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The age of the Earth is around 4.54 billion years (4.54 × 109 years ± 1%).[1][2][3] This age has been determined by radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples. The Sun, in comparison, is about 4.57 billion years old, about 30 million years older.
Following the scientific revolution and the development of radiometric age dating, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.[4] The oldest such minerals analyzed to date – small crystals of zircon from the Jack Hills of Western Australia – are at least 4.404 billion years old.[5][6][7] Comparing the mass and luminosity of the Sun to the multitudes of other stars, it appears that the solar system cannot be much older than those rocks. Ca-Al-rich inclusions (inclusions rich in calcium and aluminium) – the oldest known solid constituents within meteorites that are formed within the solar system – are 4.567 billion years old,[8][9] giving an age for the solar system and an upper limit for the age of Earth. It is hypothesised that the accretion of Earth began soon after the formation of the Ca-Al-rich inclusions and the meteorites. Because the exact accretion time of Earth is not yet known, and the predictions from different accretion models range from a few millions up to about 100 million years, the exact age of Earth is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages. The Acasta Gneiss of Northern Canada may be the oldest known exposed crustal rock.[10]
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Studies of strata, the layering of rock and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence. These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer.[11][12]
Abū Rayhān Bīrūnī (11th century CE) discovered the existence of shells and fossils in regions that were once sea floor, but were later uplifted to become dry land, such as the Indian subcontinent. Based on this evidence, he realized that the Earth is constantly changing and proposed that the Earth had an age, but that its origin was too distant to measure.[13] The principle of superposition of strata was first proposed by Avicenna (11th century). He outlined the principle while discussing the origins of mountains in The Book of Healing in 1027.[14][15] Shen Kuo (11th century) also later recognized the concept of deep time.[16]
Nicolas Steno (17th century) was one of the first Western naturalists to appreciate the connection between fossil remains and strata.[12] His observations led him to formulate important stratigraphic concepts (i.e., the "law of superposition" and the "principle of original horizontality").[17] In the 1790s, the British naturalist William Smith hypothesized 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.[18] William Smith's nephew and student, John Phillips, later calculated by such means that Earth was about 96 million years old.[19]
The naturalist Mikhail Lomonosov, regarded as the founder of Russian science, suggested in the mid-18th century that Earth had been created separately from the rest of the universe, several hundred thousand 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 Earth using an experiment: He created a small globe that resembled Earth in composition and then measured its rate of cooling. This led him to estimate that Earth was about 75,000 years old.
Other naturalists used these hypotheses to construct a history of Earth, though their timelines were inexact as they did not know how long it took to lay down stratigraphic layers. In 1830, the geologist Charles Lyell, developing ideas found in Scottish natural philosopher James Hutton, popularized the concept that the features of 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 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.
In 1862, the physicist William Thomson (who later became Lord Kelvin) of Glasgow published calculations that fixed the age of Earth at between 20 million and 400 million years.[20][21] He assumed that Earth had formed as a completely molten object, and determined the amount of time it would take for the near-surface to cool to its present temperature. His calculations did not account for convection inside the Earth, which allows more heat to escape from the interior to warm rocks near the surface.[20]
Geologists had trouble accepting such a short age for Earth. Biologists could accept that 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 cumulative selection implies great expanses of time. (Geneticists have subsequently measured the rate of genetic divergence of species, using the molecular clock, to date the last universal ancestor of all living organisms no later than 3.5 to 3.8 billion years ago).
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. The German physicist Hermann von Helmholtz (in 1856) and the Canadian astronomer Simon Newcomb (in 1892) contributed their own calculations of 22 and 18 million years respectively to the debate: they independently calculated 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.[22] Their values were consistent with Thomson's calculations. However, they assumed that the Sun was only glowing from the heat of its gravitational contraction. The process of solar nuclear fusion was not yet known to science.
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 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 Earth its current 24-hour day. His value of 56 million years added additional evidence that Thomson was on the right track.[22]
The last estimate Thomson gave, in 1897, was: "that it was more than 20 and less than 40 million year old, and probably much nearer 20 than 40".[23]
In 1899 and 1900, John Joly of the Trinity College, Dublin calculated the rate at which the oceans should have accumulated salt from erosion processes, and determined that the oceans were about 80 to 100 million years old.[22]
Rock minerals naturally contain certain elements and not others. By the process of radioactive decay of radioactive isotopes occurring in a rock, exotic elements can be introduced over time. By measuring the concentration of the stable end product of the decay, coupled with knowledge of the half life and initial concentration of the decaying element, the age of the rock can be calculated. Typical radioactive end products are argon from potassium-40 and lead from uranium and thorium decay. If the rock becomes molten, as happens in Earth's mantle, such nonradioactive end products typically escape or are redistributed. Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth assuming that a rock cannot have been in existence for longer than Earth itself.
