Changing Views of the History of the Earth

If, in the year AD 1600, you had asked an educated European how old the planet Earth was and to recount its history he would have said that it was about 6000 years old and that its ancient history was given by the biblical account in Genesis.

If you asked the same question of an educated European in AD 1900 you would have received a quite different answer. He would have answered that the Earth was ancient, that there had not been a Noachian flood, and that the species of life had not been fixed over the history of Earth. In short, Genesis was an allegory and not literal history.

The story of this great change in the conception of the history of Earth is not a simple one. The chronicle of this great change can be broken into five periods;

The pre-scientific period before AD 1600. In the pre-scientific era the Biblical account and the speculations of the Greek philosophers were accepted without great question.

The era of speculative cosmogonies ran from AD 1600-1700. In this period a number of comprehensive cosmogonies were proposed. These were long on armchair speculation and short on substantive supporting evidence. These cosmogonies were part of the new emphasis of science in seeking rational explanations of the features of the world.

The disestablishment of Genesis ran from AD 1700-1780. This period was marked by a great deal of field geology rather than grand cosmogonies. It became clear that there had been significant changes in the Earth's topography over time and that these changes could neither be accounted for by natural processes operating during the brief nor by the postulated Noachian flood. Notable observations included:

Studies of strata suggested that they were laid down by natural processes in which the sea and land had changed places several times.
Studies of earthquakes and volcanoes showed that the surface crust is subject to massive natural transformation.
Observation of rain, wind, water erosion, and sea erosion in action showed that they were forces capable of reducing mountains and creating valleys.

The catastrophist-uniformitarian debate ran from about 1780-1850. By the end of the 18'th century it was clear that the Earth had a long and varied history. Interest in major cosmogony was revived. The major debate was between the catastrophists, e.g., Cuvier, who held that the history of Earth was dominated by major catastrophic revolutions and the uniformitarians, e.g. Hutton and Lyell, who held that the history of Earth was dominated by slow relatively uniform changes in an Earth with a static over all history. During the early part of this period there was a considerable amount of activity by scriptural geologists who attempted to reconcile Genesis and geology. The efforts of the scriptural geologists failed signally; by 1830 scriptural geology was a dead issue in Science.

The modern period runs from AD 1850 to the present. The great debate was won by the uniformitarians, so much so that the degree of gradualism was overstated and the importance of catastrophes was unduly minimized. The modern period has been marked by an enormous expansion of the detailed knowledge of the geological history of the Earth and the processes that have acted during that history.

Many authors choose to present the history of a complex subject by breaking it up into major threads and following the history of each thread separately. I have chosen instead to provide a chronology of significant works and their authors with a view to providing a sense of how perspectives on Geology changed over time. The selections and comments here are not a complete exposition of the works of the authors mentioned; rather they were chosen to illustrate and exemplify changing perspectives over time.

Estimates of the Age of the Earth

In Europe the issue of the age of the Earth was not a serious one prior to the rise of science; the history of the Earth was assumed to be accounted for in Genesis. The rise of science produced a major change in attitude.

In the pre-scientific world view the issue of the age of the Earth was a theological question. The account in Genesis is replete with miracles that do not stand up under rational analysis. This did not matter; the theological perspective did not require physical rationalization. It was not ruled out, per se, but it was not necessary. It was not part of the attitude. In the new science, however, rational explanation was desirable. Ussher and Descartes illustrate the difference.

In 1640 Ussher produced his famous calculation that the Earth was created in 4004 BC. In 1637 Descartes produced a cosmogony that was highly influential for more than a century. What was the difference?

It was not in their estimates of the age of the Earth - Descartes retained the biblical date. Ussher accepted the Biblical account at face value, relying on the Biblical genealogies and on extant historical records. He implicitly assumed that the world was created much as it is now. Descartes, however, attempted to discern a physical history of the Earth. His account was plausible by the immature standards of the Science of his times; however it quite definitely did not match the Biblical account of a completed creation in six days.

In the 1700's belief in a 6000 year old Earth crumbled. Attempts to calculate the age of the Earth from physical considerations yielded estimates that ranged from 75,000 years (Buffon, 1774) to several billion years (de Maillet, Buffon).

