Radiometric dating questions
Kelvin calculated the age of the Earth by using thermal gradients, and he arrived at an estimate of about 100 million years.
For biologists, even 100 million years seemed much too short to be plausible.Huxley, attacked Thomson's calculations, suggesting they appeared precise in themselves but were based on faulty assumptions.The physicist Hermann von Helmholtz (in 1856) and astronomer Simon Newcomb (in 1892) contributed their own calculations of 22 and 18 million years respectively to the debate: they independently calculated the amount of time it would take for the Sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born.In Darwin's theory of evolution, the process of random heritable variation with cumulative selection requires great durations of time.(According to modern biology, the total evolutionary history from the beginning of life to today has taken place since 3.5 to 3.8 billion years ago, the amount of time which passed since the last universal ancestor of all living organisms as shown by geological dating.) In a lecture in 1869, Darwin's great advocate, Thomas H.material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.
Following the development of radiometric age-dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.
It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites.
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.
Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth, assuming that no rock has been intact for longer than the Earth itself.
In 1892, Thomson had been made Lord Kelvin in appreciation of his many scientific accomplishments.
His calculations did not account for heat produced via radioactive decay (a process then unknown to science) or, more significantly, convection inside the Earth, which allows more heat to escape from the interior to warm rocks near the surface.