Largest ever underwater eruption spawns massive new volcano

A huge seismic event that began in May 2018 and was felt around the world officially spawned a new underwater volcano.

Off the east coast of the island of Mayotte, a gigantic new feature rises 820 meters (2,690 feet) from the seabed, a prominence that was not there before the earthquake that rocked the island in May 2018.

“It is the largest active underwater eruption ever documented”, the researchers wrote in their article.

The new feature, believed to be part of a tectonic structure between the rifts in East Africa and Madagascar, helps scientists understand deep terrestrial processes about which we know relatively little.

Seismic rumblings from the current event began on May 10, 2018. A few days later, on May 15, a magnitude 5.8 earthquake struck, shaking the neighboring island. At first, scientists were puzzled; but it didn’t take long to realize that a volcanic event had occurred, like never before.

The signals pointed to a place about 50 kilometers from the east coast of Mayotte, French territory and part of volcanism Comoros Archipelago sandwiched between the eastern coast of Africa and the northern tip of Madagascar.

Thus, several French government institutions sent a research team to verify it; there, of course, was an underwater mountain that had never existed before.

Led by geophysicist Nathalie Feuillet of the University of Paris in France, the scientists have now described their findings in a new article.

The team began monitoring the area in February 2019. They used multibeam sonar to map an area of ​​the seabed of 8,600 square kilometers. They also placed an array of seismometers on the seabed, up to 3.5 kilometers deep, and combined it with seismic data from Mayotte.

Between February 25 and May 6, 2019, this network detected 17,000 seismic events, at a depth of about 20 to 50 kilometers below the ocean floor – a very unusual find, as most earthquakes are much shallower. 84 additional events were also very unusual, detected at very low frequencies.

Armed with this data, the researchers were able to reconstruct how the formation of the new volcano could have occurred. It started, according to their findings, with a reservoir of magma deep within the asthenosphere, the molten mantle layer located directly below the Earth’s lithosphere.

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

Under the new volcano, tectonic processes may have caused damage to the lithosphere, resulting in the formation of dykes that drained magma from a reservoir through the crust, producing earthquake swarms in the process. Eventually, this material reached the seabed, where it erupted, producing 5 cubic kilometers of lava and building the new volcano.

The low-frequency events were likely generated by a shallower, fluid-filled cavity in the crust that could have been repeatedly excited by seismic stress on faults near the cavity.

As of May 2019, the extruded volume of the new volcanic edifice was between 30 and 1,000 times greater than that estimated for other deep-water eruptions, making it the largest underwater volcanic eruption on record.

“The volumes and flows of lava emitted during the Mayotte magmatic event are comparable to those observed during eruptions on the largest hot spots on Earth”, the researchers wrote.

“Future scenarios could include another caldera collapse, underwater eruptions on the Upper Slope, or eruptions on land. Large lava flows and cones on the upper slope and on the coast of Mayotte indicate that this has happened in the past.

“Since the discovery of the new volcanic edifice, an observatory has been set up to monitor activity in real time, and return cruises continue to follow the progress of the eruption and the buildings.”

The research was published in Geosciences of nature.

About Lucille Thompson

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