|Statistic:||This current age of the planet Earth is 4.54 billion years old.|
|Statistical method:||radiometric age dating|
|Degree of error:||+/- 40 million years|
Modern geologists consider the age of the Earth to be around 4.54 billion years (4.54 years). This age has been determined by radiometric age dating of Wikipedia:Wikipedia:meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples.
Historically the age of the Earth was determined either by using creation myths in religious texts, or by Wikipedia:Wikipedia:philosophical interpretations of geologic features, most notably the Greek Wikipedia:philosophers Wikipedia:Theophrastus and Wikipedia:Xenophanes. Some Wikipedia:Biblical Wikipedia:young earth creationists believe that the earth was formed as recently as 4004 BC, whereas Wikipedia:Hindu beliefs have the Wikipedia:universe enduring for billions of years before being destroyed and recreated in an endless cycle.
After the Wikipedia:scientific revolution and the discovery of radiometric age dating, measurements of lead in uranium-rich Wikipedia:minerals showed that some were in excess of a billion years old. The oldest such minerals analysed to date – small crystals of Wikipedia:zircon from the Wikipedia:Jack Hills of Wikipedia:Western Australia – are at least 4.404 billion years old. Comparing the Wikipedia:mass and Wikipedia:luminosity of the Wikipedia:Sun to the multitudes of other Wikipedia:stars, it appears that the solar system cannot be much older than those rocks. Wikipedia:Ca-Al-rich inclusions (inclusions rich in Wikipedia:calcium and Wikipedia:aluminium) – the oldest known solid constituents within Wikipedia: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. Because the exact accretion time of the Earth is not yet known, and the predictions from different accretion models vary from several millions up to about 100 million years, the exact age of the Earth is difficult to determine.
Development of modern geologic conceptsEdit
Studies of strata, the layering of rock and earth, gave naturalists an appreciation that the 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. Wikipedia:Xenophanes interpreted fossil-bearing strata in much the same way during the 6th Century BC.
Wikipedia:Nicolas Steno (17th Century) was one of the first Western naturalists to appreciate the connection between fossil remains and strata. His observations led him to formulate important stratigraphic concepts (i.e., the "Wikipedia:law of superposition" and the "Wikipedia:principle of original horizontality"). 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. William Smith's nephew and student, John Phillips, later calculated by such means that the Earth was about 96 million years old.
The naturalist Wikipedia:Mikhail Lomonosov, regarded as the founder of Wikipedia:Russian science, suggested in the mid-18th century that the 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 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.
Other naturalists used these hypotheses to construct a Wikipedia: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 Wikipedia:Charles Lyell, developing ideas found in Scottish natural philosopher Wikipedia:James Hutton, popularized the concept 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 Wikipedia:catastrophes. Many naturalists were influenced by Lyell to become "uniformitarians" who believed that changes were constant and uniform.
Early calculations: physicists, geologists and biologistsEdit
In 1862, the physicist William Thomson (who later became Lord Kelvin) of Wikipedia:Glasgow published calculations that fixed the age of the Earth at between 24 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. His calculations did not account for the ongoing heat source in the form of radioactive decay, which was unknown at the time.
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. Wikipedia:Charles Darwin, who had studied Lyell's work, had proposed his theory of the Wikipedia:evolution of organisms by Wikipedia:natural selection, a process whose combination of random heritable variation and cumulative selection implies great expanses of time. Even 400 million years did not 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. The German physicist Wikipedia:Hermann von Helmholtz (in 1856) and the Canadian astronomer Wikipedia: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. Their values were consistent with Thomson's calculations. However, they assumed that the Sun was only glowing from the heat of its Wikipedia:gravitational contraction. The process of solar Wikipedia:nuclear fusion was not yet known to science.
Other scientists backed up Thomson's figures as well. Wikipedia:Charles Darwin's son, the astronomer George H. Darwin of the Wikipedia:University of Cambridge, proposed that the Earth and Wikipedia: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. His value of 56 million years added additional evidence that Thomson was on the right track.
In 1899 and 1900, Wikipedia:John Joly of the Wikipedia:University of Dublin calculated the rate at which the oceans should have accumulated salt from Wikipedia:erosion processes, and determined that the oceans were about 80 to 100 million years old.
