Scientist Autopsy the Aftermath of the Largest Underwater Volcanic Eruption of the Last Century

In 2012, miles of floating rock appeared in the Pacific. Now, scientists have studied the Havre seamount eruption that caused the mysterious pumice ‘raft’

New map of the Havre volcano Rebecca Carey, University of Tasmania, Adam Soule, WHOI

In 2012, the Royal New Zealand Navy happened upon something very curious—miles and miles of pumice floating in the South Pacific. As it turns out, the rocky patch, which totaled 150 square miles, was belched up from an undersea volcano.

In 2015, a team of scientists first took an in-depth look at the volcano, and aMeghan Bartels at Newsweek reports, their findings show the eruption was the largest deep sea eruption in the last century and very different from what they expected.

The eruption occurred at a volcano called Havre, which is about 600 miles off New Zealand’s North Island. The Sydney Morning Herald reports that when researchers first learned about the major eruption, they didn’t know just how large it was or the processes it involved. That’s why a team from the Woods Hole Oceanographic Institute, the University of Tasmania along with other international collaborators spent the past two years studying the volcano. Their initial 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,” University of Tasmania volcanologist Rebecca Carey tells the Sydney Morning Herald.

In 2015, the team a robotic submarine down to Havre. The "AUV," or autonomous underwater vehicle, surveyed the area for 8 to 12 hours at a time. They also employed a "ROV," short for a remotely operated vehicle, to cull some 250 hours of data. From that they created detailed maps of the volcano and examined samples retrieved from the site. The researchers have been analyzing the data from that voyage ever since.

The seafloor did not look how they expected it to. They assumed that such a large eruption would have involved a large undersea explosion. But their survey showed no mega-blowout. “Heading to the site, we were fully prepared to investigate a typical deep-sea explosive eruption,” Adam Soule, Woods Hole associate scientist, says in a press release. “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, some of them the size of a van. I had never seen anything like it on the seafloor.”

Instead, the eruption consisted of lava spewing from 14 vents located 3,000 and 4,000 feet below the surface. Violent eruptions tend to produce only pumice, but this event also created ash, lava domes, and lava flows on the seafloor. The chemical composition of these samples the robotic machines collected will help the researchers learn even more about the inner workings of Havre.

“This is often the case with work in the deep oceans,” Soule tells Ryan F. Mandelbaum at Gizmodo. “We had preconceived notions of what we’d find, but not until we got out there did we get a sense of what had happened.”

Just because the eruption wasn’t super explosive doesn’t minimize it—it produced nearly 1.5 times the amount of material that erupted out of Mount St. Helens in 1980. About 75 percent of the material reached the surface of ocean and was dispersed and the rest was spread for miles across the seafloor.

“Ultimately we believe that none of the magma was erupted in the ways we assume an explosive eruption occurs on land,” Soule says.

The research just underlines how little we understand about undersea volcanology. "We know virtually nothing about submarine volcanoes and eruption processes in the ocean, despite more than 75 percent of the Earth’s volcanoes being on the seafloor," Carey tells Bartels.

And she tells the Sydney Morning Herald that the research isn’t just applicable to geology. The eruption wiped out many undersea biological communities. Now, researchers can see how those areas are being recolonized. The eruption also produced new hydrothermal vents, which some scientists believe are where the earliest forms of life on Earth formed

Get the latest stories in your inbox every weekday.