Methods dating sediments

Florian Cajori, author of the 1908 article “The Age of the Sun and the Earth,” was a historian of science and, especially, of mathematics, and Ray Lankester, whom he quotes, was a zoologist. The first act consists in a direct attack, led by Lord Kelvin, on the extreme uniformitarianism of those such as Charles Lyell, who regarded the earth as indefinitely old and who, with great foresight (or great naivety, depending on your point of view: see the third installment of the 1900 “The Age of the Earth” article by W. Sollas), assumed that physical processes would eventually be discovered to power the great engine of erosion and uplift.The second act of the drama sees a prolonged attempt by a new generation of geologists to estimate the age of the earth from observational evidence, to come up with an answer that would satisfy the demands of newly dominant evolutionary thinking, and to reconcile this answer with the constraints imposed by thermodynamics.Lord Kelvin and his allies used three kinds of argument.The first of these referred to the rate of heat loss from the earth and the length of time it would have taken to form its solid crust.The second referred to such topics as the detailed shape of the earth (bulging slightly at the equator) and the dynamics of the earth-moon system.The third referred to the heat of the sun, particularly the rate at which such heat is being lost, compared with the total amount of energy initially available.The third act sees the entry of a newly discovered set of physical laws—those governing radioactivity.

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We, of course, know the final outcome, but we should not let that influence our appreciation of the story as it unfolds.The first argument was completely undermined after taking into account the amount of heat generated by radioactive decay.The second depended on highly dubious theories of formation of the earth and moon and plays relatively little role in this compilation.It was not until 1926, when (under the influence of Arthur Holmes, whose name recurs throughout this story) the National Academy of Sciences adopted the radiometric timescale, that we can regard the controversy as finally resolved.Critical to this resolution were improved methods of dating, which incorporated advances in mass spectrometry, sampling and laser heating.

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