A new submarine volcano is born in East Africa

The largest ever-recorded underwater volcanic eruption began on May 10, 2018 off the eastern coast of Mayotte, a group of islands off the coast of Mozambique and north of Madagascar. At that time, a very viscous and ductile volume of molten rock from the Earth’s upper mantle pierced the cooler, more fragile lithosphere layer above and shattered lava on the seabed.

That day, the people of Mayotte felt a large earthquake and a magnitude 5.8 event struck on May 15. During the following weeks, the magmatic dynamics generated a few very low frequency earthquakes and thousands of deep ones. The result of all this geophysical activity has been a new mountain on the seabed. (For more on other submarine volcanoes, see Physics today, August 2012, page 16.)

The French geological service called Nathalie Feuillet from the University of Paris and other experts to study the seismic activity. As chief scientist on a research cruise in May 2019, Feuillet led an effort to collect seismic and surface deformation data from the volcanic eruption. They discovered that the new submarine volcano is now 820 meters high and sits at the end of a series of recent lava flows that form a 50 km ridge. The geological feature is likely part of an extending tectonic structure formed by cracks and faults associated with the East African Rift to the west.

During the 2019 cruise, Feuillet and the ship’s other researchers used an echo sounder to bounce sound waves off the ocean floor over an area of ​​8,600 km2, roughly the size of Puerto Rico. These data, when fed into a digital terrain model, revealed the volcanic ridge of Mayotte, shown in the right image below. (The left image of the ocean floor, as mapped in 2014, indicates a relatively flat seabed topography.)

Credit: N. Feuillet et al., Nat. Geosci., 2021, doi: 10.1038 / s41561-021-00809-x

Find out more about the new volcano, which vomited about 5 km3 lava, co-authors Claudio Satriano, Angèle Laurent and Pascal Bernard analyzed seismic wave data collected by a network of seismometers on land and on the seabed, most of which were installed or deployed after the 2018 eruption.

The network detected some 17,000 events from February 25 to May 6, 2019, 94% of which focus on the western segment of the Mayotte Ridge and 25-50 km below the surface. 84 additional earthquakes were identified and classified as very low frequency events, with periods of 15 seconds. The very low frequency earthquakes were likely generated by a seismic source that was repeatedly excited, possibly faults destabilized by magma in a large, deep reservoir.

It is unusual to have so many earthquakes inside the Earth: many seismic events caused by volcanic activity occur above the Moho, the limit between the crust and the mantle, about 17 km away. under the surface of Mayotte. Researchers interpret deep earthquakes as evidence of geodynamic activity in the upper mantle and lower crust. Magma from the upper mantle probably filled a reservoir and pressurized the region. As the magma passed through the crust and eventually the seabed, it is said to have caused swarms of deep earthquakes. (N. Feuillet et al., Nat. Geosci., 2021, doi: 10.1038 / s41561-021-00809-x.)

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