Age of the Earth

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The age of the Earth is estimated to be 4.54 ± 0.05 billion years (4.54 × 109 years ± 1%).[1][2][3][4] This age may represent the age of the Earth's accretion, of core formation, or of the material from which the Earth formed.[2] This dating is based on evidence from radiometric age-dating of meteorite[5] material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.

Following the development of radiometric age-dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.[6] 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.[7][8][9] Calcium–aluminium-rich inclusions—the oldest known solid constituents within meteorites that are formed within the Solar System—are 4.567 billion years old,[10][11] giving a lower limit for the age of the Solar System.


Development of modern geologic concepts[edit source | edit]

For further information, see Relative dating

Studies of strata - the layering of rocks 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.[12][13]

Nicolas Steno in the 17th century was one of the first naturalists to appreciate the connection between fossil remains and strata.[13] His observations led him to formulate important stratigraphic concepts (i.e., the "law of superposition" and the "principle of original horizontality").[14] In the 1790s, 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.[15] William Smith's nephew and student, John Phillips, later calculated by such means that Earth was about 96 million years old.[16]



Early calculations[edit source | edit]

For further information, see William Thomson, 1st Baron Kelvin#Age of the Earth: geology and theology

In 1862, the physicist William Thomson, 1st Baron Kelvin published calculations that fixed the age of Earth at between 20 million and 400 million years.[17][18] He assumed that Earth had formed as a completely molten object, and determined the amount of time it would take for the near-surface temperature gradient to decrease to its present value. His calculations did not account for heat produced via radioactive decay (a then unknown process) or, more significantly, convection inside the Earth, which allows the temperature in the upper mantle to remain high much longer, maintaining a high thermal gradient in the crust much longer.[17] Even more constraining were Kelvin's estimates of the age of the Sun, which were based on estimates of its thermal output and a theory that the Sun obtains its energy from gravitational collapse; Kelvin estimated that the Sun is about 20 million years old.[19][20]

William Thomson (Lord Kelvin)

Geologists such as Charles Lyell had trouble accepting such a short age for Earth. For biologists, even 100 million years seemed much too short to be plausible. In Darwin's theory of evolution, the process of random heritable variation with cumulative selection requires great durations of time. According to modern biology, the total evolutionary history from the beginning of life to today has taken place since 3.5 to 3.8 billion years ago, the amount of time which passed since the last universal ancestor of all living organisms as shown by geological dating.[21]

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 physicist Hermann von Helmholtz (in 1856) and 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.



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".[24] In 1899 and 1900, John Joly 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]

Radiometric dating[edit source | edit]

Overview[edit source | edit]

By their chemical nature, rock minerals contain certain elements and not others; but in rocks containing radioactive isotopes, the process of radioactive decay generates exotic elements 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.[25] Typical radioactive end products are argon from decay of potassium-40, and lead from decay of uranium and thorium.[25] If the rock becomes molten, as happens in Earth's mantle, such nonradioactive end products typically escape or are redistributed.[25] Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth, assuming that no rock has been intact for longer than the Earth itself.

Convective mantle and radioactivity[edit source | edit]

In 1892, Thomson had been made Lord Kelvin in appreciation of his many scientific accomplishments. Kelvin calculated the age of the Earth by using thermal gradients, and he arrived at an estimate of about 100 million years.[26] He did not realize that the Earth mantle was convecting, and this invalidated his estimate. In 1895, John Perry produced an age-of-Earth estimate of 2 to 3 billion years using a model of a convective mantle and thin crust,[26] however his work was largely ignored.[17] Kelvin stuck by his estimate of 100 million years, and later reduced it to about 20 million years.

The discovery of radioactivity introduced another factor in the calculation. After Henri Becquerel's initial discovery in 1896, Marie and Pierre Curie discovered the radioactive elements polonium and radium in 1898; and in 1903, Pierre Curie and Albert Laborde announced that radium produces enough heat to melt its own weight in ice in less than an hour. Geologists quickly realized that this upset the assumptions underlying most calculations of the age of Earth. These had assumed that the original heat of the Earth and Sun had dissipated steadily into space, but radioactive decay meant that this heat had been continually replenished. George Darwin and John Joly were the first to point this out, in 1903.[27]

Invention of radiometric dating[edit source | edit]


Ernest Rutherford in 1908.









Arthur Holmes establishes radiometric dating[edit source | edit]

Although Boltwood published his paper in a prominent geological journal, the geological community had little interest in radioactivity.[citation needed] 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.




