When tectonic plates move away from each other, lava can to spill out onto the sea floor along mid-oceanic ridges.Credit: Doug Perrine/Nature Picture Library/SPL
Geophysicists have, for the first time, caught the ocean floor in the act of spreading apart at one of its seams. Using an array of more than 20 measuring stations laid across an 100-kilometre-long region of the Indian Ocean, they witnessed an event that released around 160 million cubic metres of lava onto the sea floor and shifted two sections of the oceanic crust apart by at least 2 metres in a matter of days.
The scale of the event, which was described today in Nature1, was “a major surprise”, according to study co-author Jean-Yves Royer, a marine geophysicist at the French National Centre for Scientific Research (CNRS) in Brest.
The oceanic crust covers nearly two-thirds of the planet, and undersea mid-oceanic ridges are responsible for its creation. As the existing crust is pulled away from the ridge by the movement of tectonic plates, new crust is produced by magma that wells up from Earth’s core and solidifies. The process has been broadly understood since the mid-twentieth century, but it had never been observed in real time.
Despite the fundamental role of mid-ocean ridges in shaping Earth’s surface, “we still know remarkably little about the frequency, magnitude and dynamics of the eruptions and tectonic processes that build them”, says Isobel Yeo, a geoscientist at the National Oceanography Centre in Southampton, UK.
Stress release
Royer and his colleagues focused on the Southeast Indian Ridge, which cuts across the Indian Ocean’s floor in a roughly east-to-west direction. The ridge separates the Antarctic plate from the Australian plate (which contain Antarctica and Oceania as well as the oceanic crust that surrounds them).
Overall, the two plates are moving away from each other by around six centimetres per year, mostly because the Australian plate is being pulled northwards. But some sections of the Australian plate might stay stationary for a while and then undergo a large burst of motion, accompanied by earthquakes. Hoping to catch one such event, in February 2024, the team placed three kinds of instrument at various locations around an 100-kilometre-long segment of the ridge.
In particular, they deployed five hydrophones, which are underwater microphones that can detect sound waves, including those produced by earthquakes. The team also set up 15 acoustic beacons, which are battery-powered stations on stilts that can both emit and detect undersea sounds. Every four hours, the beacons exchanges sound signals and measures the time it takes for the response to come back.
