For decades, geophysicists have known that most of Earth's surface is made along mid-ocean ridges, where tectonic plates pull apart and rising magma cools into fresh crust. What they could never do was watch it happen. That has now changed.
In late February 2024, a team led by marine geophysicist Jean-Yves Royer of France's National Centre for Scientific Research lowered an autonomous observatory onto the Southeast Indian Ridge, the boundary between the Australian and Antarctic plates near Amsterdam Island. The instrument suite — five hydrophones, an array of acoustic-ranging beacons, a bottom-pressure recorder and repeated seafloor mapping — was built to catch one of the rare, violent episodes in which new seafloor is born. The researchers hoped, at most, to measure a few centimetres of steady stretching.
Two months later, on 26 April 2024, the ridge obliged. A rapidly migrating swarm of small earthquakes rippled along the axial valley as sheet-like intrusions of magma, called dykes, tore through the crust in under two hours. Over the following 16 days, roughly 160 million cubic metres of lava poured onto the seafloor. The floor of the valley collapsed by about 4.2 metres as a magma reservoir some 2.5 kilometres wide and 3.6 kilometres deep drained beneath it, and the two sides of the ridge pulled apart by several metres.
The pace was startling. At its peak the seafloor moved five centimetres a minute — nearly half a million times faster than the ridge's long-term average of about 6.3 centimetres a year — before easing to a little over a centimetre a day. The few metres of horizontal motion measured in days represent 30 to 60 years of ordinary spreading, confirming that the ocean floor grows not in a smooth creep but in sudden lurches the team calls "quantum" events.
The measurements also settled a long-standing puzzle. Earthquakes alone had never accounted for how fast ridges spread; here, the faults slipped far more quietly, or aseismically, than their tremors implied. That gives scientists a rare ground truth for interpreting seismic records elsewhere.
Writing in an accompanying commentary in Nature, Ingo Grevemeyer and Lars Ruepke noted that such detailed surveys of the deep seafloor can now be carried out "as have been achieved on land." For Royer, the result "opens new horizons for marine geophysicists".