Small undersea volcanoes may unleash outsized tsunamis

For all its impact, Hunga’s caldera is quite small. The caldera left after the eruption is about four kilometers in diameter, making it one of the smallest of the 177 known calderas in a global database. What sets Hunga apart is that its caldera is unusually deep relative to its size. Relative to its diameter, the subsidence depth was comparable to that of famous calderas like Newberry in the United States. The researchers suspect that the speed of the caldera collapse may have been one of the factors behind the massive tsunami. In general, when a submarine caldera collapses, the sinking seafloor pulls the seawater above it along with it. If the seafloor hundreds of meters below the surface drops abruptly, as it did at Hunga, a large volume of water can be displaced in a short time, potentially creating a tsunami. In the paper, the team reconstructed how the collapse of the Hunga volcano unfolded. They compared a pre-eruption seafloor map made in 2015–2016 with post-eruption maps from five surveys conducted between April and October 2022. The geometry of Hunga’s magma reservoir is considered one of the factors behind the rapid collapse. The reservoir’s roof was thin and broad, so it could barely support itself once the magma drained away. The reservoir lay about two kilometers beneath the volcano, shallow compared to its considerable width of four to five kilometers. The team calculated that the roof could cave in if reservoir pressure fell by only about 30 megapascals, roughly 300 times atmospheric pressure. That means losing just a quarter of the magma present pre-eruption would be enough to bring the roof down. The Hunga caldera is thought to have collapsed even as the volcano was erupting. The sedimentary structures preserved beneath the caldera floor support this idea. A ship-based seismic survey conducted about three months after the eruption found that landslide deposits from the collapsing caldera walls were present below, within, and above the pyroclastic layers left by the eruption. The alternation of the two types of sediments suggests that debris from the broken walls was sliding down while pyroclastic material was still accumulating. If the caldera had collapsed after the eruption, the landslide deposits should lie on top of the pyroclastic layers. The researchers caution that the risks extend beyond Hunga. Hunga belongs to the Tonga-Kermadec arc, which hosts at least 74 underwater volcanoes. Of these, 20 are caldera complexes. This means the region holds numerous underwater volcanoes under conditions that may be similar to Hunga’s. Nearby, the island volcano Tofua and the submarine volcano Fonuafoʻou both resemble Hunga in overall form, structure, magma composition, and even caldera size. Yet most underwater calderas have never been surveyed as closely as Hunga. Their inaccessibility means many lack maps entirely, and others have only low-resolution data. Hunga was a rare exception. It gained attention after a new island formed during an eruption in 2014–2015. A detailed seafloor map was created at the time, in 2015 and 2016. The researchers added land-surface data created using commercial satellite imagery and calibrated with NASA’s ICESat-2 laser altimetry to construct a 3D model of the whole of Hunga. This baseline made the before and after comparison possible. Without it, tracking the changes in the caldera would have been difficult. The researchers called for detailed seafloor mapping to monitor underwater calderas. Such data would help track volcanic hazards and protect marine infrastructure. The paper argues, “Our study underscores the urgent need for high-resolution repeat seafloor mapping,” which would help in “strengthening tsunami early-warning systems and protecting critical seafloor infrastructure, particularly in poorly surveyed regions such as the southwest Pacific.” Nature Geoscience, 2026, DOI: 10.1038/s41561-026-02099-7