View source for Age of Earth

[[File:Nasa blue marble.jpg|thumb|right|200px|Earth from space]]

The '''age of the Earth''' is estimated at a little over 4.5 [[billion]] years.<ref name=Brent/> Working this out was a difficult [[problem]] to solve. For most of human [[history]], the basic [[fact]]s about the [[planet]] were unknown. The problem was tackled by [[Earth science|Earth scientists]] in the twentieth century.<ref name=Brent/>

Modern estimates are based on [[radioactive dating]] methods. The oldest [[mineral]]s on Earth&nbsp;– small [[crystal]]s of [[zircon]] from the [[Jack Hills]] of [[Western Australia]]&nbsp;– are at least 4.4 billion years old.<ref name="nature409">{{cite journal
 | author=Wilde S.A. | display-authors = etal | title=Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago | journal=Nature
 | date=2001-01-11 | volume=409 | pages=175–178
 | doi=10.1038/35051550
 | pmid=11196637
 | issue=6817
| s2cid = 4319774 }}</ref><ref>{{cite web
 | author=Valley, John W.; Peck, William H. & Kin, Elizabeth M. | url=http://www.geology.wisc.edu/outcrop/99/99_pdfs/iso_topics.pdf | year=1999 | title=Zircons are forever
 | work=The Outcrop, Geology Alumni Newsletter
 | pages=34–35
 | publisher=University of Wisconsin-Madison
 | access-date=2008-12-22
}}</ref><ref>{{cite journal
 | author=Wyche S.; Nelson D.R. & Riganti A.
 | title=4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton
 | journal=Australian Journal of Earth Sciences
 | year=2004 | volume=51 | issue=1 | pages=31–45
 | doi=10.1046/j.1400-0952.2003.01042.x }}</ref> [[Ca-Al-rich inclusions]]&nbsp;– the oldest known solid bits in [[meteorites]] formed in the solar system&nbsp;– are 4.567 [[billion]] years old.<ref>{{cite journal
 | doi = 10.1126/science.1073950
 |date=Sep 2002
 | author = Amelin, Y; Krot, An; Hutcheon, Id; Ulyanov, Aa
 | title = Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions.
 | volume = 297
 | issue = 5587
 | pages = 1678–83
 | issn = 0036-8075
 | pmid = 12215641
 | journal = Science
 | bibcode=2002Sci...297.1678A|s2cid=24923770
 }}</ref><ref name="nature436">{{cite journal
 | author=Baker, J.; Bizzarro, M.; Wittig, N.; Connelly, J.; Haack, H. | title=Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites | journal=Nature | date=2005-08-25
 | volume=436 | pages=1127–1131
 | doi=10.1038/nature03882
 | pmid=16121173
 | issue=7054 | bibcode=2005Natur.436.1127B| s2cid=4304613 }}</ref> This gives an age for the Solar System  and an upper limit for the age of [[Earth]].

== Early science ==
{{Life timeline}}
The first facts about the Earth were worked out by the [[Ancient Greek]]s. A good [[estimate]] of the Earth's [[size]] was made by [[Eratosthenes]] (276 <small>BC</small>–194 <small>BC</small>), using [[trigonometry]]. 

The first estimate of the Earth's age based on evidence was by [[Benoît de Maillet]] (1656–1738), a [[France|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 [[sediment]]ation in the sea.<ref>de Maillet, Benoît 1750. ''Telliamed: or, discourses Between an Indian philosopher and a French missionary, on the diminution of the sea, the formation of the Earth, the origin of men and animals, and other curious subjects, relating to natural history and philosophy''. Osborne, London.</ref> He hid his ideas under the [[guise]] of a talk with an [[India]]n philosopher, to avoid conflict with the [[Catholic Church]]. His work stayed in [[manuscript]] until after his death, when it was [[publish]]ed after its editor had made changes which damaged it. Now there is a better modern edition based on the manuscripts.<ref>de Maillet, Benoît 1968. ''Teliamed, or conversations between an Indian philosopher and a French missionary on the diminution of the sea''. Translated and edited by Albert V. Carozzi. University of Illinois Press, Urbana, Chicago & London.</ref> de Maillet estimated that the Earth was older than two [[billion]] years. He also recognized the true nature of [[fossil]]s, and had early ideas about [[evolution]].

== 19th century ==
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 [[uniformitarianism]] clearly meant that the Earth was [[wikt:ancient|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.<ref name="Janet">Browne, Janet 2002. ''Charles Darwin: the power of place'', volume 2 of a biography. Cape, London.</ref><sup>p314</sup> Both he and Lyell 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 2,000<sup>o</sup>C. 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 [[wikt:assumption|assumption]]s. 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.<ref name="Janet"/> [[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.<ref name="Brent"/> This, of course, would definitely not be long enough to allow for evolution. [[Oliver Heaviside]] also disagreed, and proposed an alternative geothermal model. Both Kelvin and Heaviside were eventually shown to be wrong.