Holmes' 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.[31] The growing weight of evidence finally tilted the balance in 1931, when the National Research Council of the US National Academy of Sciences 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.[32]


Modern radiometric dating[edit source | edit]

Radiometric dating continues to be the predominant way scientists date geologic timescales. Techniques for radioactive dating have been tested and fine-tuned on an ongoing basis since the 1960s. Forty or so different dating techniques have been utilized to date, working on a wide variety of materials. Dates for the same sample using these different techniques are in very close agreement on the age of the material.[citation needed]


Why meteorites were used[edit source | edit]

An age of 4.55 ± 0.07 billion years, very close to today's accepted age, was determined by Clair Cameron Patterson using uranium-lead isotope dating (specifically lead-lead dating) on several meteorites including the Canyon Diablo meteorite and published in 1956.[33]





Some meteorites are furthermore considered to represent the primitive material from which the accreting solar disk was formed.[34] Some have behaved as closed systems (for some isotopic systems) soon after the solar disk and the planets formed.[citation needed] 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.


Statistics for several meteorites that have undergone isochron dating are as follows:[36]

1. St. Severin (ordinary chondrite)
1. Pb-Pb isochron 4.543 ± 0.019 billion years
2. Sm-Nd isochron 4.55 ± 0.33 billion years
3. Rb-Sr isochron 4.51 ± 0.15 billion years
4. Re-Os isochron 4.68 ± 0.15 billion years
2. Juvinas (basaltic achondrite)
1. Pb-Pb isochron 4.556 ± 0.012 billion years
2. Pb-Pb isochron 4.540 ± 0.001 billion years
3. Sm-Nd isochron 4.56 ± 0.08 billion years
4. Rb-Sr isochron 4.50 ± 0.07 billion years
3. Allende (carbonaceous chondrite)
1. Pb-Pb isochron 4.553 ± 0.004 billion years
2. Ar-Ar age spectrum 4.52 ± 0.02 billion years
3. Ar-Ar age spectrum 4.55 ± 0.03 billion years
4. Ar-Ar age spectrum  4.56 ± 0.05 billion years

Canyon Diablo meteorite[edit source | edit]

For further information, see Canyon Diablo (meteorite)
For further information, see Age of the Solar System
Barringer Crater, Arizona where the Canyon Diablo meteorite was found.

The Canyon Diablo meteorite was used because it is both large and 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.

Fragment of the Canyon Diablo iron meteorite.



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 fact that Earth and the rest of the Solar System formed at around 4.53 to 4.58 billion years ago.[citation needed]

See also[edit source | edit]

References[edit source | edit]

