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Celebrating the age of the Earth
SIMON J. KNELL 1 & CHERRY L. E. LEWIS 2
1 Department of Museum Studies, 105 Princess Road East, Leicester LE1 7LG, UK (email: sjk8 @leicester.ac.uk) 2History of Geology Group, 21 Fowler Street, Macclesfield, Cheshire SKIO 2AN, UK (email: [email protected])
Abstract: The age of the Earth has been a subject of intellectual interest for many centuries, even millennia. Of the early estimates, Archbishop Ussher's famous calculation of 4004 BC for the date of Creation represents one of the shortest time periods ever assigned to the Earth's age, but by the seventeenth century many naturalists were sceptical of such chronologies. In the eighteenth century it was Nature that provided the record for Hutton and others. But not all observers of geology enquired about time. Many, like William Smith, simply earned a living from their practical knowledge of it, although his nephew, John Phillips, was one of the first geologists to attempt a numerical age for the Earth from the depositional rates of sediments. For more than fifty years variations of that method prevailed as geology's main tool for dating the Earth, while the physicists constrained requirements for a long timescale with ever more rigorous, and declining, estimates of a cooling Sun and Earth. In 1896 the advent of radioactivity provided the means by which the Earth's age would at last be accurately documented, although it took another sixty years. Since that time ever more sophisticated chronological techniques have contributed to a search for the oldest rocks, the start of life, and human evolution. In the attempt to identify those landmarks, and others, we have greatly progressed our understanding about the processes that shape our planet and the Universe, although in doing so we discover that the now-accepted age of the Earth is but a 'geochemical accident' which remains a contentious issue.
Establishing the Earth's age and chronology has combined with the wide adoption of uniformi- been something of an intellectual baton passed tarianism, gave the sacience new and seemingly from one academic creed to another as each rational ways to think about time. But the new seemed to offer new hope of solution or under- geologists were often reticent about discussing standing. In the seventeenth century the Earth's time in terms of years. By the 1860s, however, the age was subject to the strictures of religious and subject added the great evolutionary debate to its social orthodoxy. The product of Creation, intellectual baggage as T. H. Huxley defended theological study offered one logical path to its Darwin's billion-year estimate for natural selec- understanding, but also to its containment. This tion. Now both biologists and geologists joined pattern would repeat itself, as forces for an forces to demand a longer timescale which per- expanded timescale repeatedly did battle with mitted the playing out of those evolutionary and those who thought the Earth relatively young. uniformitarian processes necessary to explain the This was never simply a conflict between 'pro- condition of the Earth. gressive' science and a resistant society, but However, the age of the Earth had acquired a rather a reflection of the path travelled by an new interest group. Applying theoretical ideas idea through a complexity of beliefs, dogmas, developed in the arly years of the century, a theories, measurements and methodologies. number of respected physicists began to offer In the eighteenth century, Theories of the hope of at last calculating the Earth's age in Earth marshalled existing evidence, scientific years. But if their calculations were correct, then practice and analogy, to construct models for those great underpinning theories of the natural the origin and development of the planet. At the sciences could not be. With the discovery of beginning of the nineteenth century, modern radioactivity in 1896, the future of the Earth's geology emerged as a rigorous and empirical age was firmly in the court of that emerging inter- field of study then centred on stratigraphy with disciplinary subgroup, the geophysicists. In the its implied notions of relative age. This, when twentieth century these extended the timescale
From: LEwis, C. L. E. & KNELL, S. J. (eds). The Age of the Earth: from 4004BC to aB 2002. Geolog- ical Society, London, Special Publications, 190, 1-14. 0305-8719/01/$15.00 © The Geological Society of London 2001. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
2 S. J. KNELL & C. L. E. LEWIS remarkably. With Einstein's general relativity also from arithmetic, geometry, and trigono- field equations, the topic was also returning to metry, both plain and spherical; and like- cosmology, to explanations of the Earth's age in wise from a studious and laboured application the context of the age of the universe, to the realm of various sources of information, supplied of astronomers. by the observation of eclipses, by the testi- What unifies these different approaches and monies of credible authors, and by ancient their practitioners is their extrapolation of time medals, coins, monuments and inscriptions from the study of process, whether that process (Anon. 1819). was observable or modelled by theory. Early chronologers rigorously applied contemporary Chronologies were constructed as an analytical historiographic methods to chart the 'progress' tool so as to distinguish 'cause' and realize a of history. Later geologists would extrapolate 'chain of events' (Anon. 1837, p. 131). To achieve the time necessary for shaping the surface of the this, their content was selective and thematic. Earth or for the deposition or erosion of observ- To discern the age of the Earth, the only 'cred- able strata. Biologists too envisaged a time- ible authors' were those relating creation stories. dependent process of evolution. Physicists made The Bible, or rather the texts from which it was predictions and measurements from the physics written, then became the primary historical of heat and radioactive decay, and astronomers texts, key sources for the interpretation of deduced process from planetary construction time. Historians generally point to Theophilus and arrangement. In the relative calm of the of Antioch, of the second century AD, as perhaps early twenty-first century, in a field which has the first to use the Biblical record as a source seen centuries of deep controversy, we should for the construction of a chronology. The art of perhaps reflect upon the confidence our pre- chronology, however, extended back long before decessors had in their age for the Earth. They the Christian era, often deriving 'fabulous ages' too thought they had 'got it right'. of hundreds of thousands of years for particular civilizations (Anon. 1819, 1833, 1849). In the sixth century BC Zoroaster, a religious teacher Chronologies and theories who lived in Persia (now Iran), believed that the world had been in existence