Editing Age of Earth (section)

==Early calculations==
{{Further|William Thomson, 1st Baron Kelvin#Age of the Earth: geology}}
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.<ref name="England et al 2007">{{cite journal| author = England, P.| author2 = Molnar, P.| author3 = Righter, F.| title = John Perry's neglected critique of Kelvin's age for the Earth: A missed opportunity in geodynamics| journal = GSA Today |date=January 2007| volume = 17| issue = 1| pages = 4–9| doi = 10.1130/GSAT01701A.1 | doi-access = free| bibcode = 2007GSAT...17R...4E}}</ref><ref>Dalrymple (1994) pp. 14–17, 38</ref> 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 [[Earth#Heat|heat produced]] via [[radioactive decay]] (a then unknown process) or, more significantly, [[Mantle convection|convection inside Earth]], which allows the temperature in the [[upper mantle]] to remain high much longer, maintaining a high thermal gradient in the [[Earth's crust|crust]] much longer.<ref name="England et al 2007"/> Even more constraining were Thomson'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; Thomson estimated that the Sun is about 20 million years old.<ref name=":1">{{Cite book|url=https://books.google.com/books?id=s4AWPFdyrWIC|title=Lord Kelvin and the Age of the Earth|last=Burchfield|first=Joe D.|date=1990-05-15|publisher=University of Chicago Press|isbn=9780226080437|pages=69 ff|language=en|url-status=live|archive-url=https://web.archive.org/web/20180214145555/https://books.google.com/books?id=s4AWPFdyrWIC|archive-date=2018-02-14}}</ref><ref>{{cite journal|year=2000|title=Kelvin's age of the Earth paradox revisited|journal=[[Journal of Geophysical Research]]|volume=105|issue=B6|pages=13155–13158|bibcode=2000JGR...10513155S|doi=10.1029/2000JB900028|last1=Stacey|first1=Frank D.|author-link=Frank D. Stacey|doi-access=free}}</ref>

[[File:Baron Kelvin 1906.jpg|thumb|upright|left|William Thomson (Lord Kelvin)]]
Geologists such as Lyell had difficulty accepting such a short age for Earth. For biologists, even 100 million years seemed much too short to be plausible. In [[Charles Darwin]]'s theory of [[evolution]], the process of random heritable variation with cumulative [[natural selection|selection]] requires great durations of time, and Darwin stated that Thomson's estimates did not appear to provide enough time.<ref>''Origin of Species'', Charles Darwin, 1872 edition, [http://darwin-online.org.uk/Variorum/1872/1872-286-dns.html page 286]</ref> According to modern biology, the total evolutionary history from the beginning of life to today has taken place since [[Timeline of the evolutionary history of life|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.<ref name="Oldest Fossil Found">{{cite news |last=Borenstein |first=Seth |date=November 13, 2013 |title=Oldest fossil found: Meet your microbial mom |url=http://apnews.excite.com/article/20131113/DAA1VSC01.html |work=[[Excite (web portal)|Excite]] |location=Yonkers, NY |publisher=[[Mindspark Interactive Network]] |agency=[[Associated Press]] |access-date=2015-03-02 |url-status=live |archive-url=https://web.archive.org/web/20150629230719/http://apnews.excite.com/article/20131113/DAA1VSC01.html |archive-date=June 29, 2015 }})</ref>

In a lecture in 1869, Darwin's great advocate, [[Thomas Henry 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 [[Nebular hypothesis|nebula of gas and dust]] from which it was born.<ref name=Dal14-17>Dalrymple (1994) pp. 14–17</ref> 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.

In 1892, Thomson was ennobled as Lord Kelvin in appreciation of his many scientific accomplishments. In 1895 [[John Perry (engineer)|John Perry]] challenged Kelvin's figure on the basis of his assumptions on conductivity, and [[Oliver Heaviside]] entered the dialogue, considering it "a vehicle to display the ability of his [[operational calculus|operator method]] to solve problems of astonishing complexity."<ref>[[Paul J. Nahin]] (1985) [https://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4321749 Oliver Heaviside, Fractional Operators, and the Age of the Earth], [[IEEE Transactions on Education]] E-28(2): 94–104, link from [[IEEE Explore]]</ref> Other scientists backed up Kelvin's figures. Darwin's son, the astronomer [[George Darwin|George H. Darwin]], 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 acceleration|tidal friction]] to give Earth its current 24-hour day. His value of 56 million years was additional evidence that Thomson was on the right track.<ref name="Dal14-17" /> The last estimate Kelvin gave, in 1897, was: "that it was more than 20 and less than 40 million year old, and probably much nearer 20 than 40".<ref>Dalrymple (1994) pp. 14, 43</ref> In 1899 and 1900, [[John Joly]] calculated the rate at which the oceans should have accumulated [[Halite|salt]] from [[erosion]] processes and determined that the oceans were about 80 to 100 million years old.<ref name="Dal14-17" />