Rock Wikipedia:minerals naturally contain certain elements and not others. By the process of Wikipedia:radioactive decay of radioactive isotopes occurring in a rock, exotic elements can be introduced over time. By measuring the Wikipedia:concentration of the stable end product of the decay, coupled with knowledge of the Wikipedia:half life and initial concentration of the decaying element, the age of the rock can be calculated. Typical radioactive end products are Wikipedia:argon from Wikipedia:potassium-40 and Wikipedia:lead from Wikipedia:uranium and Wikipedia:thorium decay. If the rock becomes molten, as happens in the Earth's mantle, such non radioactive end products typically escape or are redistributed. Thus the age of the oldest terrestrial rock gives a minimum for the age of the Earth assuming that a rock cannot have been in existence for longer than the Earth itself.
Convective mantle and radioactivityEdit
In 1892, Thomson had been made Wikipedia:Lord Kelvin in appreciation of his many scientific accomplishments. Kelvin calculated the age of the Earth by using thermal gradients, and arrived at an estimate of 100 million years old.  He did not realize that Earth has a molten fluid mantle, and this ruined his calculation. In 1895, John Perry produced an age of the Earth estimate of 2 to 3 billions years old using a model of a molten convective mantle and thin crust.  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 Wikipedia:radioactivity. In 1898, two other French researchers, Marie and Wikipedia:Pierre Curie, discovered the radioactive elements Wikipedia:polonium and Wikipedia:radium. In 1903 Pierre Curie and his associate Wikipedia: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 heat. 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, Wikipedia:Julius Elster and Wikipedia:Hans F. Geitel, had detected radioactivity in the air and then in the soil. Other investigators found it in rainwater, snow, and Wikipedia:groundwater. Wikipedia: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.Template:Fact
Invention of radiometric datingEdit
Radioactivity, which had overthrown the old calculations, yielded a bonus by providing a basis for new calculations, in the form of radiometric dating.
Wikipedia:Ernest Rutherford and Wikipedia:Frederick Soddy 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 radioactive element decays into another element at a distinctive rate. This rate is given in terms of a "Wikipedia: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. Wikipedia:Uranium, Wikipedia:thorium, and Wikipedia: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 Wikipedia: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 Wikipedia: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 Wikipedia:alpha particles released by radioactive decay could be trapped in a rocky material as Wikipedia: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 Wikipedia:Sir William Ramsay, then at Wikipedia: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.
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 Wikipedia: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.
Arthur Holmes establishes radiometric datingEdit
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.
Wikipedia:Robert Strutt tinkered with Rutherford's helium method until 1910 and then ceased. However, Strutt's student Wikipedia: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. 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 "Wikipedia:isotopes". In the 1930s, isotopes would be shown to have nuclei with differing numbers of the neutral particles known as "Wikipedia: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 Wikipedia: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 Wikipedia: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. 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 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 datingEdit
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 usedEdit
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 unable to give a direct date of the formation of the Earth from the solar nebula because the 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 Wikipedia:plate tectonics, Wikipedia:weathering and Wikipedia: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 Wikipedia: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 Wikipedia: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 Wikipedia:Archaean lead Wikipedia:ores of Wikipedia: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 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.
Why the Canyon Diablo meteorite was usedEdit
The Canyon Diablo meteorite was used because it is a very large representative of a particularly rare type of meteorite which contains Wikipedia:sulfide minerals (particularly Wikipedia:troilite), metallic Wikipedia:nickel-Wikipedia: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 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. 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 4.53 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 "Wikipedia:helioseismic" methods which strongly agree with the radiometric dates found for the oldest meteorites.
Certain Wikipedia:Hindu puranic views assert that the Wikipedia:universe is created, destroyed, and re-created in an eternally repetitive series of cycles. In Wikipedia:Hindu cosmology, a universe endures for about 4,320,000,000 Wikipedia:years (one day of Wikipedia: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 Wikipedia:pralaya (Cataclysm), repeats for 100 Brahma years (311 trillion human years) that represents Brahma's lifespan. According to these puranic views, the current universe is 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.