The physical models were open to question and, in retrospect, were naive. The geological evidence was more serious. It became quite clear that many areas of the Earth had alternated between being land and being covered by seas, that there had been extensive slow sedimentation, that the mountains had not been created in situ as is but rather had a long history of slow deformation, and that long periods of erosion had shaped the Earth everywhere.

By the early 1800's it was generally accepted that the Earth had a long history. Its age, however, was scarcely settled. The uniformatarians (Hutton 1788, Lyell 1830) pictured the Earth as being indefinitely old.

The catastrophists (Cuvier 1812, de Beaumont 1852, Buckland 1836) accepted that the Earth was old; they disagreed with the kind of change and the rate of change that had occurred over that long history.

There was no single estimate of the Earth's age in the mid 1800's and no good way to arrive at one. There were various attempts to estimate the Earth's age, working back from sedimentation rates and other geophysical phenomena. The attempts produced estimates from about 100 million years up to several billion years. There were two major problems with such efforts. The first is that the geological history was still being reconstructed. The second is that the rates of the physical processes in question are variable and knowledge of them was incomplete.

In the late 1800's physicists, armed with a more advanced physics than that available to Descartes, made new estimates of the age of the Earth and the Sun. There were two basic questions they asked: How long would it take for the Earth to cool from its initial heat of formation to its present temperature and, given the energy sources known at the time, how long had the Sun been shining.

In 1862 Kelvin estimated the age of the Earth to be 98 million years, based on a model of the rate of cooling. This was a minimum acceptable age consistent with geology. Later in 1897 he revised his estimate downwards to 20-40 million years. This was too short for the geologists to swallow. Estimates of the age of the Sun were also too small to be consistent with geology.

Kelvin did not know about radioactivity and heating of the Earth's crust by radioactive decay; for this reason his estimates were completely wrong. Likewise, it wasn't until Einstein's theory of relativity was developed that there was a good explanation of how the Sun could have been shining as long as it had.

Prior to the development of radiometric dating geologists established the relative ages of rocks using stratigraphy (the geological column) and made crude estimates of absolute ages by taking into account sedimentation and erosion rates. Radiometric dating permits the accurate determination of absolute dates. The first radiometric dating was done in 1905; it and subsequent measurements confirmed that the Earth was several billion years old. Currently the best estimate of the age of the Earth is 4.55 billion years. An extensive chronology of the development of the radiometric dating is given below in the section Chronology of radiometric dating.

It should be understood that estimating the ages of rocks using radiometric dating is an entirely separate technique from the radiocarbon (C-14) method for dating organic remains. Radiometric dating of rocks is based on the decay of long lived isotopes of Potassium, Thorium, and Uranium. Radiocarbon dating is based on the decay of the short lived C-14 isotope and is irrelevant to determining the age of the Earth.

