Final answer:
Argon-argon dating is a form of absolute dating used to measure the argon-40 to argon-39 ratio in volcanic rocks. This method and similar radioactive dating techniques like uranium-lead dating rely on known decay rates of radioactive isotopes to determine the age of rocks.
Step-by-step explanation:
The dating method you're referring to is known as argon-argon dating, which is a form of absolute dating. In this technique, a sample is bombarded with fast neutrons in a nuclear reactor to measure the ratio of argon-40 to argon-39. This method is used specifically for dating volcanic rocks.
Radioactive dating methods, such as uranium-lead dating and potassium-argon dating, are based on the principle that radioactive isotopes decay at known and stable rates. By measuring the ratio of the original radioactive isotope to the decay products (for example, U-238 to Pb-206 or K-40 to Ar-40), scientists can determine the age of the rock sample.