Havre Caldera
High-resolution seafloor topography of the Havre caldera mapped by the autonomous underwater vehicle (AUV) Sentry shows the new 2012 erupted lavas in red. Rebecca Carey/Adam Soule

In July 2012, the Havre underwater volcano off the northern coast of New Zealand began spewing out material in what was the largest underwater eruption of the last 100 years.

Now new,research is revealing fascinating details about the event, which was larger than the devastating eruption of Mount Saint Helens in 1980 – one of the worst volcanic disasters of the 20th Century.

An international research team led by the University of Tasmania and the Woods Hole Oceanographic Institution (WHOI) examined the eruption site with robotic underwater vehicles, finding that the eruption was surprising in many ways. Their results are published in the journal Science Advances.

"We knew it was a large-scale eruption, approximately equivalent to the biggest eruption we've seen on land in the 20th Century," said Rebecca Carey, a volcanologist at University of Tasmania and co-chief of the expedition.

"Heading to the site, we were fully prepared to investigate a typical deep-sea explosive eruption," added Adam Soule, from WHOI. "When we looked at the detailed maps from the AUV, we saw all these bumps on the seafloor and I thought the vehicle's sonar was acting up. It turned out that each bump was a giant block of pumice [volcanic rock], some of them the size of a van. I had never seen anything like it on the seafloor."

In fact, so much volcanic rock had been ejected by the volcano it created a raft of floating pumice on the surface directly above that stretched for 300 miles in length and more than 30 miles in width – an area larger than Israel. One Australian Navy officer described the pumice raft as the weirdest thing he had seen in 18 years at sea, according to local press.

Analysis by the researchers suggests that the event was the largest documented underwater silicic eruption – a type of explosive eruption that releases mass quantities of thick, gas-filled lava. They also estimated the amount of material ejected was nearly 1.5 times larger than the 1980 eruption of Mount St. Helens.

"If we were able to view the eruption, we would likely have seen glowing lava oozing onto the seafloor and lots of pumice (very frothy magma) rising through the water, because it is buoyant, and then sinking and standing still as it cooled," Soule told IBTimes UK. "I imagine that at points it would have looked like a minefield with lots of pumice slowly moving through the water."

Despite the size of the eruption, it was not noticed initially by scientists, and volcanologists were not even aware Havre was an active submarine volcano.

In fact, the first signs of the eruption were spotted by a passenger on an international flight days later, who photographed the huge pumice raft from above and later notified researchers, who retrospectively analysed satellite imagery and seismic activity.

More than 70% of all the volcanic activity on Earth occurs on the seafloor, meaning it largely goes unnoticed. The Havre volcano itself is nearly a mile deep and forms part of a chain of underwater and surface volcanoes, known as the Kermadec Arc, situated between New Zealand and American Samoa.