Eon· The columns of time
Hadean
4.54 Ga → 4.03 Ga
Five hundred million years without a single rock left to show for them. The eon takes its name from the underworld, and it is the only division of Earth's history defined by the absence of what should record it.
What the evidence shows
The accretion of the terrestrial planets is dated to about 4.54 billion years, and that is where this timeline opens; hafnium and tungsten isotopes in meteorites show how fast it went, with metal separating from rock across the young Solar System in under thirty million years. What follows is a world repeatedly melted: a body the size of a small planet strikes the Earth and throws off the material that becomes the Moon, and the simulations that reproduce the Moon's Earth-like composition call for an impactor larger still than earlier work assumed. A collision on that scale leaves nothing of a surface behind. The eon ends where the oldest surviving rocks begin, which is a fact about preservation and not about the planet.
Bounds and conventions
Its older bound is the age of Earth's accretion, measured on meteorites rather than on Earth, because no terrestrial material is that old. Its younger bound moves with discovery: it follows the oldest dated rock, and every older find shortens the eon without changing anything that happened in it.
Sources
- Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
- Yin et al. (2002), Nature · A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites
- Canup (2012), Science · Forming a Moon with an Earth-like composition via a giant impact