Tonga Volcano: How ‘Seafloor Detectives’ Discovered Its Secrets


When the Hunga Tonga-Hunga Ha’apai volcano erupted in Tonga in January 2022, it became the largest eruption ever recorded by modern technology.

The blast, estimated to be hundreds of times stronger than the Hiroshima nuclear explosion, was heard in Alaska, more than 10,000 kilometers (6,000 mi) away. A plume of ash, smoke and volcanic material shot 58 kilometers (36 mi) into the sky, and hurricane-force winds were reported in Earth’s upper atmospheric layer at the edge of space.

And then the waves came: tsunami warnings were issued in the nearby Pacific island states of Fiji, Samoa and Vanuatu, as well as further afield in New Zealand, Japan, Peru, the US and Canada. The ensuing tsunami devastated Tonga with waves up to 15 meters high, killing three people and causing an estimated $90.4 million in damage.

Now a team of researchers has completed the most comprehensive study of the event to date, confirming that nearly 10 square kilometers of seafloor was moved – equivalent to 2.6 million Olympic swimming pools, and a third more than initial estimates.

However, they found that only three-quarters of this material was deposited in an area within 20 kilometers (about 12 miles) of the volcano, leaving a significant chunk unexplained. The New Zealand National Institute of Water and Atmospheric Research (NIWA), which conducted the research, believes that this missing debris could be partly explained by “air loss”. Therefore, it was only noticed after detailed mapping work was completed. The material was shot into the air and continued to circulate in the atmosphere for months, which explains why it was not on the sea floor.

But it remains unclear to researchers why exactly the eruption was so explosive.

Some of the answers came from an earlier expedition, also conducted by NIWA, that mapped the seafloor around Hunga Tonga-Hunga Ha’apai.

The NIWA research vessel set sail in April 2022 to discover why the volcano erupted so violently.  Here it is flanked by the islands - which used to be one - that remain after the explosion: Hunga Ha'apai (left) and Hunga Tonga.

Erica Spanje is a marine geology technician at NIWA and a member of the expedition that left in April. Spain describes itself as a “seabed detective,” using high-tech echolocation machines to hunt for underwater volcanoes and gather clues about the “triggers” that influence underwater eruptions.

Aboard the RV Tangaroa, NIWA’s research vessel equipped with state-of-the-art underwater surveillance technology, Spain had a dual role: operating the multibeam and taking samples of mud and rock sediment from the seabed.

The multibeam echosounder “sends out acoustic pulses to map the seafloor in 3D,” Spain says, comparing it to the echolocation used by a dolphin. “We have hydrophones that pick up that echo, and based on that we can determine how deep the sea floor is and get a sense of its shape and geometry.”

With a volcanic cone just 100 meters (328 feet) high on a small Pacific island, Hunga Tonga-Hunga Ha’apai wasn’t much to look at before the eruption. Below the surface, however, the volcano stretched 20 kilometers wide and almost 2 kilometers deep.

Satellite images show Hunga-Tonga-Hunga-Ha’apai before and after the eruption.

Continued instability in the caldera meant the crew couldn’t investigate the volcano’s vent — so an unmanned surface vessel was deployed instead. Piloted remotely by a SeaKit, based in the UK 16,000 kilometers (10,000 miles) from Tonga, the 12-metre-long (40 feet) robot found that the caldera had collapsed and was now 700 meters (2,300 feet) below the surface.

In total, 22,000 square kilometers of the seabed were scanned. Parts of the seafloor around Hunga Tonga-Hunga Ha’apai had already been mapped and by comparing the maps of the seafloor before and after the eruption, “we can begin to build a better picture of what these triggers could be” Spain says.

The results of the expedition surprised the team, says Spain. They had expected the massive eruption to have left a lot of volcanic debris on the sea floor, but in fact “the volcano looked very much like it did decades ago,” Spain says.

Instead of settling in the ocean, “much of that (volcanic material) went straight into the stratosphere,” Spain says.

Aboard the RV Tangaroa, marine geology technician Erica Spain (pictured) had two roles: to map the seafloor with a multibeam echosounder and to collect sediment samples (pictured).

The discrepancy between the size of the caldera collapse and the debris on the seafloor indicated another factor in the explosion: The volcano’s hot magma had interacted with the cool seawater to create steam. “Steam takes up a thousand times the volume of water,” says Spain. “It kind of explains why it erupted like that.”

The results of the data collected before, during and after the eruption indicated that “multiple mechanisms” were happening simultaneously, said Emily Lane, a hydrodynamics scientist at NIWA and a member of the New Zealand Tsunami Experts Panel.

The first eruption blew water out of the way and caused local waves. But changes in air pressure amplified those waves, creating a meteotsunami that travels faster than the speed of sound — and in the case of Hunga Tonga-Hunga Ha’apai, it “generated a pressure wave that traveled around the world three or four times” says Lane.

Tongatapu is the largest and most populous island in Tonga.  Vakaloa Beach Resort (pictured) on the northwest coast of the island, like many other parts of the island, was hit by tsunami waves.

Waves were also created by volcanic debris that rained into the water and the caldera collapsed, Lane says.

NIWA already has a tsunami warning system, with sensors on the seafloor around New Zealand and the South Pacific to monitor sea levels, tides and currents and report anomalies. And now the data collected from Tonga can help refine these sensors, Lane says.

“This event really changed our understanding of volcanic tsunamis because this is the first time we’ve actually been able to get modern instrumentation measurements of what happened,” says Lane.

Scientists on the RV Tangaroa used a remote underwater glider to collect data from the water column around the underwater volcano.

And while the seafloor map gave researchers a better understanding of the eruption at Hunga Tonga-Hunga Ha’apai, it also contributed to a larger project: Seabed 2030, a global Nippon Foundation initiative that aims to cover the entire seabed.

This map of the seafloor could help identify important or vulnerable ecosystems, Spain says. The information gathered can also aid in the recovery of the surrounding ocean and marine environment. For somewhere like Tonga, where about 82% of the population is engaged in subsistence fishing, it is vital to understand the impact of eruptions on aquatic life.

“We don’t know enough about the ocean and our impact on it, so mapping, observing and understanding it is incredibly important,” says Spain.

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