By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.
Potassium (K) is one of the most abundant elements in the Earth's crust (2.4% by mass).
If one of these protons is hit by a beta particle, it can be converted into a neutron.
Such numerical calibrations are crucial to understanding rates and timing of evolutionary change.
Geologists have used this method to date rocks as much as 4 billion years old.
It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40) ,decays to the gas Argon as Argon-40 (Ar-40).
One out of every 10,000 Potassium atoms is radioactive Potassium-40 (K-40).
These each have 19 protons and 21 neutrons in their nucleus.