Early methods of absolute age dating
Lord Kelvin) applied his thermodynamic principles to the problems of heat flow, and this had implications for predicting the age of a cooling Sun and of a cooling Earth.From an initial estimate of 100 million years for the development of a solid crust around a molten core proposed in 1862, Thomson subsequently revised his estimate of the age of the Earth downward.Using the same criteria, he concluded in 1899 that the Earth was between 20 and 40 million years old.Thomson’s calculation was based on the assumption that the substance of the Earth is inert and thus incapable of producing new heat.Although faced with problems of helium loss and therefore not quite accurate results, a major scientific breakthrough had been accomplished. Boltwood, working with the more stable uranium–lead system, calculated the numerical ages of 43 minerals.His results, with a range of 400 million to 2.2 billion years, were an order of magnitude greater than those of the other “quantitative” techniques of the day that made use of heat flow or sedimentation rates to estimate time. Perhaps much to their relief, paleontologists now had sufficient time in which to accommodate faunal change.Many independent estimates of the age of the Earth have been proposed, each made using a different method of analysis.Some such estimates were based on assumptions concerning the rate at which dissolved salts or sediments are carried by rivers, supplied to the world’s oceans, and allowed to accumulate over time.
With only minor indications of fossil occurrence (mainly in the form of algal stromatolites), no effective method of quantifying this loosely constructed chronology existed until the discovery of radioactivity enabled dating procedures to be applied directly to the rocks in question.
One such attempt was made by Archbishop From the time of Hutton’s refinement of uniformitarianism, the principle found wide application in various attempts to calculate the age of the Earth.