The age of the Earth in the twentieth century: a problem (mostly) solved

In the early twentieth century the Earth’s age was unknown and scientific estimates, none of which were based on valid premises, varied typically from a few millions to billions of years. This important questions was answered only after more than half a century of innovation in both theory and instrumentation. Critical developments along this path included not only a better understanding of the fundamental properties of matter, but also: (a) the suggestion and first demonstration by Rutherford in 1904 that radioactivity might be used as a geological timekeeper; (b) the development of the first mass analyser and the discovery of isotopes by J. J. Thomson in 1914; (c) the idea by Russell in 1921 that the age of a planetary reservoir like the Earth’s crust might be measured from the relative abundances of a radioactive parent element (uranium) and its daughter product (lead); (d) the development of the idea by Gerling in 1942 that the age of the Earth could be calculated from the isotopic composition of a lead ore of known age; (e) the ideas of Houtermans and Brown in 1947 that the isotopic composition of primordial lead might be found in iron meteorites; and (f) the first calculation by Patterson in 1953 of a valid age for the Earth of 4.55 Ga, using the primordial meteoritic lead composition and samples representing the composition of modern Earth lead. The value for the age of the Earth in wide use today was determined by Tera in 1980, who found a value of 4.54 Ga from a clever analysis of the lead isotopic compositions of four ancient conformable lead deposits. Whether this age represents the age of the Earth’s accretion, of core formation, or of the material from which the Earth formed is not yet known, but recent evidence suggests it may approximate the latter.