For a long time the apparent discrepancy between the age of earth and the age of the cosmos posed a great problem to geologists and astronomers alike.
Geologists had calculated, using methods like erosion or sedimentation rates, ages for earth spanning from just three million to 15 billion years. Physicists and astronomers, based mostly on the motion and energy consumption of stars, calculated an age for the universe spanning from 20 million to 10 billion years - so in many models of the cosmos earth seemed to be too young or too old to fit in.
In August 1893, during a meeting of the American Association for the Advancement of Science, geologist Charles D. Walcott (1850-1927) summarized the debate as follows:
"Of all subjects of speculative geology, few are more attractive or more uncertain in positive results than geological time. The physicists have drawn the lines closer and closer until the geologist is told that he must bring his estimates of the age of the earth within a limit of from ten to thirty millions of years. The geologist masses his observations and replies that more time is required, and suggests to the physicist that there may be an error somewhere in his data or the method of his treatment."
In 1896 the French physicist Henri Becquerel (1852-1908), based on Conrad Röntgen's research on artificially induced X-rays, discovered that naturally occurring elements, like uranium, also emit rays and in 1897 Polish physicist Marie Curie (1867-1934) coined the term radioactivity to describe this energy of unknown origin.
Fig.1. High-energy rays, derived from radioactive decay, form a halo of alteration around a mineral grain in the larger biotite-crystal, image from J. JOYLE (1909): Radioactivity and geology, an account of the influence of radioactive energy on terrestrial history.
Her husband, Pierre Curie (1859-1906), realized that this energy coming from the radioactive decay of elements and minerals found in earth´s rocks would significantly slowing down the cooling of the entire earth. As the calculation of many physicists for earth´s young age was based on the assumption of a steadily cooling earth, however as there was now a source of energy and heat, earth was cooling far more slowly than thought and therefore could be quite old. Radioactive decay or another similar long-lasting and high-energy source (nuclear fusion was discovered later) could also explain how stars could produce light and heat for very long periods of time, so the notion that stars or the sun had to be young (in most calculations younger than earth) could also be dismissed.
But even better - the discovery of radioactivity provided not only indirect evidence of an old earth, but by measuring the constant energy production it was also possible to calculate the exact age of a mineral, of a rock and even of earth - may also of the solar system and the cosmos, dating extraterrestrial matter in the form of meteorites… … (to be continued).
LEWIS, C.L.E. (2002): Arthur Homes´ unifying theory: from radioactivity to continental drift. In OLDROYD, D. R. (ed.) 2002. The Earth Inside and Out: Some Major Contributions to Geology in the Twentieth Century. Geological Society, London, Special Publications, 192: 167-183