How did William Thomson’s ideas about heat loss remain useful even after his Earth-age estimate was disproved?

Although William Thomson’s (Lord Kelvin) specific estimate of the Earth’s age—calculated at 20 to 40 million years—was disproved by the discovery of radioactivity and the realization that radioactive decay provides a continuous internal heat source, his broader thermodynamic arguments remain scientifically significant.12

Thomson’s core assertion that the Earth suffers an irreversible loss of energy remains a valid physical principle. This concept of heat loss has proven useful in understanding the evolution of the planet’s behavior over time. As the Earth’s internal temperatures have fallen, the intensity and nature of geologic processes—such as igneous activity and crustal deformation—have changed.2

Consequently, while the principle of uniformitarianism correctly holds that physical laws remain constant, Thomson’s insights help explain why the specific manifestations of those laws, including the mechanisms of plate tectonics, have evolved throughout geologic time as the planet has cooled.2