The most common isotope of uranium atoms spontaneously decays into thorium and other lighter atoms at such a rate that half of the atoms have fallen apart after 4.47 billion years. Such radioactive decays generate enough heat to keep Earth’s core nice and toasty for billions of years, explaining why Earth is so warm even if it’s way older than 20 million years. Moreover, by measuring what fraction of the uranium atoms in a rock have decayed, you can determine the age of the rock, and in this way, some rocks from the Jack Hills of western Australia have been found to be over 4.404 billion years
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