Largest Underwater Eruption Ever Recorded Gives Birth to Massive New Volcano

A massive seismic event, felt around the globe in May 2018, has officially created an underwater volcano.
A new giant feature has been discovered off the east coast of Mayotte. It rises 820m (2,690ft) above the seafloor. This is a landmark that was not there before the earthquake that struck the island in May 2018.

The researchers stated in their paper that "this is the largest active submarine volcano ever documented."

Scientists are now able to understand deep Earth processes that we don't know much about, thanks to the new feature. It is thought to be part a tectonic structure located between the East African rift and Madagascar rifts.

On 10 May 2018, the seismic rumbles from the ongoing event began. A magnitude 5.8 earthquake struck nearby islands on May 15, 2018, just a few days after the seismic rumbles of the ongoing event. Initial scientists were confused, but they soon realized that this was a rare volcanic event.

The signals indicated a location approximately 50 km from the Eastern Coast of Mayotte. This is a French territory, part of the volcanic Comoros Archipelago and sandwiched between Africa's Eastern coast and Madagascar's Northern tip.

A number of French government institutions sent a team to investigate. They found an undersea mountain that had never been seen before.

The scientists, led by Nathalie Feuillet, a geophysicist at the University of Paris, have now published their findings in a paper.

In February 2019, the team began to monitor the area. They used a multibeam sonar to map an 8,600-square-kilometer area of seafloor. They also installed a network seismometers along the seafloor up to 3.5 km deep and combined it with Mayotte seismic data.

This network detected 17,000 earthquakes between 25 February 2019 and 6 May 2019. These events were located at a depth of approximately 20 to 50 km below the ocean floor. This is a rare finding as most earthquakes are shallower. Additional 84 events were also detected at extremely low frequencies, which is highly unusual.

These data allowed researchers to reconstruct the process of formation of the new volcano. According to the researchers, it began with a magma storage deep within the asthenosphere (molten mantle layer located below Earth's crust).

Chronology of the eruption. (Feuillet et al., Nature Geoscience, 2021)

Below the new volcano, tectonic events may have caused damage the lithosphere. This resulted in dykes which drained magma up through the crust, creating swarms earthquakes. This material eventually made its way to seafloor where it erupted, generating 5 km of lava and the new volcano.

Low-frequency events are most likely caused by a fluid-filled cavity in crust. This cavity could have been repeatedly stimulated by seismic strain from faults near the cavity.

The extruded volume for the new volcanic edifice was between 30 and 1000 times greater than that of other deep-sea volcanoes. It is the largest undersea volcanic eruption yet recorded.

Researchers wrote that the volume and flux of lava emitted during the Mayotte magmatic eruption were comparable to those seen during eruptions at Earth’s largest hotspots.

Future scenarios include a new caldera collapsing, submarine eruptions up the slope or onshore volcanic eruptions. This has happened in the past, as evidenced by large lava flows and cones along the slope and onshore Mayotte.

"Since the discovery and subsequent monitoring of activity at the volcano, an observatory was established. Return cruises continue to track the evolution of the eruptions and edifices."

Nature Geoscience published the research.