Radiometric dating age of earth
Meteorites are rocks from the solar system that have fallen to Earth recently and haven’t suffered much erosion.
So, if we find a rock with equal parts Potassium-40 and Argon-40, we know that half the Potassium-40 has decayed into Argon-40, and that the rock hardened 1.3 billion years ago.
Some isotopes are unstable, and over time these isotopes “decay” into isotopes of other elements.
For example, Potassium-40 is unstable and decays into Argon-40.
The oldest reliably dated rock formation is in Greenland, where several different isotopes were used to find an age of 3.6 billion years.
To find older rocks that haven’t been eroded, we need to look beyond Earth.