Every single house on Mango Island was leveled by the tsunami that radiated when the underwater Hunga-Tonga volcano Hunga-Ha’apai exploded off the coast of Tonga in 2022. Only two buildings survived on nearby Fonoifua Island. Seven people were killed and at least 19 more were injured, and many cries for help were lost when the eruption’s underwater avalanche severed telecommunications cables. It was reportedly the largest and highest tsunami ever documented from an underwater volcanic explosion. The event was so powerful that scientists unofficially discussed creating a new “ultra” category to describe it. But even though the Tonga eruption is visible from space, there is very little data on submerged volcanoes like Hunga-Tonga Hunga-Ha’apai due to the deep sea of these geological formations and the obvious challenges of doing science at fathoms below sea level in remote areas of the ocean. However, a team led by marine geoscientist Marta Ribó from the Auckland University of Technology (AUT) in New Zealand has managed to amass an impressive body of evidence for what made the 2022 Tonga explosion so deadly. Using rare high-resolution sonar maps collected before and after the event, among other tools, Ribó and his team have determined that the Tonga eruption created its unusually powerful tsunami by blowing up the geological formation above it into almost a large contiguous block. The collapse of this mass in the post-eruption trough, which geologists call a “caldera,” helped launch volumes of ocean water at an “exceptional tsunami magnitude,” the researchers wrote, with waves up to 40 meters (131 feet) away and up to 100 kilometers (62 miles) from the explosion. “Seafloor mapping before and after the submarine [volcano] “Eruptions are very rare,” said Ribó, head of the life and earth sciences department at AUT, in a statement. “Despite their recognized threat, most underwater caldera volcanoes remain unmapped.” A bigger stir Ribó and his collaborators combined these sonar maps, taken fortuitously before the 2015 and 2016 eruptions and after 2022, with observations of the water column to calculate the total volume displaced when the volcano’s caldera collapsed. In summary, the Hunga-Tonga Hunga-Ha’apai collapse moved volcanic material equivalent to the volume of 1,500 Great Pyramids or 89,000 modern aircraft carriers, according to the UK’s National Oceanography Center (NOC), which collaborated with AUT and other institutions on this new study. The caldera sank between 490 feet (150 meters) and approximately 2,790 feet (850 meters) below sea level, moving approximately 2.1 cubic miles (8.9 cubic kilometers) of seafloor and another 1.6 cubic miles (6.85 cubic kilometers) of material from the volcano itself. High-resolution sonar maps show the caldera of Tonga volcano before the eruption (a) and after the eruption (b). The two lower images show the geomorphology of the caldera and its surroundings. Credit: Ribó, et al. Crucially, these mass balance equations omit the obvious role that the thermal expansion of ocean water into high-pressure vapor added to the power of the eruption, as molten magma spilled around the edges of this volcano’s island-sized caldera block. Volcanologists call these types of steam-powered explosions Surtseyan eruptions. Following the search, Isobel Yeo, a senior NOC volcanologist who worked with Ribó on the new study, hopes her work will inspire future expeditions to continue mapping submerged volcanic structures around the world in search of particularly dangerous candidates. In addition to his study of the collapse of the Tonga volcano, published in September in the journal Nature Geoscience, his work also produced a research report in that same issue detailing the risks involved with his findings. “Conducting repeated observations of these volcanoes is vital if we are to better understand the dangers they pose to coastal communities and critical seafloor infrastructure,” Yeo said in a statement. Techniques such as subsurface seismic profiling, for example, which uses sound waves to probe and characterize rock layers hidden beneath the ocean floor, helped the team more accurately map volcanic debris from the Tonga explosion. Ribó believes this type of mapping work should be done on hundreds more underwater volcanoes to properly assess their risk of caldera collapse. “There are at least 74 underwater volcanoes along the Tonga-Kermadec arc, with 20 caldera complexes,” he noted. “However, despite the dangers they pose, they are not well understood because they remain unexplored.”