Chronology of Writings

1510	Leonardo Da Vinci: Selections from the Notebooks of Leonardo Da Vinci. In his notebooks Da Vinci ponders fossil seashells and concludes that they could not have been laid down by the Noachian flood. He wrote: "If the Deluge had carried the shells for distances of three and four hundred miles from the sea it would have carried them mixed with various other natural objects all heaped up together; but even at such distances from the sea we see the oysters all together and also the shellfish and the cuttlefish and all the other shells which congregate together, found all together dead; and the solitary shells are found apart from one another as we see them every day on the sea-shores. "And we find oysters together in very large families, among which some may be seen with their shells still joined together, indicating that they were left there by the sea and that they were still living when the strait of Gibraltar was cut through. In the mountains of Parma and Piacenza multitudes of shells and corals with holes may be seen still sticking to the rocks..."
1594	Loys le Roy: Of the interchangeable course or variety of things in the Whole world. Le Roy accepted that land and sea could change places and that mountains could be reduced to plains and vice versa. Le Roy was vague about actual mechanisms. He can be considered as a very early uniformitarian.
1625	Nathaniel Carpenter: Geography delineated forth in two Bookes In this early work Carpenter argued that the Flood could not have been the major agent of geological change,
1634	Simon Stevin: Second Book of Geology. Stevin followed up Le Roy with arguments that wind and water sufficed as primary agents.
1637	Rene Descartes: Discours de la Methode. Descartes constructed a history of the Earth which was quite influential; it was the starting point for many later cosmogonies. Some of the main points of his system were that the Earth formed as a fiery ball, that when it cooled a crust formed over the abyssal waters, and that this crust collapsed, releasing massive volumes of water.
1640	James Ussher: A number of writers calculated the date of creation, using the Biblical chonologies, astronomical records, and historical chronologies. Of these, Ussher's date of 4004 BC is the most famous. Other dates include 3928 BC (John Lightfoot, AD 1644) and 5529 BC (Theophilus of Antioch. AD 169).
1669	Nicholas Steno: The Produmus. Steno did the basic analysis of how fossils got embedded in stone. From his field observations of the Tuscan landscape he concluded that the Flood was important but did not completely explain the observed geology.
1681	Thomas Burnet: Sacred Theory of the Earth. Burnet's famous and widely read book reworked Descartes's speculations to fit the biblical account. In his conception the antediluvian Earth was a smooth ovoid. Over time the surface dried out and the abyssal waters were heated. Eventually the surface cracked, releasing the abyssal waters in the Noachian flood.
1691	John Ray: The Wisdom of God Manifested in the Works of Creation. Ray reworked Burnet's cosmogony. One of the notable features of Ray's works was the thought he put into possible sources for the waters of the flood. Ray accepted that there had been continuous interchange between land and sea.
1693	Baron Leibnitz: Protogea. Leibnitz reworked Descartes's cosmogony. Protogea was published much later in 1749.
1695	John Woodward: An essay toward a Natural History of the Earth. Woodward came down fairly strongly for the view that the flood was an act of God that could not be accounted for by normal physical processes. He also postulated hydrological sorting to account for the ordering of fossils.
1696	William Whiston: A new theory of the Earth.... Whiston added comets to Burnet's cosmogony as the source of the waters of the flood.
1705	Robert Hooke: Lectures and Discourse of Earthquakes and Subterranean Eruptions. Hooke believed that the fossils were the remains of extinct species and could not be accounted for by the Flood. "Asking himself how the present areas of land came to be dry, he answers 'it could be from the Flood of Noah, since the duration of that which was but about two hundred natural days, or half a year could not afford time enough for the production and perfection of so many and so great and full grown shells, as these which are so found do testify; besides the quantity and thickness of the beds of sand with which they are many times found mixed, do argue that there must needs be a much longer time of seas residence of the seas above same, than so short a space can afford."
1748	Benoit de Maillet: Telliamed, or Conversations between an Indian Philosopher and a French Missionary on the Diminution of the Sea. Using Descartes's cosmology, the assumption that the earth was once entirely flooded, and the observation that the sea level was dropping three inches per century near his home, he calculated the age of the earth to be greater than 2 billion years.
1771	Peter Pallas: Observation sur la Formation des Montagnards.... Pallas made extensive observations of Russian mountains. He observed the results of processes that acted on mountains, e.g. weathering, erosion, deposition, and the fracturing and upheaval of strata. He argued for occasional catastrophic events as an origin for mountain building.
1774	Comte de Buffon: Epochs of Nature. Buffon assumed that the earth started molten, measured cooling rates of iron spheres, scaled up, and calculated the age at ~75,000 years. He himself was suspicious that this was much too young and, in manuscripts published after his death, suggested longer chronologies, including one estimate of nearly 3 billion years.
1778	Jean de Luc: Lettres Physique et Morales sur l'Histoire de la Terre et de l'Homme. De Luc's work is "transitional between the armchair speculation of the seventeenth century and the hard-nosed empiricism of the nineteenth century." De Luc accepted the biblical account, including the Noachian flood; however, he assumed that the six days of creation were six long periods of indefinite duration.
1778	John Whitehurst: An inquiry into the Original State of the Earth. Whitehurst added the notion of drastic tidal action of the moon to Woodward's cosmogony.
1779	Horace-Benedict de Saussure: Voyages dans les Alpes. De Saussure made extensive observations of the Alps. He appreciated that curved strata had originally been laid down as horizontal sheets and were later deformed.
