Earth reaches its full size, 4.54 billion years ago
What happened
Earth grew to nearly its present mass by sweeping up rock and metal from its zone of the disk over tens of millions of years. The planet's age is pinned to about 4.54 billion years, and the figure comes not from any Earth rock but from meteorites, leftover building material that was never reworked. Lead-isotope dating of the Canyon Diablo iron meteorite, treated as part of the same system as the Earth, gives an age for the Earth and meteorites of 4.54 billion years, with an uncertainty under one percent.
Why it matters
That one number anchors all of geology and much of biology. Every later date on this timeline and the others, from the first oceans to the first humans, is measured against an Earth that is 4.54 billion years old.
How we know
Earth's own surface has been recycled by plate tectonics, so no rock survives from its birth. Meteorites were never recycled, so the lead-isotope clock inside them still reads the moment the Solar System's material formed, which is taken as the age of the Earth.
Sources
- U.S. Geological Survey. Age of the Earth · Reputable sourcepubs.usgs.gov · The domain "pubs.usgs.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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