Pliny the Younger advances geochronology.

The eruption of Vesuvius volcano witnessed by Pliny the Younger in 79 CE is dated by the argon-40/argon-39 (40Ar/39Ar) method to 1938 ± 13 years (SD) before measurement in 2025 CE. Achievement of 0.7% precision and 0.4% accuracy at such a young age marks a precedent for 40Ar/39Ar dating and signals improved applicability of the method for constraining magma systems' evolution, including volcanic hazard assessment. Improvements in mass spectrometry, neutron irradiation strategy, and determination of interfering neutron-induced isotopes enable this result. The data serve to determine the decay constant for the radioactive decay of potassium-40 (40K) to 40Ar that is twice as precise as the value determined by disintegration counting, thus improving the accuracy of 40Ar/39Ar dating throughout geologic time, and serves as an important datum for intercalibrating 40Ar/39Ar with other dating methods.