In 1892, Thomson had been made Lord Kelvin in appreciation of his many scientific accomplishments. Kelvin calculated the age of Earth by using thermal gradients, and arrived at an estimate of 100 million years old.[24] He did not realize that Earth has a highly viscous fluid mantle, and this ruined his calculation. In 1895, John Perry produced an age of Earth estimate of 2 to 3 billions years old using a model of a convective mantle and thin crust.[24] Kelvin stuck by his estimate of 100 million years, and later reduced the estimate to about 20 million years.
Radioactivity would introduce another factor in the calculation. In 1896, the French chemist A. Henri Becquerel discovered radioactivity. In 1898, Polish and 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 Earth were based. These calculations assumed that Earth and Sun had formed at some time in the past and had been steadily cooling since that time. Radioactivity provided a process that generated heat. George Darwin and Joly were the first to point this out, also in 1903.[25]
Radioactivity, which had overthrown the old calculations, yielded a bonus by providing a basis for new calculations, in the form of radiometric dating.
Ernest Rutherford and Frederick Soddy, working jointly at McGill University, had continued their work on radioactive materials and concluded that radioactivity was due to a spontaneous transmutation of atomic elements. In radioactive decay, an element breaks down into another, lighter element, releasing alpha, beta, or gamma radiation in the process. They also determined that a particular isotope of a 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 and thorium have long half-lives, and so persist in 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 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 of radioactivity 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,[26] 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. Rutherford wrote,
I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in trouble at the last part of my speech dealing with the age of the earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye, and cock a baleful glance at me! Then a sudden inspiration came, and I said, 'Lord Kelvin had limited the age of the earth, provided no new source was discovered. That prophetic utterance refers to what we are now considering tonight, radium!' Behold! the old boy beamed upon me.[27]
Rutherford assumed that the rate of decay of radium as determined by Ramsay and Soddy was accurate, and that helium did not 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.[4]
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. 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.
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 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 and concluded in 1911 that the oldest (a sample from Ceylon) was about 1.6 billion years old.[28] 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 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 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 Earth was a few billion years old, and that radiometric dating was credible. Holmes published The Age of the Earth, an Introduction to Geological Ideas in 1927 in which he presented a range of 1.6 to 3.0 billion years. 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 1931, when the National Research Council of the US National Academy of Sciences finally decided to resolve the question of the age of 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.[29]
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.
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.
Today's accepted age of Earth of 4.54 billion years was determined by C.C. Patterson using uranium-lead isotope dating (specifically lead-lead dating) on several meteorites including the Canyon Diablo meteorite and published in 1956.[30]
The quoted age of 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 Earth are unable to give a direct date of the formation of Earth from the solar nebula because Earth has undergone differentiation 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 nuclide (a species of atom characterised by the number of neutrons and protons an atom contains) or an intermediate daughter nuclide 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.[31] 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 Earth as these represent the earliest formed lead-only minerals on the planet and record the earliest homogeneous lead-lead isotope systems on the planet. These have returned age dates of 4.54 billion years with a precision of as little as 1% margin for error.[32]
Statistics for several meteorites that have undergone isochron dating are as follows:[33]
1) St. Severin (ordinary chondrite) a. Pb-Pb isochron - 4.543 +/- 0.019 GY b. Sm-Nd isochron - 4.55 +/- 0.33 GY c. Rb-Sr isochron - 4.51 +/- 0.15 GY d. Re-Os isochron - 4.68 +/- 0.15 GY
2) Juvinas (basaltic achondrite) a. Pb-Pb isochron ..... 4.556 +/- 0.012 GY b. Pb-Pb isochron ..... 4.540 +/- 0.001 GY c. Sm-Nd isochron ..... 4.56 +/- 0.08 GY d. Rb-Sr isochron ..... 4.50 +/- 0.07 GY
3) Allende (carbonaceous chondrite) a. Pb-Pb isochron ..... 4.553 +/- 0.004 GY b. Ar-Ar age spectrum ..... 4.52 +/- 0.02 GY c. Ar-Ar age spectrum ..... 4.55 +/- 0.03 GY d. Ar-Ar age spectrum ..... 4.56 +/- 0.05 GY
The Canyon Diablo meteorite was used because it is a very large representative of a particularly rare type of meteorite that contains sulfide minerals (particularly troilite, FeS), 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 that provide a great separation in concentrations between parent and daughter nuclides. This is particularly true of uranium and lead. Lead is strongly chalcophilic 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 nuclides 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.[34] 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 that has not undergone plate tectonics and that 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 that have landed upon Earth have also been dated to around 4.5 billion years old by lead-lead dating. Lunar samples, since they have not been disturbed by weathering, plate tectonics or material moved by organisms, can also provide dating by direct electron microscope examination of cosmic ray tracks. The accumulation of dislocations generated by high energy cosmic ray particle impacts provides another confirmation of the isotopic dates. Cosmic ray dating is only useful on material that has not been melted, since melting erases the crystalline structure of the material, and wipes away the tracks left by the particles.
