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. Wiens has a Ph D in Physics, with a minor in Geology.His Ph D thesis was on isotope ratios in meteorites, including surface exposure dating.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.
By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.
They observed that every rock formation, no matter how ancient, appeared to be formed from still older rocks.
Comparing these rocks with the products of present erosion, sedimentation, and earth movements, these earliest geologists soon concluded that the time required to form and sculpt the present Earth was immeasurably longer than had previously been thought.
Because the exact amount of time this accretion process took is not yet known, and the predictions from different accretion models range from a few million up to about 100 million years, the exact age of Earth is difficult to determine.
It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages.