Definition
Radiometric dating is a family of techniques that determine the age of geological materials by measuring the ratio of a radioactive parent isotope to its stable daughter product. Because radioactive decay proceeds at a known constant rate (the half-life), the ratio yields the time since the material's isotope system was last reset — typically by crystallisation from a melt.
In paleoanthropology, the workhorse methods are potassium-argon (and its refinement argon-argon) for volcanic ash layers in the millions of years, uranium-series for cave deposits and tooth enamel in the hundreds of thousands of years, and radiocarbon for organic material younger than ~50,000 years.
Why it matters
How it works
For argon-argon dating, a tiny sample of feldspar crystal from a volcanic tuff is irradiated to convert potassium-39 to argon-39, then heated stepwise to release argon. The ratio of argon-40 (produced by potassium-40 decay over millions of years) to argon-39 (a proxy for original potassium content) gives the age. Modern instruments can date individual crystals, which lets analysts exclude contamination from older or younger material.
Uranium-series uses the decay of uranium isotopes in carbonate minerals — speleothems, flowstones, tooth enamel — formed during the fossil's depositional history. Radiocarbon measures carbon-14 in organic remains, calibrated against dendrochronological records and varved sediments to convert raw radiocarbon ages into calendar years.