for over 12000 In one of his popular articles, Stephen Jay years; the Roman writer Cicero relates that the Gould (1993, pp. 181-193) responded to James venerable priesthood of Chaldea in ancient Barr's (1985) definitive study of a great histor- Babylonia held the belief that the Earth emerged ical chronologer with a diatribe against previous from chaos two million years ago, while the old 'Whiggish' interpretations of this early form of Brahmins of India regarded Time and the Earth scholarship. The historiography of science had as eternal (Holmes 1913). long rejected the notion of 'progress', of the past By the sixteenth century the Bible formed a having been a succession of errors, but Gould core resource for the writing of histories in was here talking to scientists who necessarily Europe. Of particular significance was the remained wedded to, and driven by, this con- interpretation of the Second Letter of Simon cept of progress towards 'truth'. Though not Peter, which states 'One day is with the Lord as the first to do so, Barr saw chronologers not a thousand years, and a thousand years as one as religious dogmatists but as rigorous scholars day' (2 Peter 3:8) (see also Dean 1981a; Oldroyd working within the social and intellectual 1996a). It draws on Psalm 90 of the Old constraints of their time. They were partici- Testament, 'The human condition': 'Lord ... pating in the earliest of historiographic prac- To you, a thousand years are a single day'. tices- the construction of a linear timeline of Chronologers read this literally, and made events - which, in reconceived form, would later each of the six days of creation equivalent to underpin the great stratigraphic enterprise of the 1000 years. The Biblical motivation for discuss- nineteenth century. ing the Earth's limited age was to encourage the Rees's early-nineteenth-century Cyclopaedia living of 'saintly lives'. For here was not simply separated 'sacred chronology' from other histor- the birth but also the death of the Earth, the ical chronologies. Here chronology was 'one of Judgement Day, which will 'come like a thief, the eyes of history' and not simply the study and then with a roar the sky will vanish, the of ancient theological texts. Its potential sources elements catch fire and fall apart, and the Earth were many: and all it contains will be burnt up' (2 Peter As its use is extensive, the difficulty of acquir- 3:10). If this were not enough to encourage ing it is not inconsiderable. It derives necessary piety, Psalm 90 also pondered human mortality, assistance from astronomy and geography, 'over in a trice, and then we are gone.' Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
AGES OF THE EARTH 3
became very familiar to early-nineteenth-century British audiences, but he had predecessors and contemporaries across Europe (Anon. 1837, p. 133; Davies 1969, p. 13; Fuller 2001). From his own studies Hales preferred a creation date of 5411BC. But by the time he published, chronologies, however useful they were for the construction of civil histories, were no longer restricting views of the age of the Earth. Indeed scholars had, for almost a century, taken the debate beyond the religious confines of ortho- doxy. Davies (1969, p. 16), for example, suggests that scepticism of established chronologies existed amongst seventeenth-century naturalists. Though such views were rarely committed to paper, he felt sure that the London coffee houses Fig. 1. Oil Painting of Archbishop James Ussher in must have been alive with such talk. The devout 1658. Reproduced by kind permission of the governors John Ray (1627-1705), for instance, reflected and guardians of Armagh Public Library. upon the oganic origin of fossils and considered the implications for time of the depths of sedi- Scholars and particularly the clergy - long an ments and rates of denudation- a recurrent intellectual elite- used these 6000 years as a theme in the centuries to come (Davies 1969, framework. The relative merits of the Hebrew p. 59; Dean 1981a, p. 444). Massoretic text, the Samaritan Pentateuch and Despite the enforcement of religious ortho- Greek Septuagint were much debated, while doxy by the state (Davies 1969, p. 11), freedom of later studies of other civilizations revealed even expression about the origins and history of the longer chronologies and other creation stories. Earth developed rapidly in the eighteenth cen- Given the range of sources and scope for tury. In 1694, for example, Edmond Halley interpretation, it is no wonder that no definitive considered the implications of a comet impact chronology was produced. on the Earth. He imagined an Earth repeatedly Undoubtedly the most famous chronology reborn following chaotic destruction. It seemed known to us today is that published in the sev- to support a view of an eternal Earth undergoing enteenth century by James Ussher (Fig. 1) but, periods of dissolution and recreation - a notion as Fuller (2001) shows, he was not alone nor was then considered 'one of the most dangerous he the most revered in his time (see also Anon. of heresies' (Kubrin 1990, p.65; Davies 1969, 1819, 1833). A brilliant scholar, Ussher had com- p. 12). It was also a challenge to the British pleted a draft whilst at Trinity College, Dublin, establishment, much like that presented by theo- in 1597. He was then only 16 years old. Barr ries of biological transmutation a century later (1985) disentangled the complexity of Ussher's (Desmond 1989). Such ideas did not simply work, of which his Creation date of 4004 BC is undermine religious teaching but proclaimed as best known. This curiously high-resolution date a falsehood the natural social order into which is derived from traditional interpretations he the established church and wider society were had inherited, which placed 4000 years between divided. To counter such opinions of himself, the Creation and the coming of Christ, and an which were likely to affect his career prospects, adjustment to take account of evidence that Halley then spent a good deal of his time Herod (a contemporary of Christ) died in 4 BC. pursuing evidence for a finite age of the Earth. However, only one-sixth of Ussher's chronology For him, there were good socio-political reasons was concerned with the Bible, the rest was a his- for wanting to discover a particular answer tory of classical times. That we know of Ussher, which reinforced accepted views (Kubrin 1990, however, rather than one of the hundreds of p. 65). Gould pondered one such of Halley's other chronologers, is simply an accident of his- observations (Gould 1993, pp. 168-180). Under- tory (for which, see Fuller 2001). standing that the salts in the world's oceans were For one group of scholars it seemed the delivered to the sea via rivers, Halley suggested answer might be found by undertaking com- that if the rate of contribution could be parative studies of published chronologies in calculated then here would be a measure for order to locate sources of error and correction. the age of the Earth. It was an idea that was to William Hales' three-volume New Analysis of resurface in the late nineteenth century in the Chronology (1809-1812) is an example which hands of John Joly (Wyse Jackson 2001). What Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
4 S.J. KNELL & C. L. E. LEWIS surprised Gould was that Halley was consider- these curious 'registries of control' of nature. ing a maximum age for the Earth, an argument The study of these monuments is, in the against eternity, rather than a minimum. Thirty mineral kingdom, the veritable art of verifying years later, following considerable social change, the dates of nature, as the study of plant or Halley was at last able to publish his paper shell-bearing rocks is the art of verifying the discussing an Earth reborn without fear of social dates and eras of the ancient history of or religious retribution. organized beings in the kingdom of the living Halley's contemporary, Sir Isaac Newton, also world (Soulavie 1791). considered the Earth's age and chronology, He used superposition of volcanic deposits, their though without disrupting the traditional views relative state of preservation and comparative of the Church. In his posthumous Chronology of elevation above the sea as indicators of succes- Antient Kingdoms Amended he used astronomical sion, but he was not suggesting the Smithian calculations to correct those Greek and other principle of characteristic fossils. ancient chronologies which were frequently used Throughout the eighteenth century, across the as historical sources (Newton 1728). whole of Europe, Theories of the Earth mixed Rationalist Deism was an inevitable reaction conjecture, religious orthodoxy and observation to the strictures and assumptions of religious (Vaccari (2001) gives a wide overview). That orthodoxy in seventeenth and eighteenth century the Earth had undergone a succession of'revolu- Britain. It proved highly influential in intellectual tions' was a popular concept. It was critical to circles across Europe and North America. For George Cuvier's (1817) Essay on the Theory of the James Hutton, its most famous geological dis- Earth which interpreted the succession of extinct ciple, Nature provided a true record: the Earth's animals preserved in the fossil record and the wild timescale was indefinite. Hutton was born only a contortions of rocks which, following the argu- few years after it had been finally possible for ments of Steno, were believed to have originally Halley to publish his paper inferring a longer been deposited horizontally. But by the time timescale, perhaps an eternity. Hutton, how- Cuvier's Theory of the Earth was published, the ever, said nothing of eternity. On this, his great genre was attracting much criticism from a new admirer, Archibald Geike, was emphatic (Geike breed of empiricist geologists. 1899, p. 719; also Burchfield 1975; Dean 1981a, p. 454). Notable amongst Hutton's critics was Jean Andr6 de Luc who, from the same record of Relative age, uniformitarianism and time Nature, found only concordance with Biblical chronology. For de Luc (discussed by Rudwick As geology emerged as a rigorous and self- 2001) geomorphological processes created fea- contained science in the early nineteenth cen- tures in the landscape that were indisputably the tury, there remained a belief that a unifying product of time. If process rates were known then theory might permit explanation of the geologi- a mechanism for measuring time existed. cal world. Time, as a useful geological concept, It was this same kind of thinking which Georg however, did not emerge from theory but from Louis Leclerc, the Comte de Buffon, famously observation. The stratigraphic work of William used to extrapolate a longer timescale of Smith (Fig. 2), Cuvier and Alexandre Brongniart, 75000 years (discussed by Taylor 2001), a Thomas Webster and John Farey in the first timescale which he still felt too short. Buffon two decades of the nineteenth century proved applied measurement and reason, and promoted fossils as useful time markers. It was an idea that a theory based on contemporary empiricism would develop rapidly in terms of its under- (Buffon 1807). standing and application throughout the first Jean Louis Giraud, Abb6 Soulavie (1752- half of that century. Unlike Buffon and Cuvier 1813), like Nicolas Desmarest (also Taylor (and indeed Webster), Smith's goal was not 2001), interpreted temporal sequence from a intellectual or social (Knell 2000) but entirely study of volcanoes. In his book of 1791, he practical (as Torrens (2001) demonstrates). The compared volcanic deposits to the Control of knowledge he developed resulted from attempts Acts, a recent piece of legislation which ensured to solve the practical problems of coal prospect- that the dates of contracts would be registered ing. As Torrens shows, this was one of the major such that originals could be verified. He also economic challenges of the period. What Smith predicted the future role of fossils in determining actually thought of the role of time in causing the Earth's chronology: the arrangement of fossils and strata is a matter for debate (Knell 2000; Torrens 2001; Fuller All these volcanoes and their products may be 2001). For the most part he was talking about placed today in a certain order by reason of the 'natural order' of strata, though we should Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
AGES OF THE EARTH 5 have no doubt that his views changed. Certainly technique was neither infallible nor at the height by 1824 Smith's work on the Yorkshire coast, of its sophistication. In 1813, Ernst Friedrich, was giving 'strong confirmation to the geolog- Baron von Schlotheim, like others on the front- ical axiom that "Deposits of equal antiquity line of the new science, still felt fossil-based enclose analogous fossils" ', as his nephew and co- chronology a possibility rather than a reality worker, John Phillips (Fig. 2), reported to the (Schlotheim 1813). George Greenough (Fig. 2) Yorkshire Philosophical Society (Knell 2000, and a few others doubted it had any particular p. 148). That Smith should have co-authored value at all. Schlotheim, like John Farey and later such a statement is something most historians of Von Buch, called for higher resolution studies of Smith have previously denied. But as is appar- organic remains, as he felt these might reveal the ent from Torrens' paper, Smith's ideas were never epochs of revolutions in terms of spans of years. fully formed. When he and Phillips began their By the 1830s, chaotic fossil nomenclature was, detailed study of the Yorkshire coast in the 1820s more than anything, leading to stratigraphic they were testing their knowledge of sequence imprecision. The British Association for the and indicators, and the reliability of the Smithian Advancement of Science charged John Phillips method. What Torrens most clearly demonstrates with the task of sorting this out (Fig. 2). is that in an era of frequent coal prospecting Phillips epitomized the empiricism which, it scandals it was easy to understand why Smith was was claimed, had turned geology into a rigorous revered by some and yet simply not believed by science. He made an entire career out of time, others. The risks were great and the Smithian the kind of relative time which Callomon (2001)
