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How did the discovery of radioactivity change estimates of Earth’s age?

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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.

Before the discovery of radioactivity, estimates of Earth’s age were significantly underestimated, often ranging from only a few thousand to roughly 100 million years based on flawed assumptions about sedimentation rates, ocean salinity, or cooling thermodynamic models.1

The discovery of radioactivity fundamentally changed these estimates by providing a reliable method for absolute dating. Early 20th-century researchers, such as Bertram B. Boltwood, used uranium-lead systems to calculate mineral ages reaching into the billions of years, an order of magnitude greater than previous estimates. Furthermore, the realization that radioactive decay provides a continuous internal heat source invalidated earlier cooling models, such as those proposed by Lord Kelvin, which had incorrectly assumed the Earth was inert and cooling from a molten state.21

Today, through the isotopic analysis of meteorites and terrestrial samples, scientists have established that Earth is approximately 4.5 to 4.6 billion years old. This shift from a "young" Earth to one of immense geologic antiquity is a cornerstone of modern geochronology.312