Hunga Volcano's Caldera Collapse Explains 2022 Tonga Tsunami

A roughly 4-kilometer-wide caldera at the Hunga volcano in Tonga sank by about 1 kilometer during the January 2022 eruption, and researchers say that rapid collapse helped drive one of the largest tsunamis ever recorded from an underwater volcano, according to a study detailed by Ars Technica. The finding matters because Hunga's caldera is small by global standards, yet its tsunami reached heights of 40 meters within about 100 kilometers of the source — the highest ever measured from a submarine volcanic eruption.

What happened when Hunga erupted in January 2022?

The eruption on January 15, 2022, sent an eruption column more than 55 kilometers into the stratosphere, pushing vaporized seawater with it, Ars Technica reports. The pressure wave from the blast was detected worldwide. Within roughly 100 kilometers of the volcano, the resulting tsunami hit 40 meters — a scale researchers had not previously tied to a caldera this size.

How much did the caldera actually sink?

Comparisons of seafloor topography before and after the eruption, cited in the report, showed the caldera — about 4 kilometers in diameter — dropped by roughly 1 kilometer. That places Hunga among the smallest of the 177 calderas in a global database, but unusually deep relative to its width. Researchers say the subsidence depth, relative to diameter, is comparable to larger, well-studied calderas such as Newberry in the United States.

Why does collapse speed matter for tsunami size?

When a submarine caldera collapses, the sinking seafloor pulls the seawater above it downward. If that drop happens abruptly — as it did at Hunga — a large volume of water moves quickly, which researchers argue can amplify the resulting wave. The study's authors write that even a relatively small caldera can produce an outsized tsunami depending on how fast and how completely it collapses.

"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."

What do scientists still not know?

The researchers caution that submarine caldera collapses remain poorly studied because the underwater sites are hard to reach both during and after an eruption. That gap makes it difficult to predict which small volcanoes elsewhere carry similar tsunami risk.

What should coastal regions watch for next?

• Seafloor mapping efforts at other shallow submarine volcanoes to check for caldera-forming potential
• Follow-up studies on whether collapse speed, not just eruption size, should factor into tsunami warning models
• Monitoring of volcanic islands near population centers where a Hunga-scale event could recur

Glossary

Caldera: A large depression formed when the ground above an emptied magma chamber collapses. Magma reservoir: An underground pocket of molten rock that feeds a volcano; its drainage during eruption can trigger collapse above it. Subsidence: The sinking or downward settling of land or seafloor.

The Hunga case shows a volcano's raw explosive size is not the only variable that matters — how a caldera gives way afterward can determine whether a coastal region gets a routine wave or a 40-meter one.

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