Fig. 2. The British Association for the Advancement of Science at Newcastle in 1838. Oil painting on canvas by Thomas Henry Gregg. The British Association for the Advancement of Science first met in York in 1831. In 1838 the Duke of Northumberland was the President of the Meeting. Key: 1 Sir Roderick Impey Murchison; 2 Richard Owen; 3 Henry Thomas De la Beche; 4 ?; 5 John Phillips; 6 Adam Sedgwick; 7 George Bellas Greenough; 8 Charles Lyell; 9 William Buckland; 10 ?; 11 William Smith. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
6 S.J. KNELL & C. L. E. LEWIS now pursues. In the middle years of the nine- (as Morrell (2001) reveals), foreseeing the con- teenth century, Phillips was producing endless flict between the infinite timescale of uniformi- tables of facts, believing, initially at least, that tarianism and the restricted needs of those errors arose from omission and that compre- theorizing on the physics of the Earth (as hensive data would yield more accurate results. discussed by Brush 2001; Dalrymple 2001; He was later to understand the error of this Lewis 2001; Shipley 2001; Yochelson & Lewis reasoning as the wealth of data he had generated 2001; Wyse Jackson 2001). was sometimes beyond human powers to process Some have suggested that, as the century wore (Knell 2000). Phillips was so swept up in the on, geology continued to be threatened by con- mid-century fashion for statistics that one may servative and speculative philosophy. Though feel somewhat surprised when Morrell (2001) Fuller (2001) sees in Leonard Horner's 1861 tells us of Phillips' reticence to commit himself to address to the Geological Society of London a number for the age of the Earth. remnants of religious orthodoxy snapping at the Though reticent, Phillips understood the heals of science, Davies (1969, p. 210) was more human desire to create a scale for the chronol- dismissive. Was Homer 'tilting at windmills' or ogy given by relative time. He knew all attempts detecting a real threat in orthodox religion? were bound to fail due to countless factors that More than a century later, geologists continue would corrupt and distort the figures. One could the same campaign while observers ponder the only expect 'plausible inferences concerning the same question. By the middle of the nineteenth time elapsed in the production of stratified century, Victorian society was undergoing a rocks', nothing more (Phillips 1849, p. 795). crisis of faith, to which geology with its notions However, ever willing to apply a little mathe- of endless time had contributed (see Dean matics to philosophical puzzles, he made an 1981 b). However, attempts were made to under- attempt 'in a strictly philosophical spirit' in the stand the Bible in ways acceptable to contem- Encyclopaedia Metropolitana. He considered a porary interpretations of the geological evidence. number of ways in which the problem might be One came in the 1850s when Hugh Miller, the resolved, including using upright fossil plant Scottish geologist and writer, attempted to recon- remains as evidence of rapid depositional events cile the Genesis and the fossil record in The (such as the locally famous fossil tree trunks in Testimony of the Rocks (1857) (Oldroyd 1996b). the coal measures of Altofts, near Wakefield, He explained Genesis Ch. 1 as a literally visual which Phillips knew well). However, the most revelation, each "day' corresponding to a geolo- logical line of enquiry remained that which gical period. Although popular in its day, it had attracted the attention of Desmarest, was ultimately insufficiently naturalistic for the Soulavie, Buffon and de Luc, and which was scientists and too liberal an interpretation for now characterized by the uniformitarian princi- the theologians. ples of Charles Lyell (Fig. 2), John Playfair and However, by this time it was not religion Hutton: that is, the deduction of time from the that was restraining the age of the Earth, but products of geomorphological processes. Here the physics of heat and its implications for a he undertook a calculation of the time taken to cooling Earth. Now Lyell's most fundamental deposit Coal Measure flagstones within which principle - that of uniformitarianism - and all layers of rolled grains one-twentieth of an inch that rested upon it, seemed under threat. With thick could be detected. A 40 ft thickness of rock Archibald Geikie in the ascendancy as Britain's implied 9600 layers. If each layer represented a pre-eminent geologist, an unbridled supporter tide, then about 700 layers would result from of Lyell and Hutton, the geologists were not annual deposition. Thus 40 ft of rock could be going to take any restrictions on the age of deposited in approximately 13.5 years. Half of the Earth lying down. Amongst his many the Yorkshire Coal Measures consisted of sand- arguments Geikie used denudation rates, rather stones, some 1500ft of strata in all. By this than rates of sedimentation, to discuss the scale reasoning, deposition of the whole of the Coal of geological time. Based on empirical data for Measures may have lasted 1000 years. Such the mean height of the continents, and fluvial figures, combined with Phillips' belief that all sediment and solution loads, he predicted the strata ultimately originated as molten rock, complete wasting of Europe in just 4 million made him think the Earth did have a finite years, North America in 4.5 million and South age, a view he felt disagreed with Lyell (Phillips America in 7 million. It demonstrated that rates 1849, p. 798). However, by this time Phillips of denudation were not as slow as usually stated had read Joseph Fourier's (1822) ThOorie Analy- and that vast quantities of time were not required tique de la Chaleur (The Analytical Theory of to produce considerable change to the surface of Heat) and was, along with other 'theorists' the Earth (Geikie 1868). It was not that Geikie Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
