Dating the Earth

The clock

Radioactive decay turns a parent isotope into a daughter at a rate expressed as a half-life. Isotopes that decay slowly serve as geological clocks.^[1]

What the clock is timing

The date obtained is the last time the sample was melted or disturbed enough to rehomogenise its radioactive elements. It times a resetting event, not the origin of the atoms.^[1]

This is why Earth’s oldest rocks give a floor rather than an age. The Acasta Gneisses are 4.03 billion years old; detrital zircons in Western Australia reach 4.3 billion. Both are minimum ages for surviving material.^[1,2]

Why the answer came from elsewhere

Plate tectonics has recycled and destroyed Earth’s oldest crust, and no rock in its primordial state has been found. The planet cannot date its own formation.^[1]

Meteorites can. Dating of more than seventy of them places the Solar System’s formation between 4.53 and 4.58 billion years ago, and lead isotope ratios in iron meteorites give 4.54 billion years for the Earth — on the assumption that it formed at the same time as the rest of the Solar System.^[1,2]

Calibrating the scale

The same technique calibrates the geological time scale, attaching numerical ages to boundaries that are themselves defined by physical reference sections. The chart’s own caution — that these numbers are subject to revision and do not define the units — follows directly from what the method measures.^[1,2]