Eon· The columns of time

Hadean

4.54 Ga → 4.03 Ga

The Hadean is the first eon of the Earth's history, five hundred million years from which no rock survives, the only division of the time scale defined by the absence of its own archives.

Initial eon of the history of the Earth, covering five hundred million years of which no rock is preserved. Its name is formed on Hades, and it is the only division of the geological time scale defined by the absence of the archives that should record it.

What the evidence shows

The accretion of the terrestrial planets is dated to about 4.54 billion years, the point at which this timeline opens; hafnium and tungsten isotopes measured in meteorites give its speed, the separation of metal from rock having occurred within less than thirty million years across the young solar system. What follows is a world melted more than once: a body the size of a small planet strikes the Earth and detaches the material from which the Moon forms, the simulations that reproduce the Moon's terrestrial composition requiring a more massive impactor than earlier work assumed. A collision of that magnitude leaves no surface intact. The eon ends where the oldest preserved rocks begin, a limit that reports on conditions of preservation and not on the state of the planet. Only isolated grains survive, the Jack Hills zircons, eroded from rocks that no longer exist.

Bounds and conventions

The older limit is the age of terrestrial accretion, measured on meteorites and not on the Earth, no terrestrial material of that age existing. The younger limit moves with discovery: it follows the oldest dated rock, and each older find shortens the eon without altering anything that occurred within it.