AGES OF THE EARTH 7 wanted a young Earth but that a young Earth 300000000 years for the "denudation of the did not undermine uniformitarianism. Evolu- Weald?"' (Thomson 1862, p. 391; also Morrell tionary biologists were talking in terms of 2001). Since the age of the Earth was constrained Darwin's estimate of 1000 million years for the by the age of the cooling Sun, it was only to be record of natural selection, time the geologists expected that its inhabitants '... cannot continue were willing to give. As Huxley remarked in to enjoy the light and heat essential to their life, defence of geology, 'Biology takes her time for many million years longer, unless sources from geology' (Huxley quoted by Desmond now unknown to us are prepared in the great 1994, p. 370). The physicists, led by William storehouse of creation' (Thomson 1862, p. 393). Thomson- later Lord Kelvin- and his colla- Within a month of questioning Darwin's esti- borator Peter Guthrie Tait, were not followers mate in public, Kelvin delivered his now famous of Lyell or Darwin. attack on geologists and their methods for determining an age for the Earth that ignored The storehouse of creation the laws of thermodynamics. At that time Kelvin proposed the Earth's age The closing decade of the nineteenth century saw to be between 20 and 400 million years (Thom- the start of a scientific revolution that was to son 1864), but he found the problem so interest- impact on geology in a way that would transform ing that over the following three decades he it forever and prepare it for the definitive continued to pursue it, confessing that he 'would 'revolution' that occurred in the 1960s. By 1893 rather know the date of the Consistentior Status debate between geologists and physicists about than of the Norman Conquest' (Thomson 1895a, the age of the Earth was at its peak. The ages p. 227). The Consistentior Status was considered determined by geologists, using empirical meth- to be the time when the molten Earth had first ods and observing Earth processes, ranged from acquired a solid crust. As more data became 3 million years 'for the whole incrusted age of the available Kelvin reduced his estimate first to world' (Winchell 1883, p. 378), to Wilber John 100 million years (Thomson 1871), and then, in McGee's 1892 value of 15 billion years for the 1893, to only 24 million (Thomson 1895b) when whole age of the Earth, of which 7 billion had new results on the melting temperature of rocks elapsed since the end of the Palaeozoic (Yochel- became available (King 1893). This new age son & Lewis 2001). These extreme values were concurred well with the age of the Sun, reckoned largely unacceptable to both geologists and to be 20 million years. To the physicists, the physicists, and did nothing to bring the two inevitability of the Sun's decline was a compel- sides into closer agreement, although the urgent ling reason for a young Earth, and the fact that need for a reliable timescale was becoming more the two ages, Earth and Sun, now gave consistent and more evident: 'How immeasurable would be results was conclusive proof that both were right. the advance of our science could we but bring the There was little for the geologist to argue against, chief events which it records into some relation except to suggest to the physicist that 'there may with a standard of time!' (Sollas 1900, p. 717). be an error somewhere in his data or the method Lord Kelvin's interest in the age of the Earth of his treatment' (Walcott 1893, p. 639). had been prompted by his work on the age of The effort to quantify geological time affected the Sun's heat. He considered that the Sun, both the theory and practice of geology and was 'assumed to be an incandescent liquid', must largely responsible for it maturing into a profes- obey the laws of thermodynamics, therefore it sional scientific discipline. Up to that point, could not continue to radiate sunlight for ever geologists had worked out their estimates for the unless it was being provided with energy. Since age of the Earth from their empirical observa- there was no evidence for this energy, there tions of sedimentary processes, based on the was no alternative -it must be cooling down - uniformitarian premise that the rates of these and the inevitable result of cooling was that the processes did not vary over time. It was largely Sun had a finite life. In this conclusion Kelvin the work of Kelvin that made clear to geologists was supported by astronomers. the need for quantification and it was he who set Given these apparently irrefutable circum- them on a path which observed the laws of stances, it is not surprising that when Charles physics, from which they have subsequently Darwin, in the first edition of On the Origin of deviated little. A rigid adherence to Lyell's uni- Species, published his estimate of 300 million formitarian interpretation of geology was no years for erosion of the Weald, a section rela- longer possible, and although not all physicists tively high up in the geological sequence, that were united in their interpretation of Kelvin's Kelvin's response was dismissive: 'What then are data, as is evident from the views expressed by we to think of such geological estimates as his former student John Perry (Shipley 2001), Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
8 S.J. KNELL & C. L. E. LEWIS they were, on the whole, agreed on the need Based on the accumulation of helium, a by- for geologists to abandon their dogmatic atti- product of the decay process, in 1904 Ernest tudes. It was Perry, in fact, who complimented Rutherford (Fig. 3) determined the very first Kelvin in Nature for his expos6 of uniformitar- radiometric date: a fergusonite mineral gave an ianism: 'Lord Kelvin completely destroyed the age of 500Ma. More than forty years after uniformitarian geologists, and not one now Kelvin's pronouncement that the Sun would exists. It was an excellent thing to do. They are not keep shining unless new sources of energy as extinct as the dodo or the great auk' (Perry 'now unknown to us are prepared in the great 1895, p. 227). storehouse of creation', Rutherford was to That same year, 1895, William Rrntgen opportunistically turn this phrase to his advan- discovered X-rays and the following year Henri tage in order to placate Kelvin, whose age for Becquerel detected similar rays being emitted the Earth was now demonstrably in error: from uranium. The decade of discovery that 'That prophetic utterance refers to what we are ensued was a time unparalleled in the history of now considering tonight, radium!' (Eve 1939, science (Lewis 2001). The speed at which new p. 107). But Kelvin died three years later, having discoveries were made about the atom unravelled never fully accepted that a new source of heat the whole fabric of physics, and a completely new had indeed lain in the great storehouse of crea- design was needed to build it back up again. This tion- creation of the Solar System. As Martin new form of energy at last explained the dis- Rees (2001) explains, most elements, particu- crepancy between the long timescales observed larly the heavy ones like uranium, form during by geologists, and the short one predicted by a supernova explosion, the gaseous debris from Kelvin. For even if the Earth was still cooling which ultimately resulted in formation of our from a time when it had been a molten globe, the Solar System. Thus stellar nucleosynthesis could heat generated within the Earth by radioactive be viewed as Kelvin's 'great storehouse of decay had counteracted that cooling for hun- creation' in which all elements on Earth are dreds of millions, even billions, of years. prepared.
