Potassium-argon dating is a method for estimating the age of volcanic rocks by measuring the ratio of potassium-40 to argon-40 present.
The method is based on the fact that the potassium-40 isotope of potassium decays over time to form argon-40. The useful fact about these two substances is that at normal temperatures, potassium is a solid, but argon is a gas. Therefore, during volcanic eruptions, any argon that is present escapes from the rock. But after the rock solidifies, any potassium-40 that is present continues to decay, and the argon-40 that is produced cannot escape from the rock.
Thus, geologists use potassium-argon dating to measure the age of volcanic rocks. If the concentration of argon-40 is almost zero, then the rock was formed recently. If it is high relative to the amount of potassium-40 present, then the rock is old. Archaeologists and biologists are also sometimes able to use potassium-argon dating to measure the age of artifacts and fossils, when these have become trapped in or buried under volcanic rock.
The mathematical formula that is used to figure the age of the rock depends on the half-life of potassium-40 (the time it takes for half the potassium-40 in a given sample to decay). The half-live of potassium-40 is approximately 1.26 billion years (that is, 1.26x109 years).
Obviously, this formula depends on the laws of physics remaining constant over time. If the rate of radioactive decay has changed over time, the formula will not give correct dates. Most scientists believe that the rate of potassium-argon decay has not changed over the history of the earth. However, some creationists have argued that God increased the rate of potassium-argon decay during the first few days of Creation, thus causing the potassium-argon dating method to give erroneously old date readings.[Citation Needed]
Other isotopes with shorter half-lives can also be used to date objects- however, each method has its own drawbacks. For instance, the decay of carbon-14 is often used to complement the potassium-argon dating of dinosaur fossils.
Quarternary Dating Methods, by M. Walker (Wiley & Sons, 2005).
Isotopes: Principles and Applications, by G. Faure and T. Mensing (Wiley & Sons, 2005).