1787	Abraham Werner: Kurze Klassification und Beschreibung der verschiedener Gebirgsarten. Werner recognized the importance of successive advance and retreat of the oceans for creating the layers of the Earth.
1788	James Hutton: Theory of the Earth; or, an investigation of the laws observable in the composition, dissolution and restoration of land upon the globe. Hutton is traditionally credited with being the father of modern geology. He was the first modern uniformitarian. Hutton argued that the Earth was of immense antiquity, cycling through changes via slow processes sans catastrophes. The last sentence of Hutton's 1788 work is famous and is widely quoted: The result, therefore, of our present enquiry is, that we find no vestige of a beginning - no prospect of an end.
1794	Robert Townson: Philosophy of Mineralogy. Townson was one of the many catastrophists of the late 18'th and early 19'th century. He pointed out that fieldwork had revealed that the features of the surface of the Earth could not be accounted for by a single Creation and catastrophic flood but rather successions of formation and dramatic change.
1794	Richard Sullivan: A View of Nature. Sullivan was another catastrophist. He wrote: Thus succeed revolution to revolution. When the masses of shells were heaped upon the Alps, then in the bosom of the ocean, there must have been portions of the earth, unquestionably dry and inhabited; vegetable and animal remains prove it; no stratum hitherto discovered, with other strata upon it, but has been, at one time or another, the surface. The sea announces everywhere its different sojournments; and at least yields conviction that all strata were not formed at the same period.
1799	Robert Kirwan: Geological Essays. Kirwan was a scriptural geologist. Although he mostly followed the biblical account in his account the formation of the topography of the Earth took several centuries. Kirwan's virulent attacks on Hutton had the effect of making Hutton much better known than he otherwise would have been.
1812	James Hall: Transactions of the Royal Society of Edinburgh. Hall argued that Hutton's water cycles were insufficient to account for large tumbled rocks in the Alps. He proposed huge waves on a catastrophic scale that moved ice and rock.
1812	Baron de Cuvier: Dicours sur les Revolutions du Globe. Cuvier was the best known and most influential of the catastrophists. His extensive researches in the geology of the Paris basin led him to postulate a series of many global catastrophes.
1820	William Buckland: Vindiciae Geologicae. In 1820 Buckland was a scriptural geologist. Thus he wrote: Again the grand fact of an universal deluge at no very remote period is proved on grounds so decisive and incontrovertible, that, had we never heard of such an event from Scripture, or any other authority, Geology of itself must have called in the assistance of some such catastrophe, to explain the phenomena of diluvian action which are universally presented to us, and which are unintelligible without recourse to a deluge exerting its ravages at a period not more ancient than that announced in the book of Genesis.
1830	Charles Lyell: Principles of Geology. This was the work that "won" the catastrophist/uniformitarian debate. Lyell laid down four principles of uniformity: Uniformity of law (the natural laws have remained the same) Uniformity of process (same causes today as in the past) Uniformity of rate (changes occurred at the same rate as now) Uniformity of state (the Earth was much the same in the past as it is now) In modern Geology it is generally recognized that Lyell claimed too much in the last three principles. Drastic changes, albeit not as all embracing as those envisioned by the catastrophist, occur from time to time. There have been significant changes in state due to such factors as declining strength of the radioactive sources of heat, the acquisition of oxygen as a major atmospheric component, the colonization of land by life, plate tectonics, and asteroid bombardment.
1836	William Buckland: Geology and Mineralogy considered with reference to natural Theology. By 1836 Buckland had abandoned the Noachian flood as a source of major geological change. Instead he postulated numerous antediluvian catastrophes.
1852	Jean Baptiste de Beaumont: Notice sur des Systemes de Montagnes. De Beaumont was a relatively late catastrophist. He argued that as the Earth cools its volume slowly reduces. The shrinkage causes the formation of mountains via catastrophic crumpling of the surface.
1857	Hugh Miller: The Testimony of the Rocks. Miller was a very popular creationist geologist. He believed that the Noachian flood was a local flood in the Mideast and did not credit the theory that the Earth was young. On page 324 he wrote: "No man acquainted with the general outlines of Palaeontology, or the true succession of the sedimentary formations, has been able to believe, during the last half century, that any proof of a general deluge can be derived from the older geologic systems, -- Palaeozoic, Secondary [Mesozoic], or Tertiary."
1862	Lord Kelvin: On the Secular Cooling of the Earth. Using thermodynamic principles and measurements of thermal conductivity of rocks, Kelvin calculated that the earth consolidated from a molten state 98 million years ago. In 1897, he revised his estimate to 20-40 million years. Dalrymple says that Kelvin's estimates were "highly authoritative" for three decades, but notes that they were challenged by people from several fields, including T. H. Huxley, John Perry (a physicist), and T. C. Chamberlain (a geologist). All of them challenged the likelihood of Kelvin's assumptions.
1893	Charles D. Walcott: Geologic Time, as Indicated by the Sedimentary Rocks of North America. Walcott takes a detailed look at the Paleozoic sediments of the Cordilleran Sea (just east of the Sierra Nevadas), considering such things as the land area supplying sediments and the grain sizes of the sediments. He arrived at an estimate of 17.5 million years for the Paleozoic and, based on various other authors' estimates of relative ages of the other eras, 55 million years for the earth.
1905	Ernest Rutherford: In the Silliman Lectures at Yale, Rutherford suggested using radioactivity as a geological timekeeper. The idea was good but there were practical problems. Initially little was known about the physics and chemistry of radioactive elements. Instrumentation had to be improved. The next section is a chronology of key events in working out the age of the Earth using radiometric dating.

