The seafloor along a ridge in the Indian Ocean moved by up to 2.4 meters over just six days, as an underground chamber of molten rock emptied itself into cracks and onto the ocean floor. At the same time, the seabed sank by roughly four meters. Yet this dramatic shift triggered far fewer earthquakes than researchers expected.
The observation, reported by an international research team led by Jean-Yves Royer of the University of Western Brittany in Brest, marks the first time scientists have directly watched new ocean crust form. Published in the journal Nature, the findings may also resolve one of the oldest puzzles surrounding mid-ocean ridges: why earthquake activity in their centers is so much rarer than the pace of moving rock would suggest.
Mid-ocean ridges form the largest structure on Earth — a chain of underwater mountains stretching 65,000 kilometers around the globe. New crust is continuously generated there as molten rock rises from the mantle and pushes older crust outward. Despite decades of study, the exact mechanics of this process have remained largely unclear.
A rare window into the seafloor's birth
The team relied on data from the OHA-GEODAMS experiment, a network of seismic sensors and distance-measuring instruments spanning part of the Southeast Indian Ridge in the middle of the Indian Ocean. On April 26, 2024, the instruments detected a swarm of earthquakes signaling magma moving deep underground. As molten rock pushed into the upper crust, distances between measuring stations grew. Simultaneously, the valley at the center of the ridge — the actual rift between tectonic plates — collapsed downward into the emptied magma chamber. Warmer water near the seafloor later revealed the escaping molten rock, which piled up to 90 meters high.
Computer models built from this data suggest two mechanisms drove the seafloor's movement: magma physically pushing the crust apart, and the crust stretching as rocks slipped along faults running parallel to the ridge. This supports the idea that seafloor spreading at mid-ocean ridges happens in short bursts, releasing tension that has built up over decades of relative calm.
Crucially, most of this movement occurred without major shaking. The earthquakes recorded had only about a quarter of the strength that would normally be expected for such a large shift in rock. Researchers say this gliding, largely quiet process likely explains the long-standing "seismic deficit" observed at mid-ocean ridges — the puzzling scarcity of earthquakes given how quickly new crust is generated there.