Age of Earth

This article describes the historical development of the estimation of the "age of the Earth", including modern scientific dating methods. For other views, see origin belief or Dating Creation.

The age of the Earth is around 10,000 years, based on detailed scientific evidence. This calculated age represents a compromise between the oldest known terrestrial minerals – small crystals of zircon from the Jack hills of Western Australia – and astronomers' and planetologists' estimates of the age of the solar system. The radiometric age dating evidence from the zircons further confirms that the Earth is at least 4.404 billion years old. Comparing the mass and luminosity of the Sun to the multitudes of other stars, it appears that the solar system can not be much older than those rocks. Ca-Al-rich inclusions – the oldest known solid constituents within meteorites which are formed within the solar system – are 4.567 billion years old, giving an age for the solar system and an upper limit for the age of the Earth. It is assumed that the accretion of the Earth began soon after the formation of the Ca-Al-rich inclusions and the meteorites. Since the accretion time of the Earth is not exactly known yet and the predictions from different accretion models vary between several millions up to about one hundred million years, the exact age of the Earth is difficult to define.

Prescientific notions

In the centuries preceding the scientific revolution, the age of the Earth was determined from the accounts of creation by religious authority. Today some religious groups continue to accept only theological accounts regarding the age of the earth and reject all scientific evidence.

Certain Hindu puranic views assert that the universe is created, destroyed, and re-created in an eternally repetitive series of cycles. In Hindu cosmology, a universe endures for about 4,320,000,000 years (one day of Brahma, the creator or kalpa) and is then destroyed by fire or water elements. At this point, Brahma rests for one night, just as long as the day. This process, named pralaya (Cataclysm), repeats for 100 Brahma years (311 trillion human years) that represents Brahma's lifespan. We are currently believed to be in the 51st year of the present Brahma and so about 155 trillion years have elapsed since He was born as Brahma. After Brahma's "death", it is necessary that another 100 Brahma years pass until he is reborn and the whole creation begins anew. This process is repeated again and again, forever.

The Han Chinese thought the Earth was created and destroyed in cycles of over 23 million years. Westerners were more conservative. In a book published in 1654, not long before his death, Archbishop James Ussher of Armagh, Ireland, calculated from the Bible (augmented by some astronomy and numerology) that creation began on October 23, 4004 BC.

Few people in Europe had conceived the idea of deep time that stretched far into the past before the arrival of humankind, or far into the future beyond the end of humankind. One who did was Aristotle, who thought the Earth and universe had existed from eternity.

First concepts

By the 18th century, a few naturalists were trying to place the age of the Earth on a more scientific basis. The naturalist Mikhail Lomonosov, regarded as the founder of Russian science, was one of the first to undertake this exercise, suggesting in the mid-18th Century that the Earth had been created separately from the rest of the universe, several hundred thousands of years before.

Lomonosov's ideas were mostly speculative, but in 1779 the French naturalist the Comte du Buffon tried to obtain a value for the age of the Earth using an experiment. He created a small globe that resembled the Earth in composition and then measured its rate of cooling. This led him to estimate that the Earth was about 75,000 years old.

Very few of their colleagues paid them much mind. Many left the question of the age of the Earth to creation mythologies, or simply assumed that the Earth always had been, always would be. However, there were many naturalists whose studies of strata, the layering of rock and earth, gave them an appreciation that the Earth had been through many changes during its existence, however long that might be.

These layers often contained fossilized remains of unknown creatures, and there seemed to be a progression of types of such creatures from layer to layer. In the 1790s, the British naturalist William Smith pointed out that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age.

Other naturalists used this idea to construct a history of the Earth, though their timelines were inexact as they did not know how long it took to lay down such layers. Smith's nephew and student, John Phillips, later calculated by such means that the Earth was about 96 million years old.

In 1830, the geologist Charles Lyell took the next step and proposed that the features of the Earth were in perpetual change, eroding and reforming continuously, and the rate of this change was roughly constant. This was a challenge to the traditional view, which saw the history of the Earth as static, with changes brought about by intermittent catastrophes. Many naturalists were influenced by Lyell to become "uniformitarians" who believed that changes were constant and uniform.

Early calculations: physicists, geologists and biologists

In 1862, the physicist William Thomson of Glasgow published calculations that fixed the age of the Earth at between 20 million and 400 million years. He assumed that the Earth had been created as a completely molten ball of rock, and determined the amount of time it took for the ball to cool to its present temperature.

Geologists had trouble accepting such a short age for the Earth. Biologists could accept that the Earth might have a finite age, but even 100 million years seemed much too short to be plausible. Charles Darwin, who had studied Lyell's work, had proposed his theory of the evolution of organisms by natural selection, a process whose combination of random heritable variation and fundamentally non-random cumulative selection implies great expanses of time. Even 400 million years didn't seem long enough.

