Lord Kelvin and Clarence King calculated the length of time required for the Earth to cool from a white-hot liquid state; they eventually settled on 24 million years.James Joly calculated that the Earth’s age was 89 million years on the basis of the time required for salt to accumulate in the oceans.
Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in 4004 B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.No one can predict the moment when a particular grain will fall through the neck, but an estimate can be made for how long the whole pile of sand will take to fall.A similar process takes place with the radioactive decay of atoms.To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating.
Radiometric dating is largely done on rock that has formed from solidified lava.
(For a brief science review, see sidebar and figure 1.) The timepiece that allows dating is the “radioactive” decay of certain kinds of atoms from one form into another.
Radioactive decay results from unstable combinations of protons and neutrons in the atom’s nucleus.
James Hutton, a physician-farmer and one of the founders of the science of geology, wrote in 1788, “The result, therefore, of our present inquiry is, that we find no vestige of a beginning, — no prospect of an end.” Although this may now sound like an overstatement, it nicely expresses the tremendous intellectual leap required when geologic time was finally and forever severed from the artificial limits imposed by the length of the human lifetime.
By the mid- to late 1800s, geologists, physicists, and chemists were searching for ways to quantify the age of the Earth.
We will deal with carbon dating first and then with the other dating methods.