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USGS CMG InfoBank: Uranium-235

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Comment: 21:37 - 22:16 (00:39)
Source: Annenberg/CPB Resources - Earth Revealed - 10. Geologic Time
Keywords: radioisotope, uranium-235, element, lead-207, isotope, "absolute age dating"

Our transcription: To date rocks that are millions or even billions of years old, geologists work with radioisotopes that take much longer to decay, such as uranium-235.
This element makes an excellent geologic clock because it takes 713 million years for half of it to decay into lead-207.
By measuring the relative amounts of these two isotopes in a rock, it's possible to figure out the rock's age even if it is several billion years old.

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