Wikipedia:Jains, followers of Wikipedia:Jainism, believe that the earth has existed forever, that it will exist forever, and that over a period of 6 kalm life will be re-created. The Chinese believed that the Earth was created and destroyed in cycles of over 23 million years.
Several schools of thought in ancient Wikipedia:Greece had conceived of the idea of Wikipedia:deep time which stretched far into the past, or far into the future beyond the end of humankind. Wikipedia:Aristotle thought the Earth and Wikipedia:universe had existed from eternity. Prior to Aristotle, Wikipedia:Democritus (founder of the Wikipedia:Atomist school of thought) believed that the universe consisted of "Wikipedia:atoms" and "Wikipedia:void", both of which constituted the whole of the universe. He believed that these elements were eternal, never having beginning nor having an end. Wikipedia:Leucippus, a contemporary of Wikipedia:Democritus, apparently held similar views.
In modern times, Christian fundamentalists, as well as many Wikipedia:Haredi Jews, advocate Wikipedia:Young Earth creationism. Generally, they seek to provide a history of the Earth compatible with their religious texts (primarily the book of Genesis; Wikipedia:Hebrew, בראשית). Both camps normally hold that the Earth is 6,000 to 10,000 years old, based on the initial creation account of Genesis 1:1 to 2:4, as well as the genealogies of humanity provided in the Wikipedia:Bible thereafter.
There has been some question as to the time frame within the genealogies in Genesis. Some Christians (notably "Old Earth creationists" such as Hugh Ross) interpret the genealogies as non-linear, and that this may raise questions as to the amount of time allowed between ancestor/descendants. In a book published in 1654, not long before his death, Archbishop Wikipedia:James Ussher of Wikipedia:Armagh, Wikipedia:Ireland, calculated from the Bible (augmented by some Wikipedia:astronomy and Wikipedia:numerology) that creation began on Wikipedia:October 23, Wikipedia:4004 BC. Other interpretations (mostly Orthodox Jewish interpretations) point to the use of exact Hebrew grammatical construction in the genealogies; these would include: Usage of the direct object marker 'Wikipedia:eth' with wayyoled, and use of hiphil waw consecutive imperfect with the verb yalad, "beget". Other signposts of the text of Genesis being written as strict historical narrative (i.e., extensive use of Wikipedia:colophons and waw consecutives throughout) are used to account for a fairly strict genealogy in Genesis, as is confirmed by such Jewish historians as Wikipedia:Josephus. This then allows no more than several thousand years to account for human history.
Following this, many Wikipedia:Christians and Wikipedia:Haredi Jews, specifically "Young Earth creationists", interpret ancient Wikipedia:geologic events based on a Wikipedia:flood geology model (a literal interpretation of the Bible's Great Flood narrative in Genesis) to account for many large scale geologic features and most Wikipedia:fossil deposits. The concepts of 18th and early 19th century Wikipedia:catastrophism are sometimes associated with this view.
- Wikipedia:Age of the universe
- Wikipedia:History of Earth
- Wikipedia:Radiometric dating
- Wikipedia:Timetable of the Precambrian
- Wikipedia:Natural history
- ↑ Template:Cite web
- ↑ 2.0 2.1 Patterson, C., 1956, Age of meteorites and the earth: Geochimica et Cosmochimica Acta 10: 230-237.
- ↑ 3.0 3.1 Boltwood, B.B., 1907, On the ultimate disintegration products of the radio-active elements. Part II. The disintegration products of uranium: American Journal of Science 23: 77-88.
- ↑ 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 409: 175-178.
- ↑ Baker, J., Bizzarro, M, Wittig, N, Connelly, J, Haack, H, 2005, Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature 436: 1127-1131.
- ↑ England P., Molnar P., Richter P., 2007. John Perry's neglected critique of Kelvin's age for the earth: A missed opportunity in geodynamics. GSA Today v.17 (1), 4-9. doi: 10.1130/GSAT01701A.1
- ↑ 7.0 7.1 7.2 7.3 Dalrymple, G. Brent, 1991, The Age of the Earth, Stanford University Press, pp 14-17, ISBN 0-8047-2331-1
- ↑ 8.0 8.1 Template:Citation
- ↑ Joly, J. (1909). Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History, London: Archibald Constable & Co., Ltd., p. 36. ISBN 1402135777.
