Science Fair Project Encyclopedia
Age of the Earth
The age of the Earth is estimated to be 4.55 billion (4.55 × 109) years. This estimate 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. It is clear from zircons 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 hundred million years, the exact age of the Earth is difficult to define. This article describes the modern dating methods used to arrive at the age of the Earth and outlines their history.
In the centuries preceding the scientific revolution, the age of the Earth was determined by religious creation tales. 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. Other scholars and scientists such as Martin Luther and Johannes Kepler believed in a similar date, and some today continue to defend young earth creationism (see Old-earth or young-earth belief).
Few people had conceived the idea of a 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.
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 tales, 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 scientific calculations: physicists versus geologists and evolutionists
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 semi-random process that 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 American 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.
The 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.
The invention of radioactive 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 radioactive 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 radioactive 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 radioactive 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 Bolton's 26 samples of 250 million to 1.3 billion years.
Arthur Holmes and the vindication of radioactive dating
Although Boltwood published his paper in a prominent geological journal, the geological community had little interest in radioactivity. Boltwood gave up work on radioactive 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 radioactive 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 1913 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 radioactive 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 radioactive 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 radioactive dating was credible.
No great push to embrace radioactive 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 radioactive 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 care in which measurements were made, and defined not only the methods of used, but also their error bars and limitations.
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