The largest submarine volcanic eruption ever recorded began on 10 May 2018 off the eastern shore of Mayotte, one of the Comoros group of islands east of Mozambique and north of Madagascar. Then and there, a highly viscous and ductile volume of molten rock from the asthenosphere—Earth’s upper mantle—pierced the cooler and brittler lithosphere above it and erupted lava onto the seafloor.

That day, people in Mayotte felt a magnitude 4.3 earthquake, the first of many. The largest event, of magnitude 5.9, struck several days later, on 15 May. Over the next few weeks, the moving magma generated a few very low frequency earthquakes in the crust and thousands of deeper ones.1 The result of all that geophysical activity was a new mountain on the seafloor. (To learn about other submarine volcanoes, see Physics Today, August 2012, page 16.)

As the chief scientist of the May 2019 research cruise MAYOBS 1, Nathalie Feuillet of the Paris Institute of Earth Physics (IPGP) and the University of Paris led an effort to collect seismic and surface deformation data of the volcanic eruption and the ongoing earthquakes. (Mayotte is an overseas department of France.) She and her colleagues—from institutions including the French geological survey BRGM, the CNRS, and the French Research Institute for Exploitation of the Sea—found that the new undersea volcano now stands 820 m tall and lies at the end of a 50 km ridge formed by a series of recent lava flows.2 The geological feature is likely part of a tectonic structure formed by fissures and faults associated with the East African Rift to the west.

The occurrence of the 2018 eruption was unusual. No volcanic activity had ever been reported in the area before, and over the past 30 years geologists had cataloged just two small earthquakes near Mayotte, according to an earthquake database maintained by the US Geological Survey. The BRGM recorded the initial seismic activity associated with the 2018 eruption using a single seismic station on Mayotte. With their sparse observations, the BRGM suspected the earthquakes originated somewhere in the ocean east of Mayotte but couldn’t pinpoint the exact source of the tremors.

Shortly thereafter, Feuillet and other volcanologists and seismologists made plans to study the region more closely. Among other activities, they installed ocean-bottom seismometers (OBSs), instruments capable of picking up undersea earthquake activity. (For more on those devices, see “Deploying seismometers where they’re needed most: Underwater,” Physics Today online, 24 May 2019.) By the end of 2018, the project was finalized, and Feuillet and some colleagues traveled to Mayotte in February 2019 to deploy the OBSs.

The discovery of the volcano came in May 2019 when the MAYOBS 1 research cruise recovered the OBSs deployed in February; one of the devices is shown in figure 1. The researchers on the ship used a multibeam echo sounder to bounce sound waves off the ocean floor across an area of 8600 km2, slightly smaller than the size of Puerto Rico, to determine the seafloor elevation. An instrument deployed to more than 3000 m below the ocean surface looked for some trace of volcanic activity by determining the seawater’s conductivity, temperature, and chemical composition as a function of depth. Absolute pressure gauges attached to the OBSs measured the vertical deformation of the seafloor.

Figure 1.

Jérémy Gomez of the Paris Institute of Earth Physics (IPGP) helps retrieve an ocean-bottom seismometer that was deployed off the eastern coast of Mayotte, an island north of Madagascar. He and several other scientists participated in a May 2019 research cruise to collect geochemical, seismological, and seafloor topographic data after the region began experiencing volcanism and earthquakes in May 2018. (Courtesy of Eric Jacques, IPGP.)

Figure 1.

Jérémy Gomez of the Paris Institute of Earth Physics (IPGP) helps retrieve an ocean-bottom seismometer that was deployed off the eastern coast of Mayotte, an island north of Madagascar. He and several other scientists participated in a May 2019 research cruise to collect geochemical, seismological, and seafloor topographic data after the region began experiencing volcanism and earthquakes in May 2018. (Courtesy of Eric Jacques, IPGP.)

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“One evening we saw a big anomaly on a polar echogram of the water column,” says Feuillet, recalling the cruise. “It was a 2000-meter-high acoustic plume.” East of Mayotte, the source of the anomaly turned out to be a mixture of solid particles, liquid droplets, and bubbles. The jet of materials had the telltale characteristics of volcanism: highly turbid, alkaline water and elevated concentrations of molecular hydrogen, methane, and carbon dioxide. “It was one of the biggest acoustic plumes ever detected in the water column,” says Feuillet. The volcano, which had formed 10 months earlier, was erupting again.

A fuller picture of the volcano and its surroundings emerged when the seafloor topography data revealed the Mayotte volcanic ridge. A previous research cruise, led by the French Naval Hydrographic and Oceanographic Service, had fortuitously mapped the same stretch of seafloor in 2014. Figure 2a shows what was once the relatively flat seafloor topography; figure 2b, the new undersea mountain and ridge.