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 Earth and the rest of the solar system formed at around 4.53 to 4.58 billion years ago.
The radiometric date of meteorites can be verified with studies of the Sun. The Sun can be dated using helioseismic methods that strongly agree with the radiometric dates found for the oldest meteorites.[35]
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This is a learning project for exploration of scientific methods that have been used to measure the age of the Earth.
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Until the Scientific revolution there was no way for people to systematically explore the age of the Earth. People in some cultures imagined that the Earth was very old (maybe even infinitely old) and others imagined that it was young, possibly only a few thousand years old.
Even after modern science began to develop in Western Europe, exploration of topics such as the age of the Earth was inhibited by cultural momentum. When Galileo was led by his astronomical observations to "radical" ideas such as the Earth moving around the Sun, he was ordered to abandon his heliocentric ideas. This religiously-motivated ban on advocacy of heliocentrism lasted more than 100 years. Ernst Mayr suggested that new discoveries such as recognition by astronomers of the large distance to the stars was important in allowing Europeans to begin to think about both vast space and long durations of time[1]. In response to astronomical observations made using telescopes, some philosophers such as Immanuel Kant (1755, Universal Natural History and Theory of Heaven) eventually began to discuss cosmological theories in which the universe might have "infinite extent, both in space and time".
Which cultures around the world have favored either short or long ages for the Earth?
During the 1700s geologists began to interpret evidence such as sedimentary rock strata as being consistent with vast periods of time during which erosion produced layers of sediments that were compacted into rock. Buffon thought it likely that the Earth was hundreds of thousands of years old, but such estimates were based on indirect arguments linking observations of recent geological processes to interpretations of observable rock strata.
What are the deepest known sedimentary rocks?
See also: Geological structures of fossiliferrous bearing material.
Along with careful analysis of sedimentary rock strata came recognition that different types of fossil organisms are found in the various rock strata. Charles Darwin's intuitions about the time required for biological evolution made him comfortable with an interpretation of the geological evidence consistent with Earth being billions of years old.
When were the first microscopic fossils recognized?
Reading: Solution to Darwin's dilemma:
Discovery of the missing Precambrian record of life by J.
William Schopf
See also: Bacterial
fossils
In Darwin's time, physical scientists such as William Thomson performed calculations on physical processes such as the cooling of the Earth and the burning of chemical fuel by the sun. They concluded that the Earth might only be tens of millions of years old. These estimates, based on seemingly irrefutable physical principles deeply troubled Darwin. How could the geological record and a gradual evolution of the great diversity of living organisms be made compatible with such a young age for the Earth?
Marie Curie and her collaborators eventually discovered radioactivity. Radioactivity provides a source of heat for the Earth that was unknown during Darwin's life time. Nuclear fusion was eventually recognized as a natural energy source that could keep the sun burning far longer than any chemical fuel.
Assume that the Sun is made of coal. Estimate how much longer the Sun could burn at its current rate of energy release.
