Dating how old
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.Archaeologists use the exponential, radioactive decay of carbon 14 to estimate the death dates of organic material.Until the 18th century, this question was principally in the hands of theologians, who based their calculations on biblical chronology.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 mid- to late 1800s, geologists, physicists, and chemists were searching for ways to quantify the age of the Earth.
While alive, all plants and animals take C14 into their bodies.
A substantial hurdle is the difficulty of working out fossil ages.