Cross-section of the volcanic structure beneath Mayotte. This three-dimensional representation provides a conceptual model of how the sections of the magma reservoir beneath the island might be connected.
© BRGM
Volcanic eruptions that spew large volumes of magma at the surface are still rare and poorly understood. They raise a fundamental question: what form do these tens of cubic kilometres of molten rock take before an eruption? Micro-pockets scattered in the earth's crust, a concentrated magma reservoir, a layer-cake of alternating liquid magma and more solid layers? The scientific community has conflicting explanations.
A magma reservoir which could be linked to the birth of the large undersea volcano Fani Maoré, which formed in 2018
To answer this question, a research team including BRGM, CNRS, the University of Orléans and the Université de Bretagne Occidentale, as well as companies specialising in geophysics, carried out electromagnetic observations in Mayotte over a two-year period. Using this method to image molten material, they identified an imposing volume of conductive rock at a depth of 23 kilometres, under the eastern part of the island. The measurements were taken on a very large scale and are consistent with the seismological observations made in recent years by the teams of the Mayotte volcanology and seismology network, REVOSIMA (Réseau de surveillance volcanologique et sismologique de Mayotte).
The researchers then analysed samples of recent, local magmatic rocks. They subjected them to electrical conductivity tests in the laboratory, at different temperatures and under pressures close to those observed at these depths. The results obtained are compatible with the hypothesis that there is a magma reservoir under Mayotte containing up to 42% of liquid.
A scientific breakthrough in global volcanological research, published in the journal Nature
This discovery marks a turning point, to the credit of the French scientific community, which had been investigating it since the spectacular underwater eruption of 2018-2019. In less than 10 months, a new underwater volcano had emerged 50 kilometres to the east of the island. Since then, scientists have been examining the underlying magma circuits. The discovery of a large magma storage zone, part of which is liquid, deep below Mayotte, has raised new questions: did it fuel the eruption of 2018-2019? Is it part of a network of connected reservoirs? Published in Nature, this study has focused world-wide volcanological research on the Mayotte region and opens up prospects for anticipating major eruptions.
Mayotte, located in a volcanic region
Located between the East African coast and northern Madagascar, the island of Mayotte is part of the Comoros archipelago, which is well known for its volcanic activity. Eruptions are thought to have begun around 32 million years ago, before expanding in successive phases. Today, there are several active volcanic complexes in the Comoros archipelago, as evidenced by the recent eruptions of Karthala on the island of Grande-Comore to the west and Fani Maoré 50 kilometres east of Mayotte. The latter was discovered in 2019 in the ocean depths of the Comoros archipelago by French teams from the IPGP, BRGM, CNRS and Ifremer, who were trying to understand the cause of the multiple earthquakes that had been affecting the island since May 2018. Today, seismo-volcanic activity in the volcanic zone of Mayotte is closely monitored by REVOSIMA, the Mayotte volcanological and Seismological Monitoring Network set up following the crisis.
Reference
Wawrzyniak, P., Gaillard, F., Hautot, S. et al. Magnetotelluric evidence for a melt-rich magmatic reservoir beneath Mayotte. Nature (2025).
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Reservoir C1 (in orange, between 20 and 30 km deep) is the major discovery of this study: this zone of over 200 km³ contains between 22 and 42% liquid magma. Above, the conductive C2 zone (smaller and closer to the surface) completes the system. The Fani Maoré undersea volcano formed to the east of the island during the 2018-2019 eruption, building up an undersea relief in less than ten months. Magnetotelluric data can be used to identify these conductive zones in the Earth's depths, which contain a proportion of liquid magma, thanks to their ability to conduct electric current, a signature of the presence of magma.