We now know that the Earth is about 4.5 billion years old, although the planet’s core is about two years younger than that thanks to the time-dilating effects of gravity. But how did we figure that out?
Early attempts to calculate the age of the Earth using science fell just short. I 1844physicist William Thomson (aka Lord Kelvin) came up to think about how to measure the age of the Earth. He assumed that the Earth was originally a big molten blob in space. By seeing how long it would take for the planet to cool to its current state, he reasoned that he could figure out the age of the earth itself.
Doing the math years later, he estimated that the Earth was about 20-400 million years old. This contradicted many things, such as geology, Darwin’s ideas about how long it would take animals to evolve, and the fact that he only believed the Sun was smaller than it was around. 20 million years Old.
Thomson’s estimates were created before radiometric dating, which gave us a much more accurate way to age rocks.
“The ages of Earth and Moon rocks and meteorites are measured by the decay of long-lived radioactive isotopes of elements that occur naturally in rocks and minerals and decay with half-lives of 700 million to over 100 billion years for stable isotopes of . other aspects,” explains the US Geological Survey on their part website. “These techniques, firmly based in physics and collectively known as radiometric dating, are used to measure the last time the rock being dated was melted or disturbed enough to re-homogenize its radioactive elements .”
By using radiometric data we can get a much more accurate idea of old rocks. However, dating the Earth’s rocks can only give us a minimum possible age for the Earth. The oldest rock we have found so far, from the Acasta Gneiss Complex in northwestern Canada, dates from ca. 4.02 billion years since then. This gives us a minimum age, as we can safely assume that the Earth is no younger than its oldest rock. Unless you are willing (and somehow able) to analyze every rock on the planet, however, it is not possible to get an estimate of the maximum age of the planet by this method alone. The oldest rocks on Earth could have slid down into the Earth’s mantle, too, causing the Earth to age.
Instead, scientists looked at rocks from the Moon and other bodies in the solar system, where this recycling of rocks is not an issue. In 1953, geochemist Clair Cameron Patterson he examined meteorite samples from a rock fall in Arizona, which contained lead isotope abundances used for radiometric dating. These samples were between 4.53 billion and 4.58 billion years old, and the range is thought to be due to the ~50 million years that the solar system formed.
Additional measurements of space rocks, and further study of how the solar system evolved, allowed us to refine our estimates.
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