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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.Studies of strata, the layering of rocks and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence.Geologists quickly realized that this upset the assumptions underlying most calculations of the age of Earth.

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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.

Kelvin calculated the age of the Earth by using thermal gradients, and he arrived at an estimate of about 100 million years.

He did not realize that the Earth mantle was convecting, and this invalidated his estimate.

By their chemical nature, rock minerals contain certain elements and not others; but in rocks containing radioactive isotopes, the process of radioactive decay generates exotic elements over time.

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By measuring the concentration of the stable end product of the decay, coupled with knowledge of the half life and initial concentration of the decaying element, the age of the rock can be calculated.Geologists such as Charles Lyell had trouble accepting such a short age for Earth.For biologists, even 100 million years seemed much too short to be plausible.In 1895, John Perry produced an age-of-Earth estimate of 2 to 3 billion years using a model of a convective mantle and thin crust.