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SCIENCE FILE | I Didn't Know That

August 08, 1996

Q: How old are the oldest rocks on Earth?

A: The oldest rock known is a grain of zircon from Canada that is 3.96 billion years old, according to James Trefil of George Mason University. Based on the age of the moon and meteorites, however, geologists estimate that the Earth is actually about 4.6 billion years old.

Scientist date rocks by using atoms of potassium-40, which decays radioactively into argon-40 with a half-life of 1.3 billion years. Since argon-40 itself is not incorporated into minerals, Trefil says, all argon-40 atoms in a rock must have come from the decay of the potassium-40 that was there originally. Measuring it gives the date when the rock was formed.

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