How do we know Earth is about 4.54 billion years old?

Earth's age is put at about 4.54 billion years, with an uncertainty of less than 1 percent. The figure comes mainly from radiometric dating, which uses the steady decay of radioactive isotopes as a clock. Because no original Earth rock has been found, the best age comes from treating Earth and meteorites as parts of one system that formed together. [1]

Unstable parent isotopes decay into stable daughter isotopes at a fixed rate called the half-life, the time for half of a sample to decay. Measuring the ratio of parent to daughter isotopes in a rock or mineral reveals how long the decay has run. Uranium-238, for example, has a half-life of more than 4 billion years, which makes it suited to very old materials. The measured age marks the last time the rock was melted or disturbed enough to reset its isotopes. Stratigraphy, the study of rock layers, can show which layers are older but gives no exact ages. [2] [1] [3]

Plate tectonics and the rock cycle have recycled and destroyed Earth's earliest rocks. The oldest known rocks are the Acasta Gneisses in northwestern Canada at about 4.03 billion years, and the Isua rocks in Greenland at 3.7 to 3.8 billion years. Tiny zircon crystals from Western Australia reach about 4.3 to 4.4 billion years, the oldest Earth materials found so far. These rocks are lava flows and sediments, not a primordial crust, so they show that Earth's history began earlier but do not fix the moment of formation. [1] [2]

Meteorites are fragments of asteroids that have not been reworked by plate tectonics. More than 70 of them, of different types, have been dated, and they indicate that the Solar System formed between 4.53 and 4.58 billion years ago. Some meteorite materials are reported at more than 4.56 billion years, and Moon and Mars rocks date to around 4.5 billion years. [1] [2]

The most precise Earth age comes from lead. Uranium-235 and uranium-238 decay into lead-207 and lead-206, so the lead ratio shifts over time. Scientists compare the primordial lead measured in iron meteorites, such as Canyon Diablo, with the lead in Earth's oldest lead ores. The time needed to evolve from one to the other gives 4.54 billion years. This figure marks the last time lead isotopes were uniform across the inner Solar System, when lead and uranium were locked into solid bodies. [1]

Earth did not form at a single moment. Rebecca Fischer, a planetary scientist at Harvard University, told Live Science that the change from dust cloud to planet took millions of years, so the age describes an era of formation more than a specific year. [2]