Underwater Eruption Off France’s Mayotte Island Gave Rise to a New Colossal Seamount

In 2018, seismometers around the world detected mysterious rumbles emanating from a usually quiet area in the Indian Ocean between Comoros and Madagascar. At the time, researchers were astonished to find a 2,690-foot-tall underwater volcano, which is about 1.5 times the height of the One World Trade Center in New York.

The volcano was formed after the largest underwater eruption ever detected and now, scientists suspect that the volcano draws its lava from the deepest volcanic magma reservoir known to researchers, reports Laura Geggel for Live Science. The study was published in August in the journal Nature Geoscience.  

Scientists first took notice of volcanic activity about 31 miles east of the French island of Mayotte in 2018 when seismic hums, or low-frequency earthquakes, were detected by seismometers all over the globe. However, the huge underwater volcano shocked scientists because only two seismic events had been recorded near Mayotte since 1972. Before that, a layer of 4,000-year-old pumice in a lagoon nearby is the only additional evidence of an eruption ever found, per Live Science.

After researchers noticed that the island was moving eastward about 7.8 inches a year, they installed ocean-bottom seismometers and GPS systems to track the island's fascinating geological activity, per Live Science.

To understand the origin of the tremors that began in 2018, the study’s lead author Nathalie Feuillet, a marine geoscientist at the French National Center for Scientific Research, along with her team embarked on a mission—dubbed MAYOBS1—aboard the French research vessel Marion Dufrense in 2019.

"We expected to see something, but it was not certain," Feuillet tells Live Science.

The team kept an eye on the area near Mayotte Island, spanning more than 8,500 square kilometers of seafloor, with seismometers and sonar located about 3,500 meters below sea level, reports Daniel Lingenhöhl for Scientific American. The team knew there was a magmatic event east of the island, but they were not sure the magma stayed beneeth the crust or if it erupted out onto the seafloor, per Live Science.

Between February and May 2019, the team’s equipment recorded 17,000 vibrations from 20 to 50 kilometers below the sea crust. Then the vessel’s echo sounder, a system that maps out the seafloor using sound waves, detected an underwater volcano measuring about 1.2 cubic miles, Live Science reports.

Using all the combined data, the research team closely analyzed the seismic waves collected by the equipment and were able to piece together how the underwater volcano formed, per Scientific American. Before the volcano emerged 8,465 feet below sea level, the area was almost flat. Furthermore, the massive seamount did not appear in a previous geological survey conducted by the Naval Hydrographic and Oceanographic Service in France in 2014.

“To me, the claim ‘largest active submarine eruption ever documented’ misses the most important point,” William W. Sager, a geophysics expert not involved with the research, tells Scientific American. “Few submarine eruptions have been documented because they are usually hidden beneath kilometers of opaque ocean water. We know little about seamount formation, but the authors caught it as it happened. Even more important, they were able to show where the magma came from and how it got to the surface.”

The colossal underwater volcano formed from a large magma chamber right below the Earth’s crust. Tectonic plate movement tore the rock in the crust, and magma rose and formed geologic dikes, which are rock that fills larger holes and cuts through surrounding rock layers, reports Scientific American. This process created the seismic activity, and as soon as the magma reached the seafloor, it built the volcano up as its lava reservoir drained. Scientists are still monitoring the area for more tremors and volcanic activity. The most recent evidence of magma on the seafloor was recorded in January 2021, Feuillet told Live Science.

Elizabeth Gamillo | | READ MORE

Elizabeth Gamillo is a daily correspondent for Smithsonian and a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.