That means Argon atoms which are normally just gaseous become trapped in solid igneous rock.
Since the 1960s, scientists have started accounting for the variations by calibrating the clock against the known ages of tree rings.
Once the lava solidifies the molecular/crystaline structure becomes locked.
But some Potassium isotopes will decay over time to Argon anyway.
Climate records from a Japanese lake are set to improve the accuracy of the dating technique, which could help to shed light on archaeological mysteries such as why Neanderthals became extinct.
Carbon dating is used to work out the age of organic material — in effect, any living thing.