The potassium-argon (K-Ar) method is used by geologists to date rocks hundreds or even thousands of millions of years old. It is also one of the most appropriate techniques for dating early human (hominid) sites in Africa, which can be up to 5 million years old. It is restricted to volcanic rock no more recent than around 1000,000 years. (...) Potassium-argon dating, like radiocarbon dating, is based on the principle of radioactive decay: in this case, the steady but very slow decay of the radioactive isotope potassium-40 to the inert gas argon-40 in volcanic rock.