Radiometric dating makes use of the existence of radioisotopes. An example of a useful radioisotope for dating very old rocks is uranium. Uranium-238 is the most common isotope of uranium, accounting for more than 99% of uranium atoms in rocks on Earth. Uranium-238 nuclei are not stable: uranium-238 has a half life of about 4.5 billion years. Uranium-238 decays to lead-206. There are some minerals such as zircon which incorporate uranium but not lead. Most of the lead found in zircon comes from decay of uranium, so a determination of the ratio of uranium to lead allows estimation of the age of zircon-containing rock. This method was used as early as 1907 by Bertram Boltwood, who found that some Earth rocks appeared to be well over a billion years old.
Since the best estimates of the age of the Earth come from studies using radiometric dating, it is worth asking where radioactive isotopes such as uranium-238 come from. Cosmologists estimate that the universe is about 14 billion years old (see: age of the universe). About 14 billion years ago, most of the atoms of our universe formed and have remain unchanged as hydrogen and helium atoms. Most heavier atoms such as uranium apparently formed inside stars during the past 14 billion years (see: Supernova nucleosynthesis). Nucleosynthesis of uranium inside stars was followed by release of that uranium when stars exploded. The Sun is classified as a third generation star because of its relatively large amount of high atomic number atoms. The vast majority of atoms in the universe are hydrogen atoms. Stars such as the Sun fuse hydrogen atoms and produce helium atoms. An estimate of the age of the Sun can be made based on the ratios hydrogen and helium. The large amount of helium in the Sun (about 25%) is consistent with the solar system being several billion years old.
At the current rate of hydrogen --> helium fusion, how long will it take to increase the helium content of the Sun from 25% to 50%?
There are some serious limitations on the use of radiometric dating of Earth rocks as a way to estimate the age of the Earth. There may have been an extended period of time during which the Earth existed without solid minerals and rocks. It is widely assumed that when the Earth was young it was too hot to allow formation of a solid crust. Estimation of the age of the Earth by radiometric dating of Earth rocks relies on the formation of minerals under relatively cool conditions and the persistence of those minerals at cool temperatures until the time they are processed for radiometric dating.
An additional problem is that the Earth remains geologically active. Rocks and minerals near the surface of the Earth are at risk of being subjected to subduction, heating and removal of any once accessible radiometric record of the past.
Does it matter if we can precisely date the Earth? One reason to try to find and date the oldest rocks is to compare when they formed to the age of the rocks with the oldest fossils.
When the solar system formed, mineral condensations formed in space millions of years before the Sun ignited. Meteorites have been dated by radiometric methods, providing estimates of how long ago the Earth probably began to form by a process of accretion.
The age of the Earth, which is part of Earth science, was a difficult problem to solve. For most of human history, the basic facts about the planet were unknown. The problem was solved by science, but not until the field of radioactive dating was developed in the 20th century.
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The first facts about the Earth were worked out by the Ancient Greeks. A good estimate of the Earth's size was made by Eratosthenes (276BC–194BC), using trigonometry. The first estimate of the Earth's age based on evidence was by Benoît de Maillet (1656–1738), a French diplomat, philosopher and naturalist. He thought the Earth must have developed by slow, natural forces. He studied geology in the field. He could see signs of erosion on land and sedimentation in the sea.[1] He hid his ideas under the guise of a talk with an Indian philosopher, to avoid conflict with the Catholic Church. His work stayed in manuscript until after his death, when it was published after its editor had made changes which damaged it. Now there is a better modern edition based on the manuscripts.[2] de Maillet estimated that the Earth was older than two billion years. He also recognized the true nature of fossils, and had early ideas about evolution.
In the last quarter of the 19th century there was a long-running debate on the age of the Earth. In Charles Lyell's Principles of Geology (1830–33), he showed that the Earth had changed slowly, and that what we see is the result of gradual changes. This clearly meant that the Earth was ancient, though Lyell did not try to work out how old. His younger friend Charles Darwin believed this, too. Darwin saw that if evolution had taken place, it would have required a long time. Also, a huge amount of sedimentary rock lies between the early fossils in the Cambrian strata, and the present land surface. Darwin and Lyell agreed that it would have taken a very long time for so much rock to be deposited.