In a lecture in 1869, Darwin's great advocate, Thomas H. Huxley, attacked Thomson's calculations, suggesting they appeared precise in themselves but were based on faulty assumptions. Huxley was correct, and in fact Thomson's estimates would prove far too short. Thomson had attempted to root the debate in one set of facts; the geologists and evolutionists had used others; and ultimately the latter two groups were proven more correct.

He certainly managed to provoke a long and productive debate. The German physicist Hermann von Helmholtz and the Canadian astronomer Simon Newcomb joined in by independently calculating the amount of time it would take for the Sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born. They came up with a value of 100 million years, which seemed to set an upper limit on the age of the Earth that was consistent with Thomson's calculations. However, they assumed that the Sun was only glowing from the heat of its gravitational contraction. They knew of no other ways for it to produce its energy.

Other scientists backed up Thomson's figures as well. Charles Darwin's son, the astronomer George H. Darwin of the University of Cambridge, proposed that the Earth and Moon had broken apart in their early days when they were both molten. He calculated the amount of time it would have taken for tidal friction to give the Earth its current 24-hour day, and concluded that Thomson was on the right track.

In 1899, John Joly of the University of Dublin calculated the rate at which the oceans should have accumulated salt from erosion processes, and determined that the oceans were about 90 million years old.

Discovery of radioactivity

By the turn of the 20th century, Thomson had been made Lord Kelvin in appreciation of his many scientific accomplishments. He had reason to feel confident of himself, and the fact that multiple attempts to determine the age of the Earth seemed to show that it was about 100 million years old led him to feel very certain that his estimates were correct. The geologists could only suggest (correctly) that Kelvin didn't have all the facts, and they still believed that the Earth was far older than 100 million years.

The breakthrough that would ultimately resolve the conflict took place in 1896, when the French chemist A. Henri Becquerel discovered radioactivity. In 1898, two other French researchers, Marie and Pierre Curie, discovered the radioactive elements polonium and radium. In 1903 Pierre Curie and his associate Albert Laborde announced that radium produces enough heat to melt its own weight in ice in less than an hour.

Geologists quickly realized that the discovery of radioactivity upset the assumptions on which most calculations of the age of the Earth were based. These calculations assumed that the Earth and Sun had been created at some time in the past and had been steadily cooling since that time. Radioactivity provided a process that generated energy. George Darwin and Joly were the first to point this out, also in 1903.

There was the issue of whether the Earth contained enough radioactive material to significantly affect its rate of cooling. In 1901 two German schoolteachers, Julius Elster and Hans F. Geitel, had detected radioactivity in the air and then in the soil. Other investigators found it in rainwater, snow, and groundwater. Robert J. Strutt of Imperial College, London, found traces of radium in many rock samples, and concluded that the Earth contained more than enough radioactive material to keep it warm for a long, long time.

Strutt's work created controversy in the scientific community. Lord Kelvin spoke for those who still believed in the older estimates, fighting a stubborn rear-guard action in public against the new findings up to his death in 1907, though he admitted in private that his calculations had been shown to be incorrect.

Invention of radiometric dating

To feel completely vindicated, they needed to come up with new and more rigorous estimates of the age of the Earth. Radioactivity, which had overthrown the old calculations, yielded a bonus by providing a basis for new calculations, in the form of radiometric dating.

Rutherford and Soddy had continued their work on radioactive materials and concluded that radioactivity was due to a spontaneous transmutation of atomic elements. An element broke down into another, lighter element, releasing alpha, beta, or gamma radiation in the process. They also determined that a particular radioactive element decays into another element at a distinctive rate. This rate is given in terms of a "half-life", or the amount of time it takes half of a mass of that radioactive material to break down into its "decay product".

Some radioactive materials have short half-lives, some have long half-lives. Uranium, thorium, and radium have long half-lives, and so persist in the Earth's crust, but radioactive elements with short half-lives have generally disappeared. This suggested that it might be possible to measure the age of the Earth by determining the relative proportions of radioactive materials in geological samples.

In reality, radioactive elements do not always decay into nonradioactive ("stable") elements directly, instead decaying into other radioactive elements that have their own half-lives, and so on, until they reach a stable element. Such "decay series", such as the uranium-radium and thorium series, were known within a few years of the discovery of radioactivity, and provided a basis for constructing techniques of radiometric dating.

The pioneers were Bertram B. Boltwood, a young chemist just out of Yale, and the energetic Rutherford. Boltwood had conducted studies of radioactive materials as a consultant, and when Rutherford lectured at Yale in 1904, Boltwood was inspired to describe the relationships between elements in various decay series.