Fig. 3. Ernest Rutherford (1871-1937) (right), who determined the first radiometric date, with Hans Geiger, in their laboratory at Manchester University, around 1912. Credit: Science Museum/Science and Society Picture Library. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
AGES OF THE EARTH 9
Throughout history, there have always been In the United States, rejection or acceptance those slow to accept changes to the status quo, of ideas about radioactivity and its application and who fiercely resist innovation, particularly to geological problems progressed at much the when, like radioactivity, it is rapidly introduced same rate as it did in England, although in the and means the discarding of many strongly held first quarter of the twentieth century America beliefs. Having desired, in Lyell's day, an Earth lagged behind by a year or so. Since most of of almost infinite duration, geologists fought the work on radioactivity was then being done Kelvin for fifty years to be allowed an Earth that in Europe, time was needed for ideas to travel was even a hundred million years old. Within a by boat across the Atlantic and for the more few years of Rutherford's first radiometric date, important papers to be reproduced in American it became clear that, after all, geologists could journals (Yochelson & Lewis 2001). But by the again have an Earth of almost limitless extent, early 1930s, most scientists in both countries but now the demand was for less time. They essentially accepted the geological timescale pro- were being asked to accept a ten-fold increase - vided by Holmes, and little reference was made an Earth that was more than a billion years old: to any timescale provided by sedimentation or just where were all the sediments needed to fill denudation rates. Now only the astronomers such enormous amounts of time? seemed to question the validity of radiometric During this period of transition, two men were dates (Brush 2001). particularly prominent in developing the science During the Second World War, develop- of geochronology. Today John Joly is famous for ment of the atom bomb meant that almost all his attempts to date the age of the Earth from the available physicists in America were co-opted sodium content of rivers, but he was also a man to work on the Manhattan Project. The out- with an extraordinary range of interests and come for geochronology was a greatly improved abilities (Wyse Jackson 2001). Joly was quick to understanding of uranium, refined mass spectro- grasp the principles of radioactivity and to apply meters, and a number of people who had become them to geological problems, but his inability to highly skilled in the art of mass spectrometry accept the long timescales being indicated by (Lewis 2000). The superior dates now available the method was typical of many of his genera- not only pushed back the age of the Earth as tion. It required someone younger, with an open older minerals were found, but they also led three mind unfettered by the need to maintain poli- people in Europe to recognize that the new tical prestige, to solve the new problems. Arthur methods for dating the minerals also held the key Holmes' additional advantage over many of his to dating the Earth. Argument as to the timing of geological contemporaries, was that he had a events and the merits of each model led to it background in physics, but being familiar with being known for many years as the 'Holmes- all the old methods used to estimate the age of Houtermans' model for dating the Earth, as both the Earth, he also understood the difficulties Arthur Holmes and Fritz Houtermans published being encountered by geologists: their very similar versions in 1946, within months of each other. But when Gerling's 1942 paper The surprises which radioactivity had in store was translated from Russian, it seemed that he for us have not always been received as had had the idea first, so the name became even hospitably as they deserved. With the advent more cumbersome the 'Gerling-Holmes-Hou- of radium geologists were put under a great termans model' (Dalrymple 2001). obligation, for the old controversy [the need In fact, it has now come to light that the for a long time scale] was settled overwhel- principles for the model originated with Holmes mingly in their favour. But the pendulum has back in 1932 when he first proposed the concept swung too far, and many geologists feel it of 'initial ratios' (Lewis 2001), but the argument impossible to accept what they consider the as to who was 'first' is not important here. What excessive periods of time which seem inferred is interesting is the apparently simultaneous (Holmes 1913, p. 167). development of the same idea by a number of For more than four decades Holmes was in the people, not obviously in contact with each other, vanguard of geochronology, and during that time although all working in the same domain. It is a he contributed more to understanding that sci- common theme in science. Robert Strutt claimed ence than any other single individual. He pur- that he had thought of how to date minerals sued an almost evangelical crusade to persuade before Rutherford did, or certainly before Ruth- geologists and the world at large, of the value of erford published (Egerton 1948), and although radioactivity as a means for measuring the age Frederick Soddy is credited with the concept of of the Earth, and an Earth of great antiquity isotopy because he coined the term "isotope' in (Lewis 2001). 1913, the idea of several species of the same Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
