On November 11, 2018, the largest offshore volcanic event in recorded history occurred, with an origin point near the African island nation of Madagascar and the island of Mayotte, which is controlled by the French government. Despite its power, however, this event went largely unnoticed by the planet—perhaps because it was so remotely located beneath the ocean that no human was directly affected.
Now French scientists studying the eruption have come to the conclusion that the violence and strength of the eruption came from an entirely new volcano being born into existence.
Scientists have ventured out to what they believe is the epicentre of the violence, around 31 miles (50 km) east of Mayotte’s shores. There, various French agencies including the National Center for Scientific Research (CNRS) and the French Institute for Research for Exploitation of the Sea (IFREMER), worked on board the RV Marion Dufresne to investigate what had happened.
This past February, the French scientists began monitoring the area. They placed seismometers on the seafloor, over 2 miles (3.5 km) down, and used multibeam acoustic sensors to map the ocean floor. They pulled up rocks and examined them for any changes.
“They were popping as we brought them on board,” says Nathalie Feuillet, director of the Institute of Geophysics in Paris (IPGP) and leader of the expedition, in an interview with Science. That popping represented a clear sign that high-pressure gas was trapped within the black volcanic material.
The collective data, also using information pulled from Mayotte, reached one conclusion: There’s a new underwater volcano. And it’s really big—but the ocean is bigger.
According to a press release from the French government, the new volcano “is located over 3,500 meters (2.1 miles) deep. Its current size is estimated at 800 m in height (around half a mile) with a base of 4 to 5 km (2.4 miles to 3.1) in diameter. The plume of volcanic fluids 2 km (1.2 miles) in height does not reach the surface of the water.”
That there was no plume reaching anywhere near the surface could be a reason that the volcano went undetected.
The working theory is that there’s a deep magma chamber underneath the seafloor, sending molten rock upward and then contracting. That shift would cause the floor to creak and crack.
The deep, sinking magma chamber theory is supported by another piece of evidence: Mayotte itself, which according to GPS data has sunk by 5.1 inches (13 centimetres) and moved 3.9 inches (10 centimetres) east over the past year. Just that slight movement could indicate a magma chamber sinking. The island itself has experienced a string of earthquakes since last year.
It’s a theory that makes sense, but scientists aren’t willing to definitely say so yet. There are alternate ideas, like an unexpected tectonic shift. Visions of both birth and destruction can be read into the volcanic activity; it’s theorized that islands like Mayotte and Hawaii formed through similar patterns, but some also think that westward migration of the small earthquakes could trigger a tsunami.
For now, Feuillet and her team want to continue their study before reaching any conclusions. They hope to extend their time on board the Marion Dufrense by several months.