Passengers on a plane flight in 2012 saw something strange from the air: a raft of floating rock called pumice that grew to be roughly the size of Philadelphia (over 150 square miles) in the southwest Pacific Ocean. The raft hinted at an unusually large underwater volcanic eruption. In 2015, Woods Hole Oceanographic Institution (WHOI) and the University of Tasmania led an investigation to collect materials and map the volcano – and they found some surprises.

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Over 70 percent of volcanic activity on our planet happens on the seafloor, but scientists don’t always get a close-up view of events typically hidden by seawater. In 2012, the Havre volcano, northeast of New Zealand, erupted – and this time researchers got a chance to study the aftermath in what the WHOI described as “the first up-close investigation of the largest underwater volcanic eruption of the past century.” The eruption was so massive it generated a raft of pumice that could be glimpsed from space.

Related: Underwater Volcanic Eruption Creates a New Island in the Pacific Ocean

WHOI scientist Adam Soule said in the institution’s statement, “Heading to the site, we were fully prepared to investigate a typical deep-sea explosive eruption. When we looked at the detailed maps from the AUV [autonomous underwater vehicle], 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.”

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Lava came from 14 volcanic vent sites 3,000 to 4,000 feet below the surface in the eruption. Scientists had thought the explosion would generate mostly pumice, but also found ash, lava domes, and seafloor lava flows, per WHOI. Soule said, “Ultimately we believe that none of the magma was erupted in the ways we assume an explosive eruption occurs on land.” According to WHOI’s video, such research could help us better understand the planet’s evolution.

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The journal Scientific Advances published the research last week. 20 scientists at institutions in Australia, the United States, New Zealand, and Japan collaborated on the work.

Via the Woods Hole Oceanographic Institution

Images via Rebecca Carey, University of Tasmania, Adam Soule, WHOI, © Woods Hole Oceanographic Institution; Multidisciplinary Instrumentation in Support of Oceanography (MISO) Facility, © Woods Hole Oceanographic Institution; and the Sentry Group, WHOI