A New Zealand-led team has completed the fullest investigation to date into January’s eruption of the underwater Tongan volcano, with the conclusion that Hunga-Tonga Hunga-Ha’apai (HT-HH) emitted the biggest atmospheric explosion recorded on Earth in more than 100 years.

New Zealand’s National Institute for Water and Atmospheric Research (NIWA) has discovered that almost 10 km3 of seafloor was displaced–the equivalent of 2.6 million Olympic-size swimming pools and a third more than initial estimates–with two-thirds coming from the summit and the rest from the surrounding flanks.

Three-quarters of this material was deposited within 20 km of the volcano. This leaves almost 3.2 km3 unaccounted for. The project leader, NIWA marine geologist Kevin Mackay, said this missing debris could be partly explained by aerial loss.

Despite the huge displacement of material, the volcano’s flank remains surprisingly intact. However, the caldera is now 700 m deeper than before the eruption.

Further evidence from the caldera shows signs that HT-HH is still erupting. The Maxlimer USV, remotely operated from the U.K. by SEA-KIT International, detected active venting from newly formed cones, explaining why glass fragments formed from cooled molten lava were picked up during NIWA’s earlier survey.

NIWA scientists have also unraveled one of the biggest unknowns of the eruption: the pyroclastic flows. Pyroclastic flows are currents made up of dense lava, volcanic ash and gases that can reach temperatures of 1,000° C and speeds of 700 km/hr. NIWA collected 150 sediment cores, which were sent to New Zealand’s University of Otago and the National Oceanography Centre in the U.K. for analysis. These samples showed pyroclastic deposits some 80 km away from the volcano. It’s possible they could have traveled even further.

A video, shot on board NIWA’s RV Tangaroa, shows the scale of the HT-HH explosion and the findings of the resulting undersea investigations: https://youtu.be/xYhCEeIO25k.

Learn more here.