potassium-argon dating

Isotopic dating method based on the radioactive decay of potassium-40 (40K) to the stable isotope argon-40 (40Ar). Ages are based on the known half-life of 40K, and the ratio of 40K to 40Ar. The method is routinely applied to rock samples about 100,000 to 30 million years old.

The method is used primarily to date volcanic layers in stratigraphic sequences with archaeological deposits, and the palaeomagnetic-reversal timescale (see archaeomagnetic dating). Complicating factors, such as sample contamination by argon absorbed from the atmosphere, and argon gas loss by diffusion out of the mineral, limit the application of this technique.

Potassium-argon dating has made it possible to establish a chronology of hominid development (see human species, origins of), placing our earliest extant fossil ancestor, Aegyptopithecuszeuxis (from the Fayum in Egypt) at about 2.8 million years ago, and the first recognized Homo species (cranium 1470, from East Turkana, Kenya) at about 1.8 million years ago. It has also been used for dating early hominids at other African sites such as Hadar, Ethiopia, and at the Olduvai Gorge, Tanzania, where a 2-million-year chronology has been developed based on potassium-argon dating of volcanic tuff (volcanic soil or pumice) and other materials.