Chronology of Radiometric Dating

By Chris Stassen (with much owed to Dalrymple's The Age of the Earth)
Thanks also to Richard Harter for much help.

The period 1896-1905 marks the discovery of radioactivity and the realization that rocks could be dated by radioactive decay.

1896	A. Henri Becquerel discovers that uranium-bearing compounds emit invisible rays similar to X-rays. (X-rays had been discovered in 1895 by Wilhelm Roentgen.)
1898	Marie and Pierre Curie coin the term "radioactivity," prove that radioactivity is a property of atoms (as opposed to molecular composition), discover radioactivity of thorium, and identify a few of the intermediate products of the uranium and thorium decay series.
1902	Ernest Rutherford and Frederick Soddy demonstrate the exponential nature of radioactive decay.
1905	In a lecture at Harvard, Ernest Rutherford suggests that uranium/helium or uranium/lead ratios could theoretically be used to compute the age of rocks.

At this point the phenomenon of radioactive decay was still very poorly understood. The intermediate products and end-products were not known with certainty. The decay rates were entirely unknown, except for that of radium (a short-lived intermediate product which the Curies had identified and isolated). Researchers were unaware that there can be multiple isotopes of the same element, each with a different decay rate.

However, this did not prevent geologists from making several uranium/helium and uranium/lead measurements over the next few years. In many cases the work was done on rocks whose relative ages were known independently, in order to assess whether or not the element ratios correlated with relative age. It was discovered that uranium/helium is not generally reliable because helium is not retained consistently.

1907

B.B. Boltwood takes measurements that indicate lead to be a final product of uranium decay, for its abundance is strongly correlated with relative age of uranium-bearing minerals. Boltwood attempts some simple uranium/lead ages, extrapolating the uranium decay rate from the assumption of decay equilibrium and the previously measured radium decay rate. (When a decay series has reached equilibrium, the ratio of the quantity of elements present is equal to the ratio of their decay rates.)

1911

Arthur Holmes publishes several uranium/lead ages based mostly on measurements taken by Boltwood and an improved value for the uranium decay rate. These range from 340 million years (a Carboniferous sample), to 1,640 million years (a Precambrian sample).

Holmes' calculations are called chemical ages (as opposed to isotope ages) because they are derived from ratios of elements without regard to isotopes. In 1911 geologists did not know about isotopes, or about all of the intermediate decay products in between uranium and lead, or that lead was also produced by the decay of thorium. As a result of not compensating for those (then-unknown) factors, the computed ages are too high.