=== Viscous mantle ===
Right at the end of the 19th century, someone realised that if the [[mantle (geology)|mantle]] was a highly [[Viscosity|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.<ref name="kp">{{cite journal
 | title=Kelvin, Perry and the Age of the Earth
 | first1=Philip C. | last1=England | first2=Peter
 | last2=Molnar | first3=Frank M. | last3=Richter
 | journal=American Scientist | volume=95
 | issue=4 | year=2007 | pages=342–349
 | doi=10.1511/2007.66.3755 }}</ref>

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]]:
:"… much internal fluidity would practically mean infinite conductivity for our purpose".<ref>[ftp://rock.geosociety.org/pub/GSAToday/gt0701.pdf GSA Today]{{Dead link|date=January 2021 |bot=InternetArchiveBot |fix-attempted=yes }} January 2007</ref>

Kelvin stuck by his estimate of 100 million years, and later reduced the estimate to about 20 million years.

We now know that the existence of a viscous fluid under a thin crust is a much greater factor than the discovery of [[radioactivity]],<ref name="Brent"/> which was the textbook explanation for many years. The rediscovery and reexamination of Perry's work is quite recent.

== 20th century ==
In 1896 [[Henri Becquerel]] discovered [[radioactivity]].<ref>{{cite journal|author=Henri Becquerel|title =Sur les radiations émises par phosphorescence|journal=Comptes Rendus |volume = 122| pages = 420–421|year=1896|url=http://gallica.bnf.fr/ark:/12148/bpt6k30780/f422.chemindefer}}</ref><ref>Becquerel, Antoine Henri 1896. On the rays emitted by phosphorescence. ''Comptes Rendus'' '''122''', p420. [http://web.lemoyne.edu/~giunta/becquerel.html translated by Carmen Giunta]. Accessed 10 September 2006.</ref> In 1903, he shared the [[Nobel Prize in Physics]] with [[Pierre Curie|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.<ref name="Brent"/><sup>p206</sup> 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 (geology)|crust]] of the Earth, by Henry Norris Russell. He came up with 2 to 8 [[billion]] years.<ref name="Brent"/><sup>p27, table 3.1</sup> In 1949, H.E. Suess estimated 4 to 5 billion years, based on a whole array of radioactive [[isotope]]s. 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.

=== Solar System ===
The formation and [[wikt:evolution|evolution]] of the [[Solar System]] is estimated to have begun 4.568&nbsp;[[1000000000 (number)|billion]] years ago with the [[gravitation]]al collapse of a small part of a giant [[molecular cloud]].<ref>[http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo941.html Bouvier, Audrey and Meenakshi Wadhwa, "The age of the solar system redefined by the oldest Pb-Pb age of a meteoritic inclusion"]. ''Nature Geoscience,'' Nature Publishing Group, a division of Macmillan Publishers Limited. Published online 2010-08-22, retrieved 2010-08-26, doi: 10.1038/NGEO941.<br />Date based on oldest inclusions found to date in [[meteorite]]s, thought to be among the first solid material to form in the collapsing solar nebula.</ref> Basically, the whole system developed in the same period of time.

== Religious views ==
Use of the scientific method assumes uniformitarian methods for calculating Earth's age. The [[Hindu]] religion comes closest to the present-day scientific estimate.<ref name="Brent">Dalrymple G. Brent 2004. ''Ancient Earth, ancient skies: the age of Earth and its cosmic surroundings''. Stanford. p26, table 3.1</ref> Some Christians and Jews believe the [[Genesis creation narrative]] is [[wikt:literal|literally]] true, which would mean that the Earth was [[creationism|created]] between 5000 and 10,000 years ago, and the methods used to calculate age have not been consistent throughout Earth's history.<ref>{{cite web|title=Young Earth Creationism|author=Robert J. Schneider|url=http://community.berea.edu/scienceandfaith/essay08.asp|access-date=December 7, 2010}}</ref> However, these days most people think such questions are best answered by [[scientific method]]s.

== Related pages ==
* [[Geochronology]]
* [[History of the geologic time scale]]
* [[Template:Nature timeline|Nature timeline]]

== References ==
{{Reflist}}


[[Category:Geology]]
[[Category:Earth sciences]]
[[Category:History of ideas]]
[[Category:History of science]]
{{DEFAULTSORT:the solar system}}