10 S. J. KNELL & C. L. E. LEWIS element had been around for a decade pre- minute amounts of lead, this skill enabled him viously (Kauffman 1982). The process of 'dis- to finally date the 'time since the earth attained covery' is in fact rarely a 'Eureka' moment, its present mass' (Patterson 1956, p. 236). He being almost always one of a gradual evolution resolved it to be 4550 ! 70 Ma. After centuries of of ideas- the replication of memes (Dawkins controversy, it may appear that the final event 1989) that occasionally mutate. Scientists, how- went almost unnoticed, but it is only with hind- ever, do like to attribute discoveries to an indivi- sight that we now know Patterson was 'right'. dual who was first. After all, one of the basic At the time, it might only have been one more of priorities of research, particularly these days, is the many recent attempts to get it right. to be able to claim credit in the intellectual rat race. The Theory of Plate Tectonics should rank alongside Darwin's Theory of Evolution Ages of the 'oldest' and Einstein's Theory of Relativity, but perhaps because it cannot be attributed to any one indivi- There is something in the human psyche that dual, it is somehow assigned to a lower rank. drives us to look for extremes - the first and last, Regardless of who developed the model for the highest and lowest, largest and smallest, dating the age of the Earth, once it was in place youngest and oldest. With the search for the age all that remained was for techniques to progress of the Earth ticked off that list, we have looked to a point where accurate data could be obtained for other extremes. Biologists search for the first and plugged into the model to give the correct signs of life and anthropologists for the first result. Claire Patterson (Fig. 4), so named on his humans; in astronomy it is the furthest stars; in birth certificate but who frequently called himself geology the oldest rocks. Clair in an attempt to avoid the gender confusion Since development of the Theory of Plate (L. Patterson, pers. comm. 1999), was another Tectonics, we have progressed to a remarkable whose mass spectrometry skills had been honed understanding about the way the Earth has on the Manhattan Project. In 1956, after a evolved. Despite this, we still have a rather decade spent developing techniques to measure hazy view of those first 500 million years,
Fig. 4. Claire Patterson (1922--1995) who dated the age of the Earth (courtesy of the Archives, California Institute of Technology). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
AGES OF THE EARTH 11 although the hope is that the oldest rocks of the worked so hard to launch geochronology as a planet will carry a 'memory' of the processes that science. Now it is impossible to imagine how we occurred hundreds of millions of years before would manage science without them. their formation. Evidence from zircons that give Chronology is a crucial tool for the under- an age of 4.4 Ga, suggests that crust was forming standing of many disciplines. Being able to date on the Earth's surface soon after accretion events tells us much about the processes that (Kamber et al. 2001), which is considered to enabled them. For example, the key to unlock- have lasted some tens of millions of years ing geology's unifying theory was development (Hofmann 2001). But 'accretion' seems to have of a timescale of magnetic reversals seen on the persisted in the form of 'bombardment' for at floor of the oceans. Immediately it became clear least another 500 Ma, as massive impacts, known that the youngest rocks were nearest to the to have bombarded the Moon until 3.9Ga, central ridge, while the oldest were furthest away presumably also hit the Earth. No evi- and adjacent to the continents- so the oceans dence is found on Earth for this bombardment, really were opening, and the continents really but the apparent lack of survival of any in situ were drifting apart! Sea-floor spreading, hitherto crust older than 3.8Ga (Kamber et al. 2001) just a theory, became a fact. strongly supports the theory. The race to find Similarly, as Brush (2001) discusses, when the older and older crust appears to be limited by value for Hubble time, which provides an age this bolide barrier. However, history reminds us for the Universe, was seriously underestimated that for thirty years (1911-1941) the age of the in the 1930s and '40s, it resulted in a Universe Earth remained static at 1.6 Ga because, during that was younger than the Earth and stars it all that time, no minerals were found that contained. Astronomers then sought a range of indicated it might be any older. But in 1941 models, or processes, in an attempt to explain discovery of the Manitoba pegmatite, that gave this discrepancy. For example, the fact that the an age of more than 2.2Ga (Nier et al. 1941), 'timescale problem' did not exist in the Steady- made it apparent that the Earth was actually State Theory was one of its advantages. But much older than anyone had hitherto realized. because the timescale was wrong, so too were Thus the search for older and older crust should the processes used to explain formation of the not be discouraged! Universe. Until the 1950s, Holmes and his The biologist's search for the start of life on colleagues, with their persistent and consistent Earth, asks not only the question 'when?', but dating of minerals from Earth, continued to be a also 'how?'. Was it in the murky primordial thorn in the side of astronomers, in much the soup, or on the back of one of those massive same way that geologists had been a thorn in bolide impacts? Evidence for life is now found in the side of physicists during the previous cen- a remarkable variety of environments (Manning tury. It was not until the late 1950s that the 2001), and it therefore does not seem unreason- Hubble constant was revised and provided a able to anticipate that life survived even the more realistic age for the Universe, which then meteorite bombardment. In West Greenland facilitated development of current models for ~3C-depleted graphite microparticles, claimed its evolution. Today, some astronomers claim to be biogenic in origin, have been found in that the Big Bang Theory explains evolution of the oldest known sedimentary rocks that are the Universe to such an extent that they can 3.7-3.8 Ga (Kamber et al. 2001). As flexible and 'place 99 percent confidence in an extrapolation durable as life seems to be, a continued search back to the stage when the Universe was one may well push that boundary back even further. second old' (Rees 2001). In the light of history, Searching for our ancient hominid ancestors and all the bold statements discussed in this is a topic of particular interest to us all, and per- volume that have subsequently proved incorrect, haps it is a mark of the success of archaeology this is a brave assertion. to capture the public's imagination, that when asked the question 'What technique is used to date age of the Earth?' the vast majority will Conclusion answer 'carbon dating'. In the last fifteen years, refined archaeological dating techniques have im- The value that Patterson determined for the age measurably improved our ability to understand of the Earth has not changed in nearly fifty years, the time-scale of human evolution, particularly beyond that allowed for by his error range. for the last 200 000 years (Stringer 2001). The new Compatible with ages of meteorites and the dating techniques that facilitate this understand- Moon, it now seems a consistent and stable ing are the direct descendents of those established number, but is it? Like an explosive volcano, the by the early pioneers in the last century who age of the Earth debate did not die, it just lay Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021
