On November 11, 2018, seismometers around the world recorded strange seismic activity. The activity was first spotted by Twitter user @matarikipax on a US Geological Survey feed.
Now the source of the seismic activity has been identified. It was a massive underwater volcano, possibly the largest eruption ever recorded under the sea.
The activity began just off the coast of Madagascar, there was no corresponding earthquake to trigger the shaking. No one felt anything despite the seismic activity going on for 20 minutes.
Scientists thought a volcano seemed to be the best explanation, but it had been thousands of years since the last one in the region, and no pumice was seen on the surface which would be one indication of a volcanic eruption.
Then researchers mapped the seabed in the area, and confirmed the cause, a new volcano, nearly a kilometer tall and five kilometers wide.
The team of researchers found around five cubic kilometres of magma had erupted onto the seafloor. Rocks brought to the surface burst open when they were brought onboard the ship, releasing high-pressure gases trapped when the magma cooled under high pressure deep under the ocean.
The cause of the eruption is thought to be a magma chamber collapsing, pushing its contents upwards towards the surface, and letting the land above subside. Nearby, the island of Mayotte has sunk 13cm and shifted 10cm east as a result.
Photo: MAYOBS Team (CNRS/IPGP-UNIVERSITÉ DE PARIS/IFREMER/BRGM)