Massive volcano discovered under the sea

The source of a bizarre seismic event first spotted by a Wellington-based Twitter user has been identified.

It was the birth of a massive underwater volcano - perhaps the biggest eruption ever recorded under the sea.

On November 11 last year, seismometers across the world recorded a strange vibration. Twitter user @matarikipax, real name unknown, saw it on a US Geological Survey feed and called it "most unusual".

There was no earthquake to trigger the shaking, which began just off the coast of Madagascar near the French island of Mayotte.

No one felt anything despite the rumbling going on for 20 minutes, dominated by a low frequency that repeated every 17 seconds.

"I don't think I've seen anything like it," Columbia University seismologist Göran Ekström told National Geographic.

"It's like a ringing bell," French Research Institute for Development seismologist Jean Paul Ampuero told Gizmodo. "If you want to get a very low frequency, a very low tone, you need that bell to be huge."

sonar volcano
Sonar image of the volcano. Photo credit: MAYOBS Team (CNRS/IPGP-UNIVERSITÉ DE PARIS/IFREMER/BRGM)

A volcano seemed to fit the bill, but scientists were sceptical because it had been thousands of years since the last one in the region, and no pumice was seen at the surface.

French researchers recently mapped the seabed in the area, reports Science magazine, and they've confirmed the cause - a brand new volcano, 800m tall and five kilometres wide.

"We have never seen anything like this," said Nathalie Feuillet, expedition leader.

About five cubic kilometres of magma erupted onto the seabed, the French team found. Rocks brought to the surface popped open once they were brought on board, releasing high-pressure gases trapped when the magma cooled in the deep ocean.

But why did an eruption happen now? Africa itself is splitting up thanks to continental drift, which could be behind the renewed activity, one geologist told Science.

A magma chamber is suspected to be shrinking as a result, pushing its contents upwards towards the surface, and letting the land above subside. The island of Mayotte has sunk 13cm and shifted 10cm east in the past year as a result.

Since the volcano's birth in November there has been continued earthquake activity in the region, and they appear to be trending towards the island, raising fears of a tsunami.