On the q.t.: in 2018, a giant underwater volcano formed offshore the Comoros archipelago between Africa and Madagascar

Underwater volcanoes’ activities usually do not show up on the surface of the oceans. This is a photo taken about 4 months after an eruption off the Spanish island of El Hierro in 2011. From the satellite the discoloration of the water is clearly visible. Photo: NASA / <a href="https://commons.wikimedia.org/"target="_blank">Wikimedia Commons</a>

Scientists analyzed the emptying of a magma chamber of 3.4 cubic kilometers in about 30 km depth in the Earth mantle

Since May 2018, an unusual sequence of earthquakes had been recorded off-coast the island of Mayotte in the Comoros archipelago between Africa and Madagascar. Seismic activity began with a swarm of thousands of ,seemingly tectonic’ earthquakes, culminating in an earthquake of magnitude 5.9 in May 2018. Mostly since June 2018, however, a completely new form of earthquake signal had emerged that was so strong that it could be recorded up to a thousand kilometres away.

These 20 to 30 minute long signals were characterized by particularly harmonic, low frequencies, almost monochromatic, similar to a large bell or a double bass, and are called Very Long Period (VLP) signals. Although the centre of the seismic activity was located almost 35 kilometres offshore the east of the island, a continuous lowering and eastward motion of the earth’s surface at Mayotte had begun at the same time as the massive swarms of VLP events started, accumulating to almost 20 centimetres to date.

Although there was no evidence of earlier volcanic activity in the epicentre of the seismic activity, an international team of scientists had suspected magmatic processes from the beginning, as quake swarms in the upper earth crust often arise as a reaction to the rise of magma and VLPs in earlier years were associated with the collapse of large caldera volcanoes. The special frequency content of the VLP signals is caused by the resonance oscillation of the buried magma chamber. The deeper the vibrations, the larger the magma reservoir. However, the earthquake swarms under the ocean floor were much deeper than with other volcanoes and the resonance tones of the VLPs were unusually low and strong.

Sketch of the deep magma reservoir and magma upward path to form the new submarine volcano. Different types of seismic and surface deformation accompanied the 4 four indentitied phases of the unrest. Source: Cesca et al. 2020, Nature Geoscience.

The team identified different activity phases within the sequence of events from May 2018 to today. The initial swarm phase indicated a rapid upward movement of magma from a deep mantle reservoir more than 30 kilometres below the Earth’s surface. Once an open channel had formed from the Earth’s mantle to the seabed, the magma began to flow unhindered and form a new underwater volcano.

A French oceanographic campaign recently confirmed the formation of the submarine volcano, whose location coincides with the reconstructed magma rise.

In this phase, the apparent tectonic earthquake activity decreased again, while the lowering of the ground on the island of Mayotte began. Likewise, long-lasting monofrequency VLP signals started.

It was the deepest, about 30 kilometre, and largest magma reservoir in the upper mantle – more than 3.4 cubic kilometres – to date, which emptied abruptly.

„Since the seabed lies 3000 meters below the water surface, almost nobody noticed the enormous eruption. However, there are still possible hazards for the island of Mayotte today, as the Earth’s crust above the deep reservoir could continue to collapse, triggering stronger earthquakes,“ says Torsten Dahm, Head of the section Physics of Earthquakes and Volcanoes at the GFZ Potsdam who was member of the team.

Original study: Cesca, S., Letort, J., Razafindrakoto, H.N.T., Heimann, S., Rivalta, E., Isken, M.P., Nikkhoo, M., Passarelli, L., Petersen, G.M., Cotton, F., Dahm, T., 2020. Drainage of a deep magma reservoir near Mayotte inferred from seismicity and deformation. Nature Geoscience. https://doi.org/10.1038/s41561-019-0505-5

Source: Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ.

(11.02.2020, USA: 02.11.2020)