- ↑ Template:Cite book
- ↑ Dalrymple, G. Brent, 1991, The Age of the Earth, Stanford University Press, pp 74, ISBN 0-8047-2331-1
- ↑ 12.0 12.1 Dalrymple, G. Brent, 1991, The Age of the Earth, Stanford University Press, pp 77-78, ISBN 0-8047-2331-1
- ↑ Dalrymple, G. Brent, 1991, The Age of the Earth, Stanford University Press, pp 310-341, ISBN 0-8047-2331-1
- ↑ Bonanno, A., H. Schlattl and L. Paterno, (2006) The age of the sun and relativistic corrections in the EOS, Astronomy and Astrophysics 
- ↑ Template:Citation
- ↑ Josephus, Jewish Antiquities Books I–IV, Harvard Press, Cambridge, MA, 1930, p. 73; Loeb Classical Library No. 242.
- ↑ Template:Citation
- 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
- 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 - Abstract Accessed Jan. 10, 2006
- Vectorsite.net – Initial version of this article was based on a public domain text by Wikipedia:Greg Goebel
- USGS preface on the Age of the Earth
- NASA exposition on the age of Martian meteorites
- "Aging the Earth", BBC Radio 4 In Our Time series, 2003
- Baadsgaard, H.; Lerbekmo, J.F.; Wijbrans, J.R., 1993. Multimethod radiometric age for a bentonite near the top of the Baculites reesidei Zone of southwestern Saskatchewan (Campanian-Maastrichtian stage boundary?). Canadian Journal of Earth Sciences, v.30, p.769-775.
- Baadsgaard, H. and Lerbekmo, J.F., 1988. A radiometric age for the Cretaceous-Tertiary boundary based on K-Ar, Rb-Sr, and U-Pb ages of bentonites from Alberta, Saskatchewan, and Montana. Canadian Journal of Earth Sciences, v.25, p.1088-1097.
- Eberth, D.A. and Braman, D., 1990. Stratigraphy, sedimentology, and vertebrate paleontology of the Judith River Formation (Campanian) near Muddy Lake, west-central Saskatchewan. Bulletin of Canadian Petroleum Geology, v.38, no.4, p.387-406.
- Goodwin, M.B. and Deino, A.L., 1989. The first radiometric ages from the Judith River Formation (Upper Cretaceous), Hill County, Montana. Canadian Journal of Earth Sciences, v.26, p.1384-1391.
- Gradstein, F. M.; Agterberg, F.P.; Ogg, J.G.; Hardenbol, J.; van Veen, P.; Thierry, J. and Zehui Huang., 1995. A Triassic, Jurassic and Cretaceous time scale. IN: Bergren, W. A. ; Kent, D.V.; Aubry, M-P. and Hardenbol, J. (eds.), Geochronology, Time Scales, and Global Stratigraphic Correlation. Society of Economic Paleontologists and Mineralogists, Special Publication No. 54, p.95-126.
- Harland, W.B., Cox, A.V.; Llewellyn, P.G.; Pickton, C.A.G.; Smith, A.G.; and Walters, R., 1982. A Geologic Time Scale: 1982 edition. Cambridge University Press: Cambridge, 131p.
- Harland, W.B.; Armstrong, R.L.; Cox, A.V.; Craig, L.E.; Smith, A.G.; Smith, D.G., 1990. A Geologic Time Scale, 1989 edition. Cambridge University Press: Cambridge, p.1-263. ISBN 0-521-38765-5
- Harper, C.W., Jr., 1980. Relative age inference in paleontology. Lethaia, v.13, p.239-248.
- Lubenow, M.L., 1992. Bones of Contention: A Creationist Assessment of Human Fossils. Baker Book House: Grand Rapids.
- Obradovich, J.D., 1993. A Cretaceous time scale. IN: Caldwell, W.G.E. and Kauffman, E.G. (eds.). Evolution of the Western Interior Basin. Geological Association of Canada, Special Paper 39, p.379-396.
- Palmer, Allison R. (compiler), 1983. The Decade of North American Geology 1983 Geologic Time Scale. Geology, v.11, p.503-504. Wikipedia:September 12, Wikipedia:2004.