When did plate tectonics start? A lead clue points to 2.7bn years

The pattern is striking. A particular lead isotope signature (a specific mix of lead atoms) shows up again and again in rocks older than roughly 2.76 to 2.63 billion years. In younger rocks it disappears. "Despite examining thousands of rocks, we never see it again," Liebmann said in a statement.

Why is it so hard to say when plate tectonics began?

Because plate tectonics destroys its own evidence. Colliding plates push old crust back down into the mantle, and crust thrust upward erodes away. Still, older rock often becomes an ingredient in new crust and leaves a chemical trace behind. That trace can survive for billions of years in minerals such as feldspar and sulfides, which the team studied.

What does the lead fingerprint mean?

The signature points to very old crust that formed around 4 billion years ago. Back then, Earth's tectonics were probably stagnant, melting and remelting the same material to make the granite-like rocks of the first continents. Something then changed how crust was made and reworked, a shift toward the dynamic system we see today.

The team does not yet know what that "something" was, or why the change happened. Chris Kirkland, a co-author at Curtin, called it the clearest geological evidence yet for when the tectonic engine of the modern Earth "switched on." Plate tectonics shapes continents and oceans, and it influences the atmosphere and climate, so the timing matters for how our planet ended up so different from its neighbours.