Late in 1904, Rutherford took the first step toward radiometric dating by suggesting that the alpha particles released by radioactive decay could be trapped in a rocky material as helium atoms. At the time, Rutherford was only guessing at the relationship between alpha particles and helium atoms, but he would prove the connection four years later.

Soddy and Sir William Ramsay, then at University College in London, had just determined the rate at which radium produces alpha particles, and Rutherford proposed that he could determine the age of a rock sample by measuring its concentration of helium. He dated a rock in his possession to an age of 40 million years by this technique. This assumed that the rate of decay of radium as determined by Ramsay and Soddy was accurate, and that helium didn't escape from the sample over time. Rutherford's scheme was inaccurate, but it was a useful first step.

Boltwood focused on the end products of decay series. In 1905, he suggested that lead was the final stable product of the decay of radium. It was already known that radium was an intermediate product of the decay of uranium. Rutherford joined in, outlining a decay process in which radium emitted five alpha particles through various intermediate products to end up with lead, and speculated that the radium-lead decay chain could be used to date rock samples.

Boltwood did the legwork, and by the end of 1905 had provided dates for 26 separate rock samples, ranging from 92 to 570 million years. He did not publish these results, which was fortunate because they were flawed by measurement errors and poor estimates of the half-life of radium. Boltwood refined his work and finally published the results in 1907.

Boltwood's paper pointed out that samples taken from comparable layers of strata had similar lead-to-uranium ratios, and that samples from older layers had a higher proportion of lead, except where there was evidence that lead had leached out of the sample. However, his studies were flawed by the fact that the decay series of thorium was not understood, which led to incorrect results for samples that contained both uranium and thorium. However, his calculations were far more accurate than any that had been performed to that time. Refinements in the technique would later give ages for Boltwood's 26 samples of 250 million to 1.3 billion years.

Today's accepted age of the Earth of 4.55 billion years was determined by C.C. Patterson using Uranium-Lead dating on fragments of the Canyon Diablo meteorite and published in 1956.

Arthur Holmes and the vindication of radiometric dating

Although Boltwood published his paper in a prominent geological journal, the geological community had little interest in radioactivity. Boltwood gave up work on radiometric dating and went on to investigate other decay series. Rutherford remained mildly curious about the issue of the age of the Earth but did little work on it.

Robert Strutt tinkered with Rutherford's helium method until 1910 and then ceased. However, Strutt's student Arthur Holmes became interested in radiometric dating and continued to work on it after everyone else had given up. Holmes focused on lead dating, because he regarded the helium method as unpromising. He performed measurements on rock samples in 1911 and concluded that the oldest was about 1.6 billion years old. These calculations were not particularly trustworthy. For example, he assumed that the samples had contained only uranium and no lead when they were formed.

More important, in 1915 research was published showing that elements generally exist in multiple variants with different masses, or "isotopes". In the 1930s, isotopes would be shown to have nuclei with differing numbers of the neutral particles known as "neutrons". In that same year, other research was published establishing the rules for radioactive decay, allowing more precise identification of decay series.

Many geologists felt these new discoveries made radiometric dating so complicated as to be worthless. Holmes felt that they gave him tools to improve his techniques, and he plodded ahead with his research, publishing before and after the First World War. His work was generally ignored until the 1920s, though in 1917 Joseph Barrell, a professor of geology at Yale, redrew geological history as it was understood at the time to conform to Holmes's findings in radiometric dating. Barrell's research determined that the layers of strata had not all been laid down at the same rate, and so current rates of geological change could not be used to provide accurate timelines of the history of the Earth.

Holmes's persistence finally began to pay off in 1921, when the speakers at the yearly meeting of the British Association for the Advancement of Science came to a rough consensus that the Earth was a few billion years old, and that radiometric dating was credible. No great push to embrace radiometric dating followed, however, and the die-hards in the geological community stubbornly resisted. They had never cared for attempts by physicists to intrude in their domain, and had successfully ignored them so far. The growing weight of evidence finally tilted the balance in 1926, when the National Research Council of the US National Academy of Sciences finally decided to resolve the question of the age of the Earth by appointing a committee to investigate. Holmes, being one of the few people on Earth who was trained in radiometric dating techniques, was a committee member, and in fact wrote most of the final report.

The report concluded that radioactive dating was the only reliable means of pinning down geological time scales. Questions of bias were deflected by the great and exacting detail of the report. It described the methods used, the care with which measurements were made, and their error bars and limitations.

Modern radiometric dating

Radiometric dating continues to be the predominant way scientists date geologic timescales. Techniques for radioactive dating have been tested and fine tuned for the past 50+ years. Forty or so different dating techniques are utilized to date a wide variety of materials, and dates for the same sample using these techniques are in very close agreement on the age of the material. Possible contamination problems do exist, but they have been studied and dealt with by careful investigation; leading to sample preparation procedures being minimized to limit the chance of contamination. Hundreds to thousands of measurements are done daily with excellent precision and accurate results. Even so, research continues to refine and improve radiometric dating to this day.

