Dear Cryptozoologists,

I'm very sorry to report that the strange seismic signal detected last November is not, in fact, a sea serpent of any sort. In point of fact, it wasn't caused by anything living, cryptid or otherwise. Your search for evidence must continue.

Volcanologists, however, got something very exciting in that signal: it's probably a bouncing baby submarine volcano!

Dr. Anne Lemoine of the French Geological Survey, along with her colleagues Didier Bertil, Agatha Roullé, and Pierre Briole, recently released a paper analyzing that anomalous signal. The team's findings are absolutely mouthwatering: they believe that signal and the ongoing seismic crisis it's a part of might just be the largest underwater eruption we've ever detected.

To explain why such a large event is hedged by so much uncertainty, we need to have a look at the tectonic setting and available monitoring equipment.

All of the seismic excitement, which began on May 10th of 2018, is happening east of Mayotte in the Comoros Islands. This region is no stranger to interesting geological events. The Mozambique Channel, where the islands are nestled, was formed in the rifting of Madagascar from Africa around 160-117 million years ago. And things have been quite eventful since that split: the East African Rift has several branches in that area, creating the boundary between the Nubia and Somalia plates, as well as the Victoria, Rovuma and Lwandle microplates.

All of this crustal deformation is wild enough, but the area also hosts the Aswa fracture zone of Precambrian age. It isn't active anymore, but it left weaknesses in the crust which could be affecting the paths magma takes on its way up to the surface. And if that wasn't excitement enough, it's suspected there's an atypical hotspot beneath the region, causing the volcanism that gave rise to the Comoros Islands and numerous seamounts. The eruptions began around 11 million years ago and continue today.

It's all complicated. What makes studying it harder is the fact that much of it is under a kilometer or few of water. And unfortunately, the islands had limited seismic instrumentation when the crisis started, which has impacted the quality and quantity of the data collected during this intriguing sequence of events. We can't get a sharp picture of what's going on, and due to limited sensing capabilities can't see exactly what's happening. But the glimpses we've gotten suggest this is one of the most exciting volcanic events of our lifetime.

It began off the coast of Mayotte around this time last year with a cluster of earthquakes, which took residents by surprise. They live in a tectonically busy part of the world, but they're used to much more moderate seismic activity. A magnitude 5.9 earthquake jolted them hard on May 15th, letting them know this swarm was something different. The first cluster of earthquakes and tremors lasted until July 8th, migrating gradually southeastward, and rising from a depth of roughly 35 kilometers to a much shallower 15. This cluster contained all of the largest earthquakes of the series so far.

A second cluster began on June 26th, closer to the monitoring station on Mayotte, and ended on October 10th without having migrated far or caused many large seismic events. A third cluster, which began on July 5th in an area between 26 to 42 kilometers of Mayotte, didn't migrate at all, but continued on for many months, and was still ongoing when Lemoine and her team submitted their paper in late February. Aside from one brief break between July 13th to August 26th, the seimicity marched on.

And then there was that bizarre, low-frequency seismic tremor on November 11th. There had been others like it, but smaller; this one made itself known worldwide. It contained a high-frequency tremor and a few small tectonic earthquakes, followed by that low-frequency event that sent seismometers humming for twenty minutes and led to jokes about sea monsters. Volcanologists now see something quite different in that signal: magma on the move.

We haven't actually seen any magma directly, but the seismic evidence points to volcanic activity, and gives us a good idea of what may have happened. The early seismicity indicates magma beginning to find a path to the surface from a depth of around 28 kilometers. It fractured rock, creating dikes and possibly sills on its way up. We didn't know it then, but we were watching a volcanic eruption being born.

In the second phase, more magma ascended, widening the dikes and creating a conduit that extended from the top of the magma chamber to the subsurface. Once all that had taken place, the eruption proper could begin. (And by now, this all should be looking pretty familiar to those of us who were glued to Kilauea's East Rift Zone antics last summer.)

We have no real way of knowing what exactly happened on the flank of Mayotte volcano, two kilometers or more under the sea, but we know something volcanic happened. The magma may have erupted from a single point, in which case our explorations will find a brand new seamount joining the mounts that already dot the area. There may have been a fissure or few, a submarine version of the Leilani Estates eruption, spreading lava flows across the ocean floor.

It's quite possible the dike may not have punched through the sediments lying 1-2 kilometers thick on the crust. Instead, the magma it erupted may have flowed beneath them, creating a thick sill.

No matter which scenario is the case, we know that the considerable pressure of all that water kept gasses from exsolving from the magma, so nothing explosive is likely to have happened. It's an impressive eruption nonetheless! The total volume erupted so far is around 1.15 cubic kilometers, and the eruption could continue for a year or more. The research team thinks its output could end up around 2km³, making this the largest submarine eruption humans have ever recorded. We've just watched history being made.

It's believed the magma chamber is deep, so even though it's bound to deflate and possibly collapse after expelling all that magma, it's not likely to cause many problems for people on the islands. But the team can't rule out the possibility that its collapse could cause an earthquake or submarine landslide that would trigger a volcanic tsunami. And we've already seen that unstable seamounts can do the same. Those living on the Comoros Islands and the coasts of Africa and Madagascar will need to be on the alert for devastating waves in the near future.

References:

Lemoine, Anne, et al. The Volcano-tectonic Crisis of 2018 East of Mayotte, Comoros Islands. EarthArXiv, 28 Feb. 2019. Web.