Rocks keep time by decay
Radioactive isotopes decay at measured rates. Isochron and concordia methods date rocks from thousands to billions of years.
Rutherford and Boltwood realised that uranium turns into lead at a pace you can time. Clair Patterson’s lead-isotope work on meteorites (1956) gave the solar system an age of 4.55 billion years — a number that has held.
Different systems (U–Pb, K–Ar, Rb–Sr, C-14) cover different ranges. Carbon-14 is for the last ~50,000 years and is calibrated against tree rings and varves. Uranium–lead in zircon is the gold standard for deep time because it has two independent decay chains that must agree.
Anomalies happen: loss of daughter isotopes, inherited cores, contamination. They are why geochronologists use isochrons, concordia diagrams, and multiple minerals — not a single raw ratio treated as gospel. The method is not a vibe. It is a cross-checked laboratory practice.
Caveats
- A date is only as good as the closed-system assumption and the sample.
- Carbon-14 does not date dinosaur bones; the isotope is gone. That is a feature of the half-life, not a scandal.
Sources
- 1Patterson C.. Age of Meteorites and the Earth. Geochimica et Cosmochimica Acta 10, 1956.
- 2Dalrymple G. B.. The Age of the Earth. Stanford University Press, 1991.