EARTH

The Oldest Rock You Can Touch

~4.03 billion years ago (older claims to 4.3 Ga)

Archean · Eoarchean

Older crystals exist, but they are loose grains, eroded out of rocks that vanished long ago. Here is the oldest rock still standing: an outcrop in northern Canada you can walk up to and lay a hand on, formed when the planet was half a billion years old.

Why it matters

Everything earlier is reconstructed from scattered grains. Here the record becomes rock that can be mapped, and the planet's earliest history stops resting on inference.

Dating & uncertaintyDisputed

The Acasta gneiss is dated to 4.03 billion years by uranium and lead trapped in its zircon, and that age is not contested. A belt of rock in Quebec is claimed to be older still, between 4.16 and 4.3 billion years, on the strength of a clock that stopped running long ago: samarium-146, an isotope that decayed away entirely early in Earth's history and can now be read only in the neodymium it left behind. A formal objection appeared in Science, and the argument has run for fifteen years.

Sources

1. O'Neil et al. (2008), Science · Neodymium-142 evidence for Hadean mafic crust
2. Andreasen et al. (2009), Science · Comment on Neodymium-142 evidence for Hadean mafic crust
3. Sole et al. (2025), Science · Evidence for Hadean mafic intrusions in the Nuvvuagittuq Greenstone Belt

Earth history as one day

02:41:4521 h 18 min before midnight

if Earth’s 4.54 billion years were compressed into a single day