Post-eruption imaging of the Hunga caldera gives new details of its collapse. The Hunga volcano during a 2009 eruption. Credit: TELUSA FOTUKrakatau, a volcanic island in Indonesia, killed more than 30,000 people when it erupted in 1883.

One might assume that volcanic debris posed the major threat, yet many of the victims died from a tsunami caused by the volcanic eruption instead. Volcanoes at sea can pose hazards quite different from those on land, as their impact can vary depending on how the volcanic activity interacts with seawater. In the deep ocean, high water pressure can suppress explosive volcanic activity, making eruptions less destructive.

But closer to the surface, the lower water pressure and the high temperatures of volcanic material can cause violent explosions. Yet scientists still do not fully understand the conditions under which volcanic activity can cause a tsunami. To better understand these risks, an international team of researchers examined the 2022 Hunga eruption in Tonga, focusing on the rapid caldera collapse that may have amplified its powerful tsunami.

Calderas form when the ground above a magma reservoir collapses as the magma drains away. Comparisons of seafloor topography before and after the eruption showed that a roughly 4-kilometer-wide caldera sank by about a kilometer during the eruption. The researchers argue that the rapid collapse of the entire caldera may have enhanced the massive tsunami.

Even a relatively small caldera like Hunga may be capable of producing a dangerous tsunami, depending on the speed and nature of its collapse. This suggests that other underwater volcanoes may also have the potential to generate dangerous tsunamis. Still, the paper warns: “The dynamics of submarine caldera-forming eruptions and, thus, the hazards they pose are not well understood, owing to the relative inaccessibility of the resulting calderas and eruption products.”A roof too thin to hold The Hunga volcano’s eruption on January 15, 2022, ranks among the most powerful volcanic events this century.

The eruption column soared more than 55 km, pushing vaporized seawater up into the stratosphere, and the pressure wave from the explosion was detected across the globe. The accompanying tsunami is estimated to have reached heights of 40 meters within about 100 km from the source. It is the highest tsunami ever recorded from an underwater volcanic eruption.

For all its impact, Hunga’s caldera is quite small. The caldera left after the eruption is about 4 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 2 kilometers beneath the volcano, shallow compared to its considerable width of 4 to 5 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-e...