How is potassium-40 used to calculate the ages of rocks?
![A periodic table element box for potassium shows atomic number 19, symbol K, atomic weight 39.0983, electron configuration [Ar]4s1, and name potassium. Icons indicate it is an alkali metal, solid at room temperature, body-centered cubic crystal structure, and has strongly basic higher-valence oxides. A key explains the symbols for alkali metals, solid state, body-centered cubic structure, and strong basicity.](https://cdn.britannica.com/81/22381-050-E3372C94/properties-Potassium.jpg)
Potassium-40 is a radioactive isotope used to determine the age of rocks through a process known as potassium-argon dating. This method relies on the predictable radioactive decay of potassium-40 into argon-40 and calcium-40.123
Because potassium is a common constituent in many rock-forming minerals, scientists can measure the ratio of the parent isotope (potassium-40) to the daughter product (argon-40) within a sample to calculate its age. While potassium-40 also decays into calcium-40, the calcium-potassium method is rarely used because non-radiogenic calcium is naturally abundant in rocks, which masks the presence of the radiogenic variety. Conversely, argon is a gas that typically escapes from rocks during volcanic processes, meaning any argon-40 found trapped within a mineral is generally assumed to have been produced by the decay of potassium-40 since the rock formed.13
Advanced techniques, such as the argon-40–argon-39 method, allow researchers to analyze smaller samples and account for potential inaccuracies caused by mineral alteration or inherited daughter products.3