Figure 2.

A new volcano formed east of the East African Rift off the eastern shore of Mayotte after lava erupted through the seafloor in mid 2018. (a) This elevation map from measurements collected during a 2014 oceanographic research cruise, shows a relatively flat seafloor topography. (b) More recent data from a 2019 cruise and seismic stations (black triangles in inset) reveal the formation of an undersea volcanic ridge. (Adapted from ref. 2; inset from ref. 3.)

Figure 2.

A new volcano formed east of the East African Rift off the eastern shore of Mayotte after lava erupted through the seafloor in mid 2018. (a) This elevation map from measurements collected during a 2014 oceanographic research cruise, shows a relatively flat seafloor topography. (b) More recent data from a 2019 cruise and seismic stations (black triangles in inset) reveal the formation of an undersea volcanic ridge. (Adapted from ref. 2; inset from ref. 3.)

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To learn more about the new volcano, which spewed about 5 km3 of lava during the 2018 eruption, coauthors Wayne Crawford and Jean-Marie Saurel of IPGP and the University of Paris and other seismologists on the team analyzed the seismic-wave data collected by a network of seismometers on land and on the seafloor; most of them were installed or deployed after the 2018 eruption.

From 25 February to 6 May 2019, the network detected some 17 000 earthquakes, 94% of which cluster on the western segment of the Mayotte ridge and 25–50 km below the seafloor. An additional 84 earthquakes were identified by IPGP and University of Paris coauthors Claudio Satriano, Angèle Laurent, and Pascal Bernard as very low frequency events lasting up to about 30 minutes, with seismic-wave energy detected below 0.10 Hz. The very low frequency earthquakes can be generated by a seismic source that’s been repeatedly excited, possibly faults destabilized by magma from the upper mantle that pressurize a large deep reservoir.

To have so many earthquakes deep in Earth’s interior is rare. Many seismic events caused by volcanic activity occur in the brittler crust rather than the deeper, more ductile mantle. At Mayotte, the crust descends about 17 km below the surface. Below that brittle–ductile transition zone, molten rock more easily deforms and is therefore less likely to crack and instigate tremors in response to seismic energy.

Still, Feuillet and her colleagues suspect that magma activity may have caused the deep earthquakes that were observed below Mayotte. Most of them were clustered beneath a caldera structure, a large, low topographical region that formed when an ancient volcano erupted and collapsed. The many faults and fissures of the caldera form channels through which the magma could have easily navigated from a deep upper-mantle reservoir to the seafloor.

The researchers infer that the Comoros archipelago, with the African continent to the west, is part of a tectonic zone where the crust is pulling and sliding apart. Some earthquakes could be the result of the transfer of tectonic deformation from the East African Rift to an area of Madagascar with rifting in the crust.

As the region stretches, the lithosphere is susceptible to fracturing, which provides more pathways for the magma to reach the surface. Once the magma travels through the weakened crust and reaches the seafloor, it can instigate swarms of deep earthquakes. That interpretation is supported by another recently published paper by Océane Foix, Feuillet, and their colleagues. They used a tomographic method to construct a more detailed picture of the new volcano’s plumbing.3 

The new volcano is 50 km east of Mayotte. Feuillet and her colleagues suspect that the main magma reservoir is 5–10 km east of the island and about 70 km below the seafloor. Another eruption from the reservoir, if it’s closer to the island, could be more dangerous than the last one. The next goal is to develop a warning system that would alert everyone in the region, especially Mayotte’s 270 000 residents, of a future eruption as early as possible.

On land, GPS instruments and seismometers are collecting real-time data. But a permanent underwater observatory closer to the source would provide better-quality measurements. Newly funded instrumentation includes submarine pressure gauges to more closely monitor how the seafloor deforms in response to subsurface magma activity.

Feuillet and other colleagues have organized several cruises to Mayotte to collect data and monitor the ongoing seismic and volcanic activity. Under a new research framework named the Mayotte Volcanological and Seismological Monitoring Network, scientists are keeping the residents and leaders of the islands informed of the evolution of the situation through monthly bulletins, daily reports, and a Facebook page.

A January 2021 cruise found evidence of new lava flows, but when Feuillet and her team returned in May, that flow had stopped. They’ve since measured some seismic activity and surface deformation, although at a much lower rate. Feuillet says, “We are still monitoring this area to better understand if the eruption is continuing or not at the site of the new volcano.”

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