It’s hard to find rocks on the surface of the Earth that have not been altered over time.
Many independent measurements have established that the Earth and the universe are billions of years old.
Geologists have found annual layers in ice that are easily counted to multiple tens of thousands of years, and when combined with radio isotope dating, we find hundreds of thousands of years of ice layers.
The half-life is the time that it takes for half the radioactive sample to change from one element into the other.
Some isotopes have short half-lives of minutes or years, but Potassium-40 has a half-life of 1.3 billion years.
Once the rock hardens, however, all the Argon-40 is trapped in the sample, giving us an accurate record of how much Potassium-40 has decayed since that time.