Underwater Volcano Builds an Underwater Mountain

Underwater Volcano

It took just six months for a brand-new volcano to come into being off the island of Mayotte, between Madagascar and Mozambique in the Indian Ocean. For months, inhabitants of the French island had felt small earthquakes almost every day—and a large, magnitude 5.8 earthquake in May of last year—but the island’s only seismometer wasn’t enough for local authorities to pinpoint the source

In February, the Institute of Geophysics in Paris (IPGP) led a scientific investigation into the phenomena. Researchers were stunned when they examined imagery created during a recently concluded mission: there it was, a brand-new mountain about 800 metres high and five kilometres wide.

They sent an expedition to the site, which placed six seismometers on the ocean floor. After analyzing the data a few months later, researchers found a cluster of earthquake activity 20-50 km into the Earth’s crust.

The team suspects that a deep magma chamber fed molten rock to the ocean floor before shrinking, creating the cracking and creaking of the surrounding crust that Mayotte’s residents felt as earthquakes. This hypothesis is supported by the fact that the island has sunk by 13 cm and shifted 10 cm east within the last year.

Also, it appears that about five cubic kilometres of magma erupted onto the ocean floor.

This information came from a map of the floor made by multibeam sonar on the expedition’s ship. Sonar data also show that plumes of bubble-rich water emerged from the volcano, explaining why local fisherman had recently come across large numbers of dead fish.

It’s an exciting time for scientists and locals. After all, it’s been 4,000 years since the area has had any volcanic activity at all.

There’s plenty more to do. Scientists will now analyze the chemical composition of water samples to understand more about the volcano—like the depth of the magma before it erupted, and how likely it is that the volcano will explosively erupt again.

Researchers also think that this birth of a volcano explains a mysterious seismic hum, detected by seismometers around the World months before. Heard as an ultra-low frequency, the hum wasn’t consistent with the mixed-frequency waves associated with earthquakes, but it’s better explained by the eruption and cooling of magma from a baby volcano beneath the ocean.


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