by Ralph W. Matthews, Ph.D.
Before 1955, ages for the Earth based on uranium/thorium/lead ratios were generally about a billion years younger than the currently popular 4.5 billion years. The radiometric evidence for a 4.5 b.y. old Earth is reviewed and deficiencies of the uranium/lead method are discussed. The basic theory of radiometric dating is briefly reviewed. Since 1955 the estimate for the age of the Earth has been based on the assumption that certain meteorite lead isotope ratios are equivalent to the primordial lead isotope ratios on Earth. In 1972 this assumption was shown to be highly questionable. Despite this, the momentum gained in the two decades prior to 1972 has made 4.5 b.y. a popularly accepted “universal constant” even though the foundations on which it was based have been virtually removed. Some evidence is also presented to show that radiometric results that are in agreement with the accepted geological time scale are selectively published in preference to those results that are not in agreement.
Basics
The geological time scale and an age for the Earth of 4.5 b.y. rely heavily on the uranium/thorium/lead radiometric dating methods.1Because it is not generally appreciated that the assumptions on which the radiometric estimates are based are a virtually impossible sequence of events, let us refresh our minds on the fundamentals of the method by turning to the hourglass analogy (Fig. 1). This system of measuring time works well providing that:
- the hole does not clog up,
- the sand always flows at a known and reproducible rate,
- we know how much sand is in the bottom at the beginning,
- no sand is added or subtracted during the timing run. In other words, it has to be a closed system….
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