The Pacific is the biggest ocean on Earth, but it’s getting smaller every day. Australasia and the Americas are inching closer together, and in about 350 million years the Pacific will effectively close.

That’s when plate tectonics — the process driving all that slow motion, and one that geologists have assumed to be continuous — may grind to a halt.

Plate tectonics is the movement of enormous sections of Earth’s crust—the plates. New crust forms where plates separate on the seafloor, and existing crust sinks into the mantle when a neighboring plate overrides it at what’s called a subduction zone.

Today, most subduction zones are in the Pacific, and they’ll vanish along with that ocean. Contrary to widespread opinion, others are unlikely to replace them elsewhere, say Paul G. Silver of the Carnegie Institution of Washington and Mark D. Behn of Woods Hole Oceanographic Institution in Massachusetts.

That would stop plate tectonics worldwide — at least for a geological while.

Silver and Behn point to the Tethys Ocean, an ancient sea that shrank to nothing when squeezed by Africa and India drifting against Eurasia. The disappearing act spawned no new local subduction zones, showing that once lost, the zones aren’t readily replaced.

Plate tectonics may have already taken a global hiatus 900 million years ago, when several continents collided to form the supercontinent Rodinia. The team says various geological indicators suggest that during Rodinia’s 140-million-year existence, the world’s plates were at a standstill.

The research was detailed in the journal Science earlier this year.