Hidden magma body beneath Mayotte revealed by electromagnetic imaging

Researchers detected an immense electrically conductive body within Mayotte’s crust, centered at about 23 ±1 km (14 ±0.6 miles) below sea level. The structure spans more than 200 km³ (48 miles³), comparable to the entire volume of Mount Fuji.

Magnetotelluric imaging, which measures variations in the Earth’s natural electromagnetic fields, revealed conductivity values far higher than those of solid rock, pointing to the presence of molten material.

Laboratory measurements on Mayotte’s magma were used to link conductivity with melt content. The results indicate that 22–42% of the reservoir is molten — an exceptionally high proportion for crustal systems. Most reservoirs contain less than 10% melt and behave as semi-solid mushes. This discovery shows that Mayotte’s deep crust remains active, capable of storing and supplying eruptible magma.

Such a high melt fraction challenges assumptions about the stability of oceanic crust. It suggests that beneath Mayotte, magma is not locked in a rigid framework but flows through a partially liquid network.

This dynamic state means the reservoir can feed major eruptions when pressure and pathways align, making Mayotte one of the few places on Earth where a deep, melt-rich body has been directly imaged and experimentally confirmed.

Tracing the link to the Fani Maoré eruption

The deep reservoir likely connects to the vent that produced the Fani Maoré submarine eruption (2018–2021). Located about 35 km (22 miles) east of Mayotte, the eruption released roughly 6.5 km³ (1.6 miles³) of lava — one of the largest submarine events ever recorded. Its sudden onset, accompanied by intense seismic swarms and ground subsidence of several centimetres, surprised scientists and residents alike.

Bathymetric surveys conducted by French research vessels revealed a new volcanic cone rising 800 m (2 625 feet) above the seafloor. The magnetotelluric data indicate that the deep melt reservoir beneath Mayotte served as the eruption’s source. Seismic and deformation data further suggest a vertical conduit connecting the reservoir to the eruption site, forming a direct magma pathway from mid-crustal depth to the ocean floor.

Petrological analyses of the erupted lava support this link. The chemical compositions align with differentiation processes expected in a magma body containing 20–40% melt, confirming that the eruption originated from a deep, long-lived reservoir rather than a shallow magma pocket.

Together, the findings explain the island’s persistent ground deformation and continued seismic unrest since 2018.

Why does this challenge the standard view of magma chambers

For over a decade, the transcrustal magmatic system model has dominated volcanology. It describes magma reservoirs as crystal-rich “mush zones” containing only small amounts of liquid melt. Such systems are hard to detect geophysically because the low melt content produces weak conductivity and limited seismic anomalies.

The reservoir beneath Mayotte, by contrast, contains up to 42% melt and behaves more like a liquid than a crystalline mush. This suggests that large, melt-dominated bodies can persist for long periods in the crust, even far from tectonic boundaries. The discovery doesn’t invalidate the mush model but expands it, showing that magmatic systems form a continuum — from rigid, crystal-dominated reservoirs to highly molten, eruptible ones like Mayotte’s.

Recognizing where a system lies on this spectrum helps explain why some volcanoes remain dormant while others erupt repeatedly. Mayotte demonstrates that even in oceanic regions once thought stable, deep crustal magmatism can evolve into large-scale eruptions when the right conditions align. The finding also raises the possibility that similar hidden reservoirs exist beneath other volcanic islands, waiting to be detected.

Hazards and monitoring in a restless region

Although the 2018–2021 eruption occurred entirely underwater, its effects on Mayotte were significant. Hundreds of small earthquakes were recorded, and satellite data revealed several centimetres (about an inch) of ground subsidence. These signals reflected magma withdrawal from the deep reservoir and its movement toward the new submarine vent.

Today, the Bureau de Recherches Géologiques et Minières (BRGM) and the Volcanological and Seismological Monitoring Network of Mayotte (REVOSIMA) continue to monitor the area.

Their seismic and geodetic instruments detect persistent microearthquakes and slow deformation, suggesting that the magmatic system remains active. Ongoing magnetotelluric, seismic, and GPS campaigns aim to determine whether magma continues to accumulate or migrate upward.

Understanding the reservoir’s state is vital for risk assessment. A body containing hundreds of cubic kilometres of partially molten rock stores vast thermal energy and pressure. While future eruptions are likely to remain submarine, seafloor deformation or slope collapse could generate local tsunamis or open new vents closer to the island.

Continuous observation remains essential for hazard forecasting and early warning in the western Indian Ocean.

How scientists revealed what lies below

Magnetotelluric imaging measures fluctuations in the Earth’s electromagnetic field caused by lightning and solar activity. By comparing the strength of electric and magnetic components at different frequencies, scientists infer the subsurface’s electrical resistivity, which drops sharply in the presence of molten or saline materials.

In the Mayotte study, researchers deployed both land-based and ocean-bottom magnetotelluric sensors to create a three-dimensional conductivity model of the crust. They then compared these field data with laboratory experiments on basaltic melts derived from Mayotte’s lava.

Heated to over 1 100°C (2 010°F) to simulate magmatic conditions, the samples provided a conductivity baseline that allowed researchers to estimate the melt fraction of the reservoir.

Alternative explanations, such as saline fluids or graphite layers, were tested but ruled out. These materials could not reproduce the depth, shape, or intensity of the observed conductivity anomaly.

Combined with seismic and geochemical evidence, the magnetotelluric model provides one of the clearest images ever obtained of a mid-crustal magma body beneath an oceanic island.

This success demonstrates how integrating electromagnetic, seismic, and gravity methods can illuminate the pathways through which magma ascends. Each dataset reveals a different physical property, and together they trace the full evolution from deep storage to eruption.

A window into Earth’s hidden magmatic engine

The Mayotte reservoir offers a rare glimpse into how Earth stores and moves magma deep within its crust. It confirms that large quantities of molten rock can persist at depths greater than 20 km (12 miles), maintaining eruption potential even in regions considered geologically stable. For researchers, this represents both a technological milestone and a conceptual breakthrough in understanding oceanic volcanism.

Mayotte’s deep magma body may serve as a natural laboratory for studying how molten rock interacts with the crust and seawater. Continued monitoring will help scientists observe how reservoirs recharge, cool, or trigger new eruptions. These real-time insights could improve eruption forecasting in submarine volcanic zones worldwide.

Ultimately, the discovery bridges the gap between theory and observation. It proves that processes once thought unreachable can now be imaged directly, revealing that Earth’s interior remains alive — dynamic, molten, and constantly reshaping the planet from within.

References:

¹ Magnetotelluric evidence for a melt-rich magmatic reservoir beneath Mayotte – Pierre Wawrzyniak et al. – October 29, 2025 – https://doi.org/10.1038/s41586-025-09625-4

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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