  1. "Age of the Earth". U.S. Geological Survey. 1997. Archived from the original on 23 December 2005. Retrieved 2006-01-10. CS1 maint: discouraged parameter (link)
  3. Manhesa, Gérard; Allègre, Claude J.; Dupréa, Bernard; Hamelin, Bruno (1980). "Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics". Earth and Planetary Science Letters. 47 (3): 370–382. Bibcode:1980E&PSL..47..370M. doi:10.1016/0012-821X(80)90024-2. Unknown parameter |last-author-amp= ignored (help)CS1 maint: discouraged parameter (link)
  4. Braterman, Paul S. (2013). "How Science Figured Out the Age of Earth". Scientific American. Archived from the original on 2016-04-12. CS1 maint: discouraged parameter (link)
  5. Hedman, Matthew (2007). "9: Meteorites and the Age of the Solar System". The Age of Everything. University of Chicago Press. pp. 142–162. Archived from the original on 2018-02-14. CS1 maint: discouraged parameter (link)
  6. 6.0 6.1 6.2 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 (134): 77–88. doi:10.2475/ajs.s4-23.134.78. CS1 maint: discouraged parameter (link)
    For the abstract, see: Chemical Abstracts Service, American Chemical Society (1907). Chemical Abstracts. New York, London: American Chemical Society. p. 817. Retrieved 2008-12-19. CS1 maint: discouraged parameter (link)
  7. Wilde, S. A.; Valley, J. W.; Peck, W. H.; Graham C. M. (2001-01-11). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago". Nature. 409 (6817): 175–178. doi:10.1038/35051550. PMID 11196637.
  8. Valley, John W.; Peck, William H.; Kin, Elizabeth M. (1999). "Zircons Are Forever" (PDF). The Outcrop, Geology Alumni Newsletter. University of Wisconsin-Madison. pp. 34–35. Archived (PDF) from the original on 2009-02-26. Retrieved 2008-12-22. CS1 maint: discouraged parameter (link)
  11. Baker, J.; Bizzarro, M.; Wittig, N.; Connelly, J.; et al. (2005-08-25). "Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites". Nature. 436 (7054): 1127–1131. Bibcode:2005Natur.436.1127B. doi:10.1038/nature03882. PMID 16121173.
  13. 13.0 13.1 Stiebing, William H. (1994). Uncovering the Past. Oxford University Press US. ISBN 978-0-19-508921-9.
  14. 14.0 14.1 Brookfield, Michael E. (2004). Principles of Stratigraphy. Blackwell Publishing. p. 116. ISBN 978-1-4051-1164-5.
  15. Fuller, J. G. C. M. (2007-07-17). "Smith's other debt, John Strachey, William Smith and the strata of England 1719–1801". Geoscientist. The Geological Society. Archived from the original on 24 November 2008. Retrieved 2008-12-19. CS1 maint: discouraged parameter (link)
  16. Burchfield, Joe D. (1998). "The age of the Earth and the invention of geological time". Geological Society, London, Special Publications. 143 (1): 137–143. Bibcode:1998GSLSP.143..137B. CiteSeerX doi:10.1144/GSL.SP.1998.143.01.12.
  17. 17.0 17.1 17.2 England, P.; Molnar, P.; Righter, F. (January 2007). "John Perry's neglected critique of Kelvin's age for the Earth: A missed opportunity in geodynamics". GSA Today. 17 (1): 4–9. doi:10.1130/GSAT01701A.1.
  18. Dalrymple (1994) pp. 14–17, 38
  19. Burchfield, Joe D. (1990-05-15). Lord Kelvin and the Age of the Earth. University of Chicago Press. pp. 69 ff. ISBN 9780226080437. Archived from the original on 2018-02-14. CS1 maint: discouraged parameter (link)
  21. Borenstein, Seth (November 13, 2013). "Oldest fossil found: Meet your microbial mom". Excite. Yonkers, NY: Mindspark Interactive Network. Associated Press. Archived from the original on June 29, 2015. Retrieved 2015-03-02. CS1 maint: discouraged parameter (link))
  22. 22.0 22.1 22.2 Dalrymple (1994) pp. 14–17
  23. Paul J. Nahin (1985) Oliver Heaviside, Fractional Operators, and the Age of the Earth, IEEE Transactions on Education E-28(2): 94–104, link from IEEE Explore
  24. Dalrymple (1994) pp. 14, 43
  25. 25.0 25.1 25.2 Nichols, Gary (2009). "21.2 Radiometric Dating". Sedimentology and Stratigraphy. John Wiley & Sons. pp. 325–327. ISBN 978-1405193795.
  26. 26.0 26.1 England, Philip C.; Molnar, Peter; Richter, Frank M. (2007). "Kelvin, Perry and the Age of the Earth" (PDF). American Scientist. 95 (4): 342–349. CiteSeerX doi:10.1511/2007.66.3755. Archived (PDF) from the original on 2010-07-02. CS1 maint: discouraged parameter (link)
  28. Rutherford, E. (1906). Radioactive Transformations. London: Charles Scribner's Sons. Reprinted by Juniper Grove (2007) ISBN 978-1-60355-054-3.
  30. Dalrymple (1994) p. 74
  31. The Age of the Earth Debate Badash, L Scientific American 1989 esp p95 Archived 2016-11-05 at the Wayback Machine
  32. Dalrymple (1994) pp. 77–78
  33. Patterson, Claire (1956). "Age of meteorites and the earth" (PDF). Geochimica et Cosmochimica Acta. 10 (4): 230–237. Bibcode:1956GeCoA..10..230P. doi:10.1016/0016-7037(56)90036-9. Archived (PDF) from the original on 2010-06-21. Retrieved 2009-07-07. CS1 maint: discouraged parameter (link)
  34. Carlson, R. W.; Tera, F. (December 1–3, 1998). "Lead-Lead Constraints on the Timescale of Early Planetary Differentiation" (PDF). Conference Proceedings, Origin of the Earth and Moon. Houston, Texas: Lunar and Planetary Institute. p. 6. Archived (PDF) from the original on 16 December 2008. Retrieved 2008-12-22. Cite uses deprecated parameter |booktitle= (help)CS1 maint: discouraged parameter (link)
  35. Dalrymple (1994) pp. 310–341
  36. Dalrymple, Brent G. (2004). Ancient Earth, Ancient Skies: The Age of the Earth and Its Cosmic Surroundings. Stanford University Press. pp. 147, 169. ISBN 978-0-8047-4933-6.
  37. Terada, K.; Sano, Y. (May 20–24, 2001). "In-situ ion microprobe U-Pb dating of phosphates in H-chondrites" (PDF). Proceedings, Eleventh Annual V. M. Goldschmidt Conference. Hot Springs, Virginia: Lunar and Planetary Institute. Bibcode:2001eag..conf.3306T. Archived (PDF) from the original on 16 December 2008. Retrieved 2008-12-22. Cite uses deprecated parameter |booktitle= (help)CS1 maint: discouraged parameter (link)

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