Massive Earthquake Swarms Signalled the Birth of a Volcano as Huge Magma Chamber Empties Onto Seafloor

The events leading up to the birth of a new volcano have been recorded by scientists, with researchers showing how around 7,000 earthquakes were produced in the lead up to an eruption, where a huge magma chamber was emptied out onto the seafloor.

Researchers led by Simone Cesca, from the GFZ German Research Centre for Geosciences, started investigating unusual seismic activity off the coast of the French island of Mayotte, in the western Indian Ocean. Starting in May, 2018, what was once a "seismically quiet area" had started experiencing earthquake swarms.

Over the course of a year, scientists tracked events taking place in the area. Their findings, published in Nature Geoscience, provide a "detailed picture of a deep, rare magmatic process" that ended with a new volcano being born.

"We [soon] recognized in late May 2018 a seismic swarm in a region where seismicity has been moderate in previous years," Cesca told Newsweek. "The largest event, with magnitude Mw 5.9, was [the] largest ever recorded in the region." He said the characteristics of the swarm suggested magma was the underlying cause. "Later observations, such as long period seismic signals, which are typically observed at volcanoes, confirmed our [suspicions]."

These new forms of earthquake signals were so strong they could be detected from thousands of miles away. They would last for between 20 and 30 minutes and were characterized by low frequencies. Similar signals had previously been associated with large caldera volcanoes.

Using GPS systems, the team also found that there was surface deflation recorded on Mayotte.

Researchers believe what they were recording was the movement of magma deep beneath the Earth's surface.

magma path
Diagram showing the path of the magma that led to the new volcano. Cesca et al. 2019, Nature Geoscience

The initial earthquake swarms are thought to relate to the rapid upward movement of magma from a deeper reservoir, about 18 miles below the surface. After the channel had opened up, the magma flowed up to form a new volcano. "It is the deepest and largest magma reservoir in the upper mantle (more than 3.4 cubic kilometres) to date, which is beginning to empty abruptly," study co-author Eleonora Rivalta, also from GFZ, said in a statement.

Cesca added: "The magma started to propagate from a deep chamber upward, first intruding in the crust, and producing earthquakes, but not yet an eruption. When the magma reaches the seafloor, then it can flow more easily and the chamber drainage is faster. This produced a stronger surface deformation and, for example, subsidence at Mayotte island." He said that while there is some seismic activity there still, it appears the magma outflow is ending.

It is not clear how often new volcanoes form. Cesca said that on geological timescales it is not that rare, but on a human scale it is. The event studied, he added, is the largest submarine eruption ever monitored.

The team now hopes to look for similar, undiscovered processes that are taking place in remote regions of Earth, while continuing to monitor earthquake and volcanic processes across the planet.

The underwater volcano was about 30 miles from the coast of Mayotte, in the western Indian Ocean. iStock