The new volcano and the island that moves

 By Eniday Staff

Pierre Passot was an unlikely candidate to become supreme commander of the French navy – having grown up some 600 kilometres from the nearest sea port – and an even less likely candidate to become the absolute ruler of Mayotte, an island near the Comoros archipelago in the vast channel between Mozambique and Madagascar. However, this is what happened on 25 April 1841, when Madagascan sultan Adriantsoulim decided to sell Mayotte to the French (and their well-armed ships…), rather than leave it vulnerable to the Comorans, who had been raiding it for decades…

Even Passot, though, would have been amazed to know that Mayotte had the strange habit of moving about. It’s not that it makes long or sudden journeys, but Mayotte does move. During 2018, for example, it shifted 10 centimetres to the east and sank 13 centimetres into the seabed. Researchers from the Institut de Physique du Globe de Paris (IPGP) have caluclated that since Passot first set foot on the island, Mayotte has moved more than 300 metres closer to Madagascar and sunk into the seabed by the same amount. Why does this happen? It’s all to do with volcanoes, but not those on the island – now all extinct – but the ones on the surrounding seabeds, which make up an area of extraordinarily intense and complex seismic activity. In fact, so much is going on down there that it was recently possible to witness first-hand the creation of a new volcanic phenomenon. In June, a team of researchers identified and monitored a massive lava flow at 3500 metres below sea level, just a short distance away from a new volcano that emerged just a month earlier.

The birth of an underwater volcano from a depth of 3,500 meters has never been experienced anywhere else in the world (MAYOBS2 team)

A new volcano from the depths of the sea

Nothing too unusual in that, you might say, except that the lava flow covers nearly 9 square kilometres of the seabed and has an average thickness of 75 metres. That would be enough to blanket cities the size of Rome or Paris in two metres of molten rock or, to put it another way, is several thousand times the volume of ash that destroyed Pompei and Herculaneum about 2000 years ago. The volcano discovered in May lies 50 kilometres to the east of Mayotte and rises around 800 metres from the seabed, Its plume of white-hot fluid – no black smoke like on Stromboli as we’re underwater – reaches another 2 kilometres high, peaking at about 700 metres below the surface of the ocean. Still nothing to write home about? What about the fact that the volcano was not there last year and nor was the mass of lava that appeared in June. Most likely, the volcano appeared in Autumn or Winter 2018, after the last campaign of seismic measurements in that stretch of sea.

First pictures of the sea bottom showing the new lava flow: pillow lava and lava ropes (MAYOBS4 team)

The hypothesis put forward by the researchers is that the area between Mayotte and where the volcano appeared conceals a magma chamber – a huge underground reservoir whose contents flow through channels formed beneath the seabed to feed the volcano and the lava column observed minute by minute since mid-June. The chamber gradually empties to fuel the new volcano, as with all its predecessors on the seabed, that are apparently no longer active. It is this emptying that is thought to have caused the eastward drift of Mayotte – a bit like an inflatable buoy that dances on the surface of the water when youremove the plug. It’s a complex phenomenon, difficult to describe and even harder to explain.

Bathymetric data recorded during the MAYOBS2 and MAYOBS4 campaigns. The new lava flow can be seen in the image on the right (MAYOBS4 team)

But scientists agree that what is happening in Mayotte is absolutely unique, has nothing to do the rise of a ‘hotspot’ as in the creation of Vulcano or Stromboli off the coast of Sicily, or, on a larger scale, with Réunion in the Indian Ocean. Nor is it similar to the cases of subduction observed in the Mediterrean or the Antilles. The only thing that’s certain, right now, is that what is happening is absolutely epic in scale: a story that is still to be written.

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Eniday Staff