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Here’s what you’ll learn when you read this story:
Most of Earth’s crust lies beneath its oceans, so it’s no surprise that most of the destruction and renewal of the crust also happens in the depths.
Researchers deployed a geodetic observatory to monitor an area known for its hot temper—and became the first to observe this phenomenon in action.
Because these eruptions of magma involve little to no seismic activity, the new observation also explains the absence of earthquake data in previous studies.
Most of Earth’s surface lies beneath the ocean. And while endless blue stretches of waves may seem peaceful aboard a cruise or from a beach chair on the shore, there’s unseen turmoil going on in the deep. Tectonic boundaries tense up. Submerged crust cracks, then gapes. Magma gushes in and cools down until it becomes new crust. The planet heals and keeps renewing itself.
Recently, when marine geophysicist Jean-Yves Royer of the French National Center for Scientific Research (CNRS) set off for the Southeast Indian Ridge between Antarctica and Australia, he released an underwater observatory capable of capturing this phenomenon in action. Royer and his research team had developed the Observatory with Hydro-Acoustics and Geodesy near Amsterdam Island (OHA-GEODAMS) experiment specifically for this purpose, but when the time came to lower the tripod and transponder to the bottom of the ocean, they weren’t sure whether they would succeed.
“Over hundreds of thousands of years, this interplay of magmatic and tectonic deformation shapes abyssal hills, which cover most of the ocean floor,” the researchers said in a study recently published in Nature. “Little is known, however, on how these processes unfold on short timescales. Seafloor spreading is thought to proceed as a succession of ‘quantum’ events of extension, involving earthquakes and [magma] intrusions.”
By “quantum,” Royer meant that these events are separate and distinct from each other. They are also not frequent occurrences—tension between tectonic plates can accumulate for decades before an outburst that tears open the crust, and it was uncertain whether that would happen in the Saint Paul-Amsterdam volcanic plateau where the geodetic observatory was lowered.
The observatory’s five autonomous hydrophones were sensitive enough to observe the entire plateau, however. Mid-ocean ridges (MOR)—which span 40,400 miles (65,000 km) around Earth—rise up from divergent plate boundaries where tectonic plates keep shifting. There, magma that will later harden into basalt erupts from cracks in the seafloor as the two sides pull away from each other. Cataclysmic events can happen if the axis of a ridge fails and magma invades.
The researchers were focused on the mid-ocean ridge known as the East Pacific Rise, which is rapidly spreading in the southeastern Pacific basin. Lower and heavier than tectonic plates, divergent boundaries are often found at extreme depths, which OHA-GEODAMS was built to survive. The team lowered the observatory and began to wait.
And then, it happened. Near the end of April 2024, OHA-GEODAMS detected enormous sheets of magma oozing out from below, filling the cracks they created with about 5.3 billion cubic feet (150 million cubic meters) of molten rock. And following this initial surge, the subterranean activity only continued.
As underground magma sheets spread, they typically trigger earthquakes—events that occur once or twice a year on the Saint Paul-Amsterdam plateau. Sometimes, however, these seismic signals are faint or entirely absent. This is because “aseismic” (non-earthquake) movements often occur along mid-ocean ridge (MOR) faults, which explains why previous studies of the region lacked sufficient seismic data. Instead of shaking the ground, these quiet events are driven by slow fault slips and magma forcing its way into the cracking crust. But once the magma sheet observed by Royer’s team slowed and stopped spreading, it finally gave way to detectable seismic activity (primarily strike-slip earthquakes, which involve horizontal movements of the crust).
These earthquakes were triggered by the massive movement of magma, which had drained the subsurface reservoir and caused the seafloor above to collapse. This sudden collapse destabilized nearby fault lines that had lain dormant for years, reawakening them and causing the fractured rock to grind together once more. What Royer and his team were witnessing was far from a typical episode of seafloor rifting, even though the crust was tearing apart at an astonishing five centimeters per minute, which is roughly half a million times faster than normal. Instead, they were observing a rare, coupled tectonic-magmatic event driven by both fault slippage and massive magma intrusion—a phenomenon never before documented in this way.
For Royer, this discovery provides a crucial piece of a long-standing geological puzzle, and helps explain how mid-ocean ridges release decades of silently building tension in sudden, discrete bursts. “Spreading segments undergo decades of quiescent stretching as abutting transforms accrue displacements by frequent slip events,” Royer said. “The stresses that build up in the MOR segment are released during quantum events of seafloor spreading that involve both [magma intrusion] and fault slip. This may be the primary mechanism by which MOR normal faults accumulate displacement, potentially explaining their seismic moment deficit.”
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