In the first edition of On the origin of species (1859), Darwin had estimated that the erosion of the Sussex Weald must have taken 300 million years.[3]p314 Both he and Lyle were surprised when the physicist William Thomson (Lord Kelvin) said the Earth could not be as old as they thought. He made the calculation based on how long it must have taken the Earth to cool down to its present temperature, given a starting point of 2,000oC. Kelvin's result was based on the idea that the near-surface geothermal gradient reflects the conductive cooling of the solid Earth
He did this calculation a number of times, making various assumptions. In 1862 his estimate was between 20 and 400 million years; but in 1866 he reduced the top estimate to 100 million years, and attacked Darwin and Lyle for not taking notice of his calculation. We know Darwin was anxious and worried that this would not be long enough to allow for evolution.[3] Thomas Henry Huxley remarked that Kelvin's calculations were good, but his assumptions were wrong. In 1897 Kelvin did the calculation one last time, and came up with 20 to 40 million years.[4] This, of course, would definitely not be long enough to allow for evolution.
Right at the end of the 19th century, someone realised that if the mantle was a highly viscous (sticky) fluid, that would make a great difference to the calculations. In 1895, John Perry, a former assistant to Kelvin, produced an age of Earth estimate of 2 to 3 billion years old using a model of a convective mantle and thin crust.[5]
In Kelvin’s model, heat on the Earth’s surface is derived from the cooling of a shallow outer crust, assuming a solid Earth. But if the conductivity inside the Earth were much higher than at the surface, then the Earth's core and lower mantle would also cool. This would provide a huge store of energy to the surface. In that case, Kelvin’s estimate of the age of the Earth would be too low by many times.
Perry's main reason was that convection in the fluid, or partly fluid, interior of the Earth would transfer heat much more effectively than would conduction:
Kelvin stuck by his estimate of 100 million years, and later reduced the estimate to about 20 million years.
We now know that the assumption of a viscous fluid under a thin crust is a much more factor that the discovery of radioactivity, which been the textbook explanation for many years. The rediscovery and reexamination of Perry's work is quite recent.
In 1896 Henri Bequerel discovered radioactivity.[7][8] In 1903, he shared the Nobel Prize in Physics with Pierre and Marie Curie "in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity".
Eventually, it was realized that radioactivity was a major source of heat inside the Earth.[4]p206 In 1921 came the first modern estimate, using radiometric dating. It was based on uranium-lead dating: the rate of decay of uranium to lead in the crust of the Earth, by Henry Norris Russell. He came up with 2 to 8 billion years.[4]p27, table 3.1 In 1949, H.E. Suess estimated 4 to 5 billion years, based on a whole array of radioactive isotopes. This is close to the time we estimate today, which has been refined further to about 4,560 million years.
The calculation makes use of convection in a viscous fluid as well as radioactivity, so it combines Perry's idea with the effect of radioactivity, even though Perry's contribution had been forgotten. The later knowledge of plate tectonics made it quite certain that the lower mantle was a viscous fluid.
The formation and evolution of the Solar System is estimated to have begun 4.568 billion years ago with the gravitational collapse of a small part of a giant molecular cloud.[9] Basically, the whole system developed in the same period of time.
Many societies assumed the Earth had always been as it is now. Some religions raised the question of its age: the Hindu religion got closest to the present-day scientific estimate.[4] Some Christians and Jews believe the Genesis creation narrative is literally true, which would mean that the Earth was created between 5000 and 10,000 years ago.[10] However, these days most people think such questions are best answered by scientific methods.
   | Vectorsite.net - Greg Goebel / In The Public Domain |
| | USGS preface on the Age of the Earth - Geologic Time: Age of the Earth |
 | TalkOrigins.org - The Age of the Earth |
| | Zircon ages from W. Australia - Absract - Wiley InterScience :: Session Cookies |
| | Wgeology.wisc.edu - Zircons Are Forever |
| | Origin of Earth and Moon Conference - Origin of the Earth and Moon |
| | 11th Annual W.M. Goldschmidt Conference - Eleventh Annual V. M. Goldschmidt Conference Main Page |
| | NASA exposition on the age of Martian meteorites - Meteorite Yields Evidence of Primitive Life on Early Mars |
| | "Aging the Earth", BBC Radio 4 In Our Time series, 2003 - BBC - Radio 4 In Our Time Presented By Melvyn Bragg - Ageing the Earth |
| | Old-earth or young-earth belief - Get Answers - Answers in Genesis |
| | The Radiometric Dating Game - - Radiometric Dating Game - |
| | Is the Earth 4.5 Billion Years Old - Is Earth 4.5 Byr Old? |
| | The Case of the KBS Tuff - The KBS Tuff |
| | How old is the earth? - Chapter 4: Unlocking the Geologic Record - Answers in Genesis |
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