Even though Holmes' ages are incorrect, they eventually prove to be much better estimates than the best ones previously available to geologists (which were based on non-uniform and unreliable processes such as rates of sedimentation). Holmes' ages for Phanerozoic (Cambrian or later) samples are within 20% of the values given by modern methods. In the early 1900s, however, Holmes' results appeared to be at odds with other methods in common use, and they were not met with immediate acceptance from all quarters.

1913	J.J. Thompson observes that neon atoms have two different atomic weights (20 and 22), using equipment he calls a "positive-ray" apparatus. The existence of isotopes is confirmed. Unfortunately, it would take a long time to accumulate significant knowledge on the isotopes relevant to geological dating. Chemical dating methods won't entirely give way to isotope dating methods until almost 1940.
1917	J. Barrell publishes a Phanerozoic time scale based on chemical ages produced by Holmes (1911), and interpolations involving less quantitative methods. The divisions in the time scale fall fairly close to today's accepted values. For example, Barrell placed the Cenozoic-Mesozoic (Cretaceous-Tertiary) boundary at 55-65 million years ago (today's value: 65 million years ago), and the base of the Cambrian at 360-540 million years ago (today's value: 570 million years ago).
1920	F.W. Aston improves upon Thompson's (1913) positive-ray apparatus, and invents what he calls a "mass spectrograph." Using this device, he discovers a third isotope of neon with atomic weight 21. Aston devotes the remainder of his life to improving the design and precision of his device, and over time discovers 212 of the 287 naturally occurring isotopes.

The early period was one of developing knowledge and technique and of assessing the ages of individual rocks and formations. However, researchers were beginning to realize that the same methods hold promise for assessing the Earth's age.

Calculating an age for the Earth introduces additional complexity: even if it is a given that accurate ages for rocks can be obtained, there is no guarantee that the age of any given rock would be the age of the Earth. It would be necessary to either find rocks which formed at the same time as the Earth, or else come up with dating techniques that could "look back" through more recent events to the Earth's formation.

1921

Henry Russell calculates a maximum chemical age of eight billion years for the Earth's crust, based on estimates of its total uranium and lead content. Using the age of the oldest known (at that time) Precambrian minerals as a minimum for the Earth's age, Russell said:

Taking the mean of this and the upper limit found above from the ratio of uranium to lead, we obtain 4 x 10⁹ years as a rough approximation to the age of the Earth's crust.
(Russell 1921, quoted in Dalrymple 1991)

1927

Arthur Holmes publishes a booklet on the age of the Earth, which becomes fairly popular. The booklet contains a revised version of Russell's calculation, based on different estimates of the total quantity of uranium and lead in the Earth's crust. Holmes suggests that the age of the Earth is between 1.6 and 3 billion years. Twenty years after the first serious attempts at radioactive-decay ages (Boltwood 1907), the total number of computed mineral ages is still small enough that Holmes can summarize them all in one short table.

In between 1921 (Russell's estimate) and roughly World War II, a number of similar chemical ages for the Earth's crust were computed and published. These include: 3.4 billion years (Rutherford 1929); 4.6 billion years (Meyer 1937); and 3 to 4 billion years (Starik 1937).

