Most of our planet’s crust is forged in a thalassic factory human eyes never see.
Across 65,000 kilometers (40,400 miles) of the ocean floor, a system of mid-ocean ridges marks the boundaries where Earth’s tectonic plates meet.
There, in ocean depths far from the light of the Sun, cataclysmic upheavals rend the ocean floor asunder, flooding the open wound with magma that then cools and hardens to form a brand new section of planetary crust.
More than two-thirds of Earth’s skin was born in this way – and now, for the first time, scientists have finally managed to record the process in action.
“We did not dream of capturing such a massive event, and were hoping to at least measure the steady stretching of the ridge (maybe a few centimeters) that allows stresses to build up between events, like a loaded spring,” marine geophysicist Jean-Yves Royer of the French National Center of Scientific Research told ScienceAlert.
“Instead, we were treated to a once-in-forty-year event and measured several meters of displacements in both directions!!”
The extraordinary observations are described in a new paper published in Nature.
Because tectonic boundaries are lower than the relatively lighter, more buoyant interiors of the tectonic plates, the vast majority lie under the water that covers most of Earth’s surface at depths profoundly inhospitable to human exploration.
Human ingenuity, however, should not be underestimated.
In late February 2024, Royer and his colleagues deployed the Observatory with Hydro-Acoustics and Geodesy near Amsterdam Island (OHA-GEODAMS) experiment at the Southeast Indian Ridge between Australia and Antarctica.
They had spent years building an underwater observatory designed to catch one of the rare, violent episodes in which new seafloor is born.
Their goal was audacious.
Seafloor spreading is not a slow, gradual creep. Instead, tectonic tension builds over decades before being unleashed in rare, violent bursts – discrete episodes the researchers call “quantum” events.
The team built an observatory consisting of five autonomous hydrophones positioned to monitor the entire Saint Paul-Amsterdam volcanic plateau, hoping to catch one of these cataclysmic episodes.
They weren’t entirely sure what to expect – but then, just months later in April 2024, the seafloor split open.
“‘Fortune favors the bold,’ as the saying goes,” Royer said.
“What happened in those 16 days at the end of April 2024 was that the ridge axis failed and let the accumulated magma underneath propagate into the oceanic crust.”
Vast sheet-like intrusions of magma, known as dikes, tore through the crust in less than two hours, injecting an estimated 150 million cubic meters (5.3 billion cubic feet) of magma into the crust.
As the dikes spread, they triggered earthquakes, reawakened long-dormant faults – fractures where blocks of Earth’s crust can suddenly slip past one another – and drained the magma reservoir beneath the ridge.
“As a result, the seafloor above it collapsed very rapidly,” Royer said.
Eventually the dikes reached the seafloor itself, allowing lava to erupt onto the ocean floor while continuing to drain the magma reservoir, causing the seafloor to collapse further.
“Overall, the floor of the valley that marks the ridge axis collapsed by 4.2 meters (13.8 feet), slipping on its bordering faults,” Royer said. “This is the first time a quantum event of seafloor spreading involving both diking and faulting is observed by the hour.”
Managing to capture such a rare event mere months after deploying the technology was only the start of the surprises.
“We did not dream of capturing such a massive event.” – marine geophysicist Jean-Yves Royer
Seafloor spreading at mid-ocean ridges is estimated to occur at a rate of about 6.3 centimeters (2.5 inches) per year, if the movement is continuous.
This eruption was the perfect demonstration that seafloor spreading is not continuous, but occurs in giant lurches.
At its peak, the ridge was pulling apart at 5 centimeters every minute – nearly half a million times faster than its long-term average.
“The measured horizontal displacements (2 to 4 meters) are equivalent to 30 to 60 years of continuous spreading at 6.3 centimeters per year, which gives an idea of the recurrence of these quantum events,” Royer explained.
And the researchers were finally able to resolve a long-standing mystery.
Scientists have long known how quickly tectonic plates move apart. But, in previous work, when they added up all the movement produced by recorded earthquakes, the numbers never matched. There simply wasn’t enough earthquake activity to account for the observed rate of seafloor spreading.
Royer and his colleagues found the missing motion. Although earthquakes accompanied the event, their models showed the faults actually slipped far more than the earthquakes alone suggested.
Related: Scientists Discover 73 Volcanic Calderas Hidden Across The Ocean Floor
Most of the event’s movement occurred aseismically – silently, without generating strong seismic waves.
This means these observations provide a ground truth against which seismic data can now be tested, helping scientists better understand how the ocean floor spreads even when they aren’t fortunate enough to witness it directly.
“We have shown that, with a bit of luck, but also a bit of flair, measuring seafloor spreading is now possible,” Royer said.
“So it opens new horizons for marine geophysicists.”
The findings have been published in Nature.
This article was fact-checked by Michael Irving and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
