how do we date fossils and other very old rocks that are many millions if not billions of years old?
In: Planetary Science
Very few fossils are dated directly. Instead they are given dates relative to rocks that can be dated using radioactive decay.
Sedimentary rocks containing fossils rarely contain enough radioactive material for dating, but it is possible to date volcanic rocks that cooled from magma that sometimes occur in a rock sequence. So if you have a lava flow somewhere below the fossils, you can usually assume the fossil is younger than the lava because it was deposited on top. If you then have another lava flow above the fossils, you can say it is younger than the fossils and therefore the fossil’s age lies somewhere between the two dates for the lava.
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