Potassium-argon dating, method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock. The potassium-argon dating method has been used to measure a wide variety of ages.

Why is potassium 40 useful for radioactive dating?

The very slow decay of potassium 40 into argon are highly useful for dating rocks, such as lava, whose age is between a million and a billion years. The decay of potassium into argon produces a gaseous atom which is trapped at the time of the crystallization of lava.