Absolute vs relative age dating
Then after another 5,000 years half of the remaining parent isotope will have decayed.
Index fossils are fossils that are known to only occur within a very specific age range.
These isotopes break down at a constant rate over time through radioactive decay.
By measuring the ratio of the amount of the original (parent) isotope to the amount of the (daughter) isotopes that it breaks down into an age can be determined.
A few days ago, I wrote a post about the basins of the Moon -- a result of a trip down a rabbit hole of book research.
Here's the next step in that journey: the Geologic Time Scales of Earth and the Moon.