EARTH, AGE OF. — The methods used until a few decades ago to determine the age of the Earth are now seen to lack any scientific rigor.
These methods were based mainly on the rate of deposition of sediments in aqueous environments, or on the salinity of present-day oceans, or on calculations of the progressive cooling of the Earth’s sphere, or again on the rate at which exogenous agents erode mountain ranges. The values obtained by these methods are much lower than those derived today by other means. The first of these methods, already glimpsed by Herodotus, consisted in establishing, on the basis of currently observed phenomena, the time required for the deposition of a known thickness of materials. However, the problematic reconstruction of a type section representing all the sedimentary layers deposited in various geological periods involves insurmountable problems: the rate of sedimentation depends on the environment and varies from case to case; moreover, there would always remain the doubt that materials from some of these layers had been removed by erosion in earlier times and redeposited elsewhere.
The other methods are equally open to criticism. The calculation of the Earth’s age based on the salinity of the seas attempts to determine the time necessary for rivers (assuming their total flow has remained unchanged over time) to pour into the oceans quantities of salts sufficient to transform the original fresh waters into the present-day waters, whose average salinity is known. The study of the progressive cooling of the Earth’s sphere involves—among numerous sources of error—the problematic knowledge of an initial temperature and of the quantity of heat generated by the still-active disintegration of radioactive elements dispersed at inaccessible depths.
The current methods for determining geological time, though far more precise despite various sources of error, are based on the radioactive disintegration of uranium (especially the two isotopes U I and AcU), which leads to the formation of radioactive lead (specifically the two isotopes Pb²⁰⁶ and Pb²⁰⁷). Radioactive materials are contained in varying degrees in all rocks, and it is assumed that the accumulation of residual elements (helium and lead) began from the moment they were enclosed in the rock during its consolidation.
Since the initial quantity of uranium remains practically constant due to the slowness of the disintegration process (half-life = 4.4 × 10⁹ years), and since the quantity of the final product formed each year can therefore be considered constant, by accurately determining the percentages of uranium and lead present together, one can calculate the age in years of a rock using the formula:
\[
\text{Age in years} = \frac{\text{Lead (in grams)}}{\text{Uranium (in grams)} \times 1.3 \times 10^{-10}}
\]
where 1.3 × 10⁻¹⁰ represents the quantity of lead (in fractions of a gram) generated in one year by one gram of uranium. The oldest rocks on which this method has been tested have been found to be close to two billion years old.
Another modern method is based on the study of the size and color of pleochroic halos, a type of halo that forms around zircon grains.