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The Earth is very old. But how old, exactly? And how can we know with any degree of confidence? As Henry Reich describes in the video above, the process of scientifically estimating the age of the Earth revolves around, essentially, finding the oldest piece of the planet we can, then figuring out how old that piece is.
Finding super old rocks is conceptually straightforward, but practically difficult. The processes of plate tectonics mean that the Earth is constantly recycling its rock, breaking it down into magma in the interior before pumping it back up to the surface once more. But old rocks do exist, says Reich, and the oldest rock we know is a tiny piece of zircon found in western Australia.
The process of figuring out a rock's age often falls to the scientific techniques of radiometric dating, the most famous of which is radiocarbon dating. This process focuses on the ratio between the number of carbon-14 and carbon-12 isotopes in any once-living being: that ratio indicates how long it's been since that being was alive. But carbon is not the only element that can be dated—a whole host of others exist. In uranium-lead dating, for instance, the radioactive decay of uranium into lead proceeds at a reliable rate.
Based on the very old zircon rock from Australia we know that the Earth is at least 4.374 billion years old. But it could certainly be older. Scientists tend to agree that our little planet is around 4.54 billion years old—give or take a few hundred million.