Why meteorites were used

The quoted age of the Earth is derived, in part, from the Canyon Diablo meteorite for several important reasons and is built upon a modern understanding of cosmochemistry built up over decades of research.

Most geological samples from the Earth are not not able to give a direct date of the formation of the Earth from the solar nebula because the Earth has undergone stratification into the core, mantle and crust, and this has then undergone a long history of mixing and unmixing of these sample reservoirs by plate tectonics, weathering and hydrothermal circulation.

All of these processes may adversely affect isotopic dating mechanisms because the sample cannot always be assumed to have remained as a closed system, by which it is meant that either the parent or daughter nucleide or an intermediate daughter nucleide may have been partially removed from the sample, which will skew the resulting isotopic date.

To mitigate this effect it is usual to date several minerals in the same sample, to provide an isochron. Alternately, more than one dating system may be used on a sample to check the date.

Some meteorites are furthermore considered to represent the primitive material from which the accreting solar disk was formed. Some have behaved as closed systems (for some isotopic systems) soon after the solar disk and the planets formed. To date, these assumptions are supported by much scientific observation and repeated isotopic dates, and it is certainly a more robust hypothesis than that which assumes a terrestrial rock has retained its original composition.

Nevertheless, ancient Archaean lead ores of galena have been used to date the formation of the Earth as these represent the earliest formed lead-only minerals on the planet and record the earliest homogeneous lead-lead isotope sytems on the planet. These have returned age dates of 4.54 billion years with a precision of as little as 1%.

Why the Canyon Diablo meteorite was used

The Canyon Diablo meteorite was used because it is a very large representative of a particularly rare type of meteorite which contains sulfide minerals (particularly troilite), metallic nickel-iron alloys, plus silicate minerals.

This is important because the presence of the three mineral phases allows investigation of isotopic dates using samples which provide a great separation in concentrations between parent and daughter nucleides. This is particularly true of uranium and lead. Lead is strongly chalcophile and is found in the sulfide at a much greater concentration than in the silicate, versus uranium.

Because of this segregation in the parent and daughter nucleides during the formation of the meteorite, this allowed a much more precise date of the formation of the solar disk and hence the planets than ever before.

The Canyon Diablo date has been backed up by hundreds of other dates, from both terrestrial samples and other meteorites. The meteorite samples, however, show a spread from 4.53 to 4.58 billion years ago. This is interpreted as the duration of formation of the solar nebula and its collapse into the solar disk to form the sun and the planets. This 50 million year time span allows for accretion of the planets from the original solar dust and meteorites.

The moon as another extraterrestrial body which has not undergone plate tectonics and which has no atmosphere, provides quite precise age dates from the samples returned from the Apollo missions. Rocks returned from the moon have been dated at a maximum of around 4.4 and 4.5 billion years old.

Martian meteorites which have landed upon the Earth have also been dated to around 4.5 billion years old by lead-lead dating.

Altogether the concordance of age dates of both the earliest terrestrial lead reservoirs and all other reservoirs within the solar system found to date are used to support the hypothesis that the Earth and the rest of the solar system formed at around 4.58 to 4.53 billion years ago.

References

Carlson R.W. & Tera F., 1998. Lead-Lead Constraints on the time scale of early planetary differentiation. Origin of Earth and Moon Conference, Lunar and Planetary Society. PDF abstract
Powell, James Lawrence, 2001, Mysteries of Terra Firma: the Age and Evolution of the Earth, Simon & Schuster, ISBN 0-684-87282-X
USGS (1997) Age of the Earth U.S. Geological Survey Accessed Jan. 10, 2006
Terada, K & Sano Y., 2001. In-situ ion microprobe U-Pb dating of phosphates in H-chondrites. in Proceedings of the 11th Annual W.M. Goldschmidt Conference, Lunar and Planetary Society. PDF abstract
Valley, John W., William H. Peck, Elizabeth M. King (1999) Zircons Are Forever, The Outcrop for 1999, University of Wisconsin-Madison Wgeology.wisc.edu – Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago Accessed Jan. 10, 2006
Wilde S.A., Valley J.W., Peck W.H. and Graham C.M. (2001) Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature, v. 409, pp. 175-178.
Wyche, S., D. R. Nelson and A. Riganti (2004) 4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton, Australian Journal of Earth Sciences Volume 51 Zircon ages from W. Australia - Absract Accessed Jan. 10, 2006

External links

TalkOrigins.org
Vectorsite.net – Initial version of this article was based on a public domain text by Greg Goebel
USGS preface on the Age of the Earth
NASA exposition on the age of Martian meteorites