AGE OF THE EARTH. - The methods used until a few decades ago to determine the age of the earth are revealed today as totally lacking in scientific rigor.
They were based mainly on the rate of deposition of sediments in an aqueous environment, or on the salinity of the present oceans, or on a calculation of the progressive cooling of the terrestrial sphere, or again on the rate at which exogenous agents erode mountain chains; the values obtained with such methods are much lower than those obtained today by other means. The first of the methods cited, already glimpsed in Herodotus, consisted in fixing, on the basis of presently observable phenomena, the time necessary for the deposition of a known thickness of materials; but the problematic reconstruction of a type section representing all the piles of sediments deposited in the various geological periods entails insoluble problems: the rate of sedimentation depends in fact on the environment and varies from case to case; there would then always remain the doubt that the materials of some of these piles had been, in earlier times, removed by erosion and redeposited.
The other methods are subject to equally easy criticisms. The calculation of the age of the earth on the basis of the salinity of the seas seeks to establish the time necessary for rivers (their global discharge supposed unchanged over time) to pour into the oceans quantities of salts such as to change the primitive fresh waters into the present waters whose average salinity is known. The study of the progressive cooling of the terrestrial sphere implies - among the numerous causes of error - the problematic knowledge of an initial temperature and of the quantity of heat developed by the disintegration, still active, of the radioactive elements scattered at depths that are not accessible.
The present methods of determining geological time, much more precise although subject to various causes of error, are founded on the radioactive disintegration of uranium (especially of the two isotopes UI and AcU), which leads to the final formation of radioactive lead (respectively of the two isotopes Pb²⁰⁰ and Pb²⁰⁷). Radioactive materials are contained in varying measure in all rocks, and it is supposed that from the moment when they were enclosed in the consolidating rock the accumulation of the residual elements (helium and lead) began.
Since the initial quantity of uranium remains practically constant because of the slowness of the disintegration process (period = 4.4 × 10⁹ years), and since therefore the quantity of the final product formed each year can also be considered constant, by determining exactly the percentages of uranium and lead in each other's presence, one can calculate the age in years of a rock with the formula:
Age in years = frac{Lead (in grams)}{Uranium (in grams) × 1.3 × 10^{-19}}
in which 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 was tested proved to be close to two billion years old.
Another modern method is based on the study of the dimensions and color of the "pleochroic halos," a sort of halo that forms around grains of radioactive zircon, abundant in the mica of eruptive rocks, and due to the development of helium; this method too gives values close to those provided by the lead isotopes.
The earth therefore has an age (from the time when a solid terrestrial crust formed to today) of at least two thousand million years; of these, ca. 1500 are to be assigned to the Archeozoic era, in whose most recent part the first traces of life appear; the other 500 million are to be divided according to the accompanying little table.
| Duration of the geological eras in millions of years | |
|---|---|
| Quaternary or Neozoic era | 0.5 — 1 |
| » Tertiary or Cenozoic | 50 — 60 |
| » Secondary or Mesozoic | 130 — 150 |
| » Primary or Paleozoic | 300 |
| » Archeozoic | 1300 — 1500 |
| Total ca. | 1800 — 2000 |
At present, however, these values tend to increase as a consequence of the perfecting of research methods and of the discovery of even older rocks: it seems therefore, from the most recent results, that the earth has an age (again from the solidification of the terrestrial crust to today) of not less than two and a half billion years.
For the age of the solar system, of the galactic system and of the universe, v. UNIVERSO.
Bibliography3 works
Bruno Accordi