A couple of months after a French-led research team anchored a set of underwater instruments across a stretch of seafloor, the crust below them cracked open and began sinking beneath a flood of fresh lava.
For the first time, researchers recorded a seafloor spreading event in real time, at the Southeast Indian Ridge near Amsterdam Island in the Southern Indian Ocean. The findings, from a team at Geo-Ocean, part of the CNRS and the University of Brest, were published in Nature.
The event began with a swarm of earthquakes that tore through the valley floor, spreading outward from the center of the ridge in both directions at 6.5 to 10 feet (two to three meters) per second.
Within hours, the valley floor had already dropped more than 3.3 feet (a meter). Over six days, it sank a total of 13.8 feet (4.2 meters).
In their paper, the authors write that they “were lucky enough to capture a notable seafloor spreading event” not long into their experiment.
New Ocean Floor Formed at the Southeast Indian Ridge
Mid-ocean ridges are the underwater seams where new ocean floor forms. Over geological time, crust created along this ridge system has formed about two-thirds of Earth's surface. But almost no one has directly measured one of these seams while it's actively spreading, because the ridges sit kilometers underwater in places research ships rarely reach.
To try anyway, the team deployed an observatory called OHA-GEODAMS across the ridge axis, using five underwater microphones, 15 seafloor beacons, and a single pressure sensor planted in the valley floor to track the smallest shifts in the crust below.
Read More: Rare Arctic Seafloor Footage Captures Narwhals, Backward-Swimming Fish, and Panicked Copepods
Lava Poured Out as the Valley Floor Sank
The team's instruments pointed to a pocket of molten rock, sitting roughly 2.2 miles (three and a half kilometers) beneath the ridge, that began to drain. That magma forced its way sideways into cracks called dikes, which spread along the ridge axis and eventually broke through to the seafloor.
Over about 16 days, an estimated 5.2 billion to 5.6 billion cubic feet (148 to 160 million cubic meters) of lava poured out, in places piling up more than 295 feet (90 meters) thick.
Nearby faults slipped too. Some of that movement showed up as standard earthquakes, but researchers calculated that roughly three-quarters of it produced no detectable shaking at all.
Quiet Fault Movement Released Decades of Strain
Mid-ocean ridges move steadily over time, but they do not produce as many earthquakes as that motion would seem to predict. Here, most of that gap came from quiet slips rather than sudden jolts, with faults absorbing years of built-up motion all at once.
In this single event, the ridge released the equivalent of roughly 39 years of ordinary plate motion, compressed into little more than two weeks. A nearby fault system ruptured within minutes of the dike's activity, including one strong, magnitude 5.9 quake, suggesting the eruption disturbed the surrounding seafloor for kilometers around.
The researchers note that their instruments only captured one ridge segment, one type of event. Other mid-ocean ridges spread with little or no magma involved and are dominated by earthquakes instead, and the team says those systems should be watched next.
This stretch of ridge had built up decades of motion, released within days, because a small research team happened to have instruments in place at the right time. The ocean floor grows a few centimeters a year, about the pace of a fingernail. But it doesn't grow at a steady crawl — it waits, strains, and then, every so often, releases that strain all at once.
Read More: An Underwater Volcanic Eruption Brewing in the Bismarck Sea May Cause a New Island to Rise
Article Sources
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- This article references information from a study published in Nature: Anatomy of a seafloor spreading event captured by in situ seismogeodesy