1927b	F.W. Aston makes the first measurements of the isotopic ratios of "common lead." At this time it was already known that lead found in association with uranium had a relatively low atomic weight, but it seemed that all other lead (known as "common lead") had the same atomic weight. (The lighter atomic weight of lead in association with uranium is due to enrichment in ²⁰⁶Pb from decay of ²³⁸U. ²⁰⁶Pb is lighter than the atomic weight of common lead, which is about 207.2.)
1937	Alfred Nier begins to make a series of careful measurements on the isotopic composition of common lead. He discovers that the isotopic ratios of common lead can vary significantly, even in cases where the atomic weight does not. The most common radiogenic lead isotopes -- ²⁰⁸Pb (from ²³²Th) and ²⁰⁶Pb (from ²³⁸U) -- have on average roughly the same atomic weight as "common lead." As long as both are added in approximately equal amounts, the isotopic composition (relative to ²⁰⁴Pb) would be changed but the atomic weight would not. Nier concludes that the variations in isotopic composition of "common lead" are due to mixture in varying degrees between radiogenic lead and "primeval" lead (which existed in a fixed, but at this point in time unknown, isotopic ratio at the time of formation of the Earth).
1941	Alfred Nier obtains and measures some ancient Pb ores which have the lowest ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb ratios of any rocks found to date. (²⁰⁴Pb is not produced by radioactive decay, while all other stable isotopes of lead are. The lower the ratio of other lead isotopes to ²⁰⁴Pb, the less radiogenic lead is present.) Nier speculates that these represent approximately the "primeval" Pb isotope ratios.
1941b	E. Gerling uses Nier's (1941) "primeval" lead isotope ratios to create lead isotopic growth curves, and uses these to estimate a minimum age for the Earth's crust of 3.2 billion years. In doing so, Gerling devises the basic technique which will eventually produce an accurate age for the Earth and solar system. Unfortunately, Gerling's original calculations are incorrect primarily because Nier's ancient lead ore is not truly "primeval" in composition. Though Gerling's result is within 30% of the actual age of the Earth, it is merely a good measurement of the age of Nier's samples rather than the age of the planet itself.
1944	During World War II, intense research on the atomic bomb leads to fantastic improvements in equipment for identifying and analyzing isotopes. It becomes possible to detect minute quantities of specific isotopes, and to measure their abundance with high precision.
1946	Alfred Nier improves on the design of the mass spectrometer and his machine shop builds dozens of the devices. The widespread availability of this equipment allows a much larger number of researchers to enter into the study of isotope geology. By the early 1950s, universities all over the world have laboratories dedicated to performing isotopic age assessments.
1946b	Arthur Holmes produces calculations based on Nier's (1941) data. Holmes was unaware of Gerling's (1941b) work and attempted a slightly different technique. Holmes' computations result in a wide range of values; when plotted on a histogram, an obvious peak in the measurements occurs at about 3.3 billion years (a figure similar to Gerling's). Holmes' computation involves the assumption that lead on Earth had been separated once long ago and the individual units had been allowed to evolve along independent isotopic growth curves. Due to that assumption being incorrect, Holmes mis-interprets scatter around a single growth curve as a number of independent growth curves. His work on tracing the "independent" curves back to their mutual intersection does not yield meaningful results.
1946c	F. Houtermans independently performs calculations that are similar to Holmes' (1946b) and flawed in essentially the same way. His work is noteworthy in that he is the first to emphasize that the data on different isotopic growth curves would be co-linear if they started at the same point, and for these lines he coins the term "isochrones" (now known as "isochrons").

By 1946 equipment and understanding of the decay process are sufficiently mature to generate an accurate assessment of the age of the Earth. It had been amply established that isotope dating can yield precise and meaningful results. However, the major remaining problem is still the same as that of almost thirty years prior: exactly how to apply the techniques, and what to apply them to, in order to obtain an age for the Earth.

The evaluation of lead isotopic growth curves (somewhat unfairly to Gerling, known as the Holmes-Houtermans Model) holds promise, for it can look back through recent events to a point of origin. However, the key -- and still missing -- data needed in order to use such a method would be the lead isotopic ratios at the time of the Earth's formation (i.e., that of "primeval" lead).

