An absolute dating technique similar to radiocarbon dating but applicable to much older deposits. It is used to determine the age of volcanic rock strata containing or sealing archaeological objects rather than to date the artefacts themselves. The technique works by measuring the ratio of radioactive potassium‐40 to the stable gas argon‐40 in volcanic rocks. Over time 40K decays to become 40Ar at a known rate, its half‐life being 13 000 million years. In volcanic rocks any argon present will have escaped when the rock was last molten but will start to accumulate again when it solidifies. Thus by carefully measuring the amount of 40K and 40Ar present in a sample it is possible to work out how long ago it was that the rock solidified. Because of the long half‐life of 40K the amount of argon present in samples less than 1 million years old is insufficient to measure, but the technique is very useful for work on early hominin deposits associated with volcanic layers.
Subjects: Science and Mathematics — Archaeology.