variance in time readings between the old methods of dating the Earth and the radiometric methods.

Strutt, in place of Holmes who was then working in Burma, once again put forward the arguments in favour of radiometric dating, but the response of William Sollas, Professor of Geology at Oxford, was typical. He was overwhelmed at the amount of time now available after the paucity previously on offer: “the geologist who had before been bankrupt in time now found himself suddenly transformed into a capitalist with more millions in the bank than he knew how to dispose of”. Sollas urged caution, and urged geologists to substantiate the new techniques advocated by physicists “before committing themselves to the reconstruction of their science”. But the die-hards were growing smaller in number, and the meeting ended with a general acceptance that the Earth was probably around 1500 million years old.

PRINCIPLES

Holmes worked for many decades on furthering development of the geological time scale, as well as making major contributions to early ideas on continental drift, the formation of granites, the geology of Africa, and unravelling the Precambrian. He also wrote one of the most popular geological text books of all time: Principles of Physical Geology. Today, the geological time scale has become the framework onto which we hang all geological events. Thanks to Arthur Holmes, we learnt how to tell geological time from isotopic clocks and using those clocks we have revealed the true age of Mother Earth, discovered many of her internal mysteries and developed a unifying theory that explains all geological processes – just as he predicted we would. His contribution to our science cannot be overstated.

* Dr Cherry Lewis is Honorary Research Fellow at the University of Bristol and biographer of Arthur Holmes.

Further reading

• Soddy, F 1913. Intra-atomic charge. Nature, 92, pp399-400.
• Holmes, A 1911. The Association of Lead with Uranium in Rock-Minerals, and its Application to the Measurement of Geological Time. Proceedings of the Royal Society of London. Series A, 85, 578 (9 June) pp248-256.
• Boltwood, B B 1907. On the ultimate disintegration products of the radio-active elements. Part II. The disintegration products of uranium. American Journal of Science, 23, pp77-88.
• Holmes, A 1913. The Age of the Earth. Harper & Brothers, London and New York.
• Lewis, C L E 2000. The Dating Game: one Man’s search for the age of the Earth. Cambridge University Press.
• Lewis, C L E 2001. Arthur Holmes’ vision of a geological timescale. In: Lewis, C. L. E. & Knell, S J (eds). The Age of the Earth: from 4004BC to AD 2002. Geological Society Special Publication 190, pp121-138.

HOLMES STORIES

Did you know Arthur Holmes? Have you any stories about him? The Pardee Symposium in Holmes’s honour this October is anxious to record people’s recollections. If you have an anecdote that you think might be illuminating or amusing, or both, please email it to [email protected].