12 S. J. KNELL & C. L. E. LEWIS dormant for fifty years until science had pro- BARR, J. 1985. Why the world was created in 4004 BC: gressed sufficiently to question the underlying Archbishop Ussher and Biblical chronology. assumptions on which it was based. Our im- Bulletin of John Rylands University Library Man- proved understanding of the extreme chemical chester, 67, 575-608. BRUSH, C. 2001. Is the Earth too old? The impact of geo- differentiation that occurred during separation of chronology on cosmology, 1929-1952. In: LEWIS, the crust from the mantle reveals that although C. L. E. & KNELL, S. J. (eds) The Age of the Earth: the mechanisms for enriching uranium and lead in from 4004BC to AD2002. Geological Society, the crust are quite different, the result is that, on London, Special Publications, 190, 157-176. average, the overall enrichment is much the same BUFFON, LECLERC, G. L. COMTE DE. 1807. Epoques de for both elements (Hofmann 2001). la Nature, Paris. [Translated extract in MATHER, As Hofmann explains, although this 'geo- K. F. & MASON, S. L. (eds) 1939. A Source Book chemical accident' served Patterson well and in Geology. McGraw-Hill, New York, 65-73]. apparently provided him with the "right' answer, BURCHFIELD, J. O. 1975. Lord Kelvin and the Age of the Earth. Macmillan, London. terrestrial lead isotopes, such as those that con- CALLOMON, J. H. 2001. Fossils as geological clocks. In: tributed to the ocean sediments used by Patter- LEWIS, C. L. E. & KNELL, S. J. (eds) The Age of son to represent 'average Earth lead', can only the Earth: from 4004BC to AD2002. Geolog- provide an approximate age. The ages of other ical Society, London, Special Publications, 190, events, such as formation of meteorites now 237-252. considered to be 4.56 Ga, the timing of core dif- CUVIER, G. 1817. Essay on the Theory of the Earth, ferentiation around 4.40Ga and discovery of [translated by Robert Jameson], Edinburgh. 4.40 Ga zircons, place the 'age of the Earth' into DALRYMPLE, G. I. 2001. The age of the Earth in the a wide band of 160Ma. But even so, only the twentieth century: a problem (mostly) solved. In: LEWIS, C. L. E. 8z KNELL, S. J. (eds) The Age of the lower limit of this 'wide band' falls outside Pat- Earth: from 4004 BC to AD 2002. Geological Soci- terson's error range of 170 Ma, on his original ety, London, Special Publications, 190, 205-221. estimate of 4.55Ga. According to Hofmann, DAVIES, G. L. [HERRIES] 1969. The Earth in Decay: however, it may be possible to reduce the error A History of British Geomorphology 1578-1878. to perhaps as little as 10 Ma. Macdonald, London. The extinct decay products of short-lived DAWKINS, R. 1989. The Selfish Gene (second edition). radioactive nuclides are now being used to Oxford University, Oxford. interpret the chronology of the earliest history DEAN, D. R. 1981a. The age of the earth controversy: of the solar system, so perhaps they will also beginnings to Hutton. Annals of Science, 38, 345-456. provide further constraints on the formation DEAN, O. R. 1981b. 'Through science to despair': history of the Earth. Until then, the age of the geology and the Victorians. In: PARADIS, J. 8¢ Earth remains controversial and the search for POSTLEWAIT, T. (eds) Victorian Science and Vic- a refined value continues. It could make for inter- torian Values. Annals of the New York Academy of esting discussion at the turn of the next century. Sciences, 364, 111-116. DESMOND, A. 1989. The Politics of Evolution. Uni- We would like to thank all the authors who have versity of Chicago, Chicago. contributed to making this volume the rich and varied DESMOND, A. 1994. Huxley: The Devil's Disciple. book it has become, and on whom we have extensively Michael Joseph, London. drawn for this paper. As editors, we would like to EGERTON, A. C. 1948. Lord Rayleigh 1875-1947. Obi- thank not only the referees of this paper, M. Taylor tuary Notices of Fellows of the Royal Society for and M. Stoker, but also all those referees who have 1947, 503-538. willingly given of their time and expertise to comment EVE, A. S. 1939. Rutherford. Cambridge University, on the wide range of papers put before them. Thank Cambridge. you all. It made our task a lot easier! FOURIER, J. 1822. Thdorie Analytique de la Chaleur. Paris. FULLER, J. G. C. M. 2001. Before the hills in order stood: the beginning of the geology of time in References England. In: LEWIS, C. L. E. 8z KNELL, S. J. (eds) The Age of the Earth: from 4004BC to aD 2002. ANON. 1819. Chronology. In: REES, A. (ed.) Cyclo- Geological Society, London, Special Publications, paedia, Vol. 7 [unpaginated]. 190, 15-23. 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HALES, W. 1809-1812. A New Analysis of Chronology PHILLIPS, J. 1849. Geology. Encyclopaedia Metropoli- (three vols). London. tana, Vol. 6. HOFMANN, A. W. 2001. Lead isotopes and the age of REES, M. J. 2001. Understanding the beginning and the the Earth- a geochemical accident. In: LEWIS, end. In: LEWIS, C. L. E. & KNELL, S. J. (eds) The C. L. E. & KNELL, S. J. (eds) The Age of the Earth: Age of the Earth: from 4004Bc to A02002. from 4004BC to A02002. Geological Society, Geological Society, London, Special Publications, London, Special Publications, 190, 223-236. 190, 275-283. HOLMES, A. 1913. The Age of the Earth. Harper, RUDWICK, M. 2001. Jean-Andr6 de Luc and nature's London and New York. chronology. In: LEWIS, C. L. E. & KNELL, S. J. KAMBER, B. S., MOORBATH, S. & WHITEHOUSE, M. J. (eds) The Age of the Earth: from 4004BC to 2001. The oldest rocks on Earth: time constraints AD2002. Geological Society, London, Special and geological controversies. In: LEWIS, C. L. E. 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