How we know the age of the Earth
10 steps · 1669 – 2001
From reading layers of rock to counting lead atoms: three centuries of clues, mistakes and disputes on the way to 4.55 billion years.
▶ Play on the globe- 11669 · Florence, Italy · Discovery
Steno's principles of stratigraphy
Steno realises strata are laid down one on top of another: what lies below is older. Rocks are a book that can be read in order.
🌍 View on the globe - 21788 / 1830 · Siccar Point, Scotland, United Kingdom · Idea or paradigm
Uniformitarianism and deep time
At Siccar Point Hutton sees vertical layers capped by horizontal ones and grasps that today's slow processes needed immense time: «no vestige of a beginning».
🌍 View on the globe - 31796-1815 · Bath, England, United Kingdom · Discovery
Stratigraphy and index fossils: first geological map
Digging canals, William Smith finds that each layer carries its own fossils. With them he matches rocks across England and draws the first geological map.
🌍 View on the globe - 41822-1841 · London, United Kingdom (Geological Society) · Discovery
Geologic time scale
Geologists order history into periods: Cambrian, Jurassic, Cretaceous… They know what comes before and after, but not how many years each lasts.
🌍 View on the globe - 51858-1859 · Down House, Kent / Linnean Society, London, England · Discovery
Evolution by natural selection
Evolution by natural selection needs hundreds of millions of years to work. The age of the Earth becomes a problem for biology.
🌍 View on the globe - 61862 · Glasgow, United Kingdom · Discovery
Age of the Earth from cooling (Kelvin)
Kelvin works out how long the Earth would take to cool and gets a few tens of millions of years at most. A blow to Darwin… and a mistake, because an unknown heat source was missing.
🌍 View on the globe - 71896 · National Museum of Natural History, Paris, France · Discovery
Natural radioactivity
Radioactivity supplies the heat Kelvin left out and, above all, a clock: unstable atoms decay at a fixed rate.
🌍 View on the globe - 81905-1907 · Yale University, New Haven, United States · Discovery
Uranium-lead radiometric dating
Boltwood measures how much lead accompanies uranium in different minerals and gets ages of up to 2,200 million years. The clock works, and the Earth is far older than Kelvin said.
🌍 View on the globe - 91956 · Caltech, Pasadena, United States · Discovery
Radiometric age of the Earth: 4.55 billion years
Patterson dates lead from a meteorite, formed at the same time as the Earth, in a clean room he built himself: 4.55 billion years. The figure still stands.
🌍 View on the globe - 102001 · Jack Hills, Western Australia, Australia · Discovery
Jack Hills zircons: the oldest terrestrial mineral
A 4,404-million-year-old Australian zircon grain is the oldest known piece of Earth, and hints that there were oceans soon after the planet formed.
🌍 View on the globe