1953	Clair C. Patterson produces accurate "primeval" lead isotopic measurements from minerals of the Canyon Diablo meteorite which contain very little (less than ten parts per billion) uranium. Meteorites provide the final solution to the puzzle, for they both are "rocks which formed at the same time as the Earth," and provide the important data which allows lead isotope computations to look back to the formation of the Earth. There had previously been no way to directly assess the age of the Earth; once meteorites were involved, suddenly there were several independent means. In a recent issue of the Caltech Alumni Magazine, Clair Patterson discussed the ideas that led up to the measurement: [Harrison] Brown had worked out this concept that the lead in iron meteorites was the kind of lead that was in the solar system when it was first formed, and that it was preserved in iron meteorites without change from uranium decay, because there is no uranium in iron meteorites. [...] There are two isotopes of uranium that decayed to two different isotopes of lead, and there's also thorium, which decays to another isotope of lead. So you have three different isotopes of lead. And the whole thing gets mixed up. You've got all these separate age equations for the different isotopes of uranium and different isotopes of lead that were formed. [...] If we only knew what the isotopic composition of primordial lead was in the Earth at the time it formed, we could take that number and stick it into this marvelous equation that the atomic physicists had worked out. And you could turn the crank and blip--out would come the age of the Earth. (Patterson 1997)
1953b	F.G. Houtermans uses Patterson's (1953) data and the lead isotopic ratios of young terrestrial sediments, to compute a rough age for the Earth of 4.5 ± 0.3 billion years. These represent the first publication of the right value by a valid calculation. However, Houtermans' calculations are essentially isochrons based on two data points (one data point for iron meteorites, another for young terrestrial sediments). Without additional data to tie the Earth and meteorites to a common source, the computed values are not guaranteed to be meaningful.
1956	Clair C. Patterson publishes an isochron age for the solar system (and therefore the Earth) of 4.55 ± 0.07 billion years. The age computation is based on Pb isotope analysis of five meteorites. Patterson points out that data for young Earth sediments fall on the same isochron; this implies that the Earth shares a common origin with the dated meteorites. Though only a few meteorites had been dated at this point in time, and the individual meteorite ages that did exist were not very precise, they also agree with the isochron age.
1998	A lot of data has been collected since Patterson's (1953, 1956) and Houtermans' (1953b) works. Precision of instruments has improved. Many more meteorites have been sampled and dated. Moon rocks have been sampled and dated. Decay constants have been measured with more accuracy. New techniques have been devised, tested, and applied. The arrival of this new data has two effects: (1) some new data can be used to improve the precision of the original computations; and (2) new independent measurements confirm the original ones. Purely by coincidence, all of the adjustments (for example, current values of decay constants) to Patterson's 1956 computation have canceled each other out. Today's best estimate of the age of meteorites (4.55 ± 0.02 billion years) is identical to Patterson's value except for the smaller error range. That value has been confirmed dozens of times over. The best estimate of the age of the Earth today is the same as that for meteorites: 4.55 ± 0.02 billion years. In the event that one wishes to be extra cautious in reporting a value, using the very generous error range of 4.5 ± 0.1 billion years is almost certain to encompass future changes as well. For further detail on this topic, I strongly recommend G. Brent Dalrymple's The Age of the Earth.

References

Most of the references and quotations in the Chronology have been been taken from the Catastrophism by Richard Huggett. This work is a synoptic view of changing perspectives both of change in the inorganic and organic world. Dalrymple's Age of the Earth is a standard source for understanding how the age of the Earth is determined.

Russell, H.N., 1921. A superior limit to the age of the Earth's crust in Proceedings of the Royal Society of London, series A, vol. 99, pp. 84-86.

Dalrymple, G. Brent, 1991. The Age of the Earth. California: Stanford University Press, ISBN 0-8047-1569-6.

Patterson, C.C., 1953. "The isotopic composition of meteoritic, basaltic and oceanic leads, and the age of the Earth" in Proceedings of the Conference on Nuclear Processes in Geologic Settings, Williams Bay, Wisconsin, September 21-23, 1953. pp. 36-40.

Patterson, Clair C., 1997. Duck Soup and Lead in Engineering & Science (Caltech Alumni Magazine) volume LX, number 1, pp. 21-31.

Russell, H.N., 1921. A superior limit to the age of the Earth's crust in Proceedings of the Royal Society of London, series A, vol. 99, pp. 84-86.

Acknowledgements

I particularly want to thank Mark Isaak who supplied a number of references which were not available to me, Chris Stassen for supplying the section on the history of radiometric dating, and Andrew MacRae who supplied information about Hugh Miller's The Testimony of the Rocks.

Changing Views of the History of the Earth

Contents

Introduction

Estimates of the Age of the Earth

Chronology of Writings

Chronology of Radiometric Dating

References

Acknowledgements