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Why did Lord Kelvin underestimate the age of Earth?

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Faux amphibolite — geology A rock formation called “faux amphibolite,” from the Nuvvuagittuq greenstone belt in northern Quebec, dated to 4.28 billion years ago.

Lord Kelvin (William Thomson) underestimated the age of the Earth because his calculations were based on the assumption that the planet was an inert body cooling from an originally molten state.12

Applying the principles of thermodynamics, Kelvin treated the Earth as a heat machine that could not be a perpetual motion machine, concluding that it must have a finite, relatively short lifespan. By assuming the Earth lacked any internal mechanism to generate new heat, he calculated that the planet was between 20 million and 100 million years old.231

This estimate was proven incorrect following the discovery of radioactivity in the late 19th century. Researchers, including Marie and Pierre Curie, realized that the spontaneous disintegration of radioactive elements—such as uranium and thorium—continuously generates heat within the Earth’s interior. Because Kelvin did not account for this internal heat source, his model failed to reflect the true, much greater age of the Earth.132