Tonga’s explosive volcano ‘surprisingly intact’

Change of the seabed

The submarine volcano Tonga which produced a spectacular eruption in January remains surprisingly intact.

A New Zealand-led team has just finished mapping the flanks of the seamount, which many people thought had been torn apart by the ferocity of the event.

But structurally, Hunga-Tonga Hunga-Ha’apai has not changed much.

The Tonga eruption produced the largest atmospheric explosion recorded on Earth in over a century.

It has generated tsunamis in the Pacific and other ocean basins around the world. It even lifted the clouds over the UK, 16,500 km away. Fortunately, only a handful of people lost their lives in the Kingdom of Tonga.

New Zealand’s National Institute for Water and Atmospheric Research (NIWA) has now successfully approached a vessel to map the post-eruption shape of Hunga-Tonga Hunga-Ha’apai (HTHH) and of the surrounding seabed.

Although there has clearly been a lot of ash deposition and sediment movement, the volcano continues to hold its own.

Expedition leader, NIWA marine geologist Kevin Mackay, said he was surprised by what he saw in sonar data from the research vessel (RV) Tangaroa.

“Given the violence of the January 15 eruption, I expected the building to collapse or blown up, and it didn’t,” he said.

“While the volcano appeared intact, the seafloor showed dramatic effects from the eruption. There is fine sandy mud and deep ash undulations up to 50 km from the volcano, with carved valleys and huge piles of sediment.”


The Tangaroa in front of the two parts of the rim of the caldera which rest above the surface of the ocean

From their 22,000 km2 survey, the Tangaroa team calculates that around 6-7 km3 of material has been added to the seabed.

This is ash and rock that was initially ejected by the volcano into the air, but then fell back into the water and traveled down the sides of the seamount to sink to the ocean floor.

These density fluxes, or pyroclastics, were the main factor in generating the tsunami waves that inundated the local islands, Mr Mackay told BBC News.


The volcano and surrounding seabed were last mapped in 2016

The 1.8 km high HTHH was last surveyed in 2016. Combining previous data with new information from Tangaroa allowed scientists to create a “difference map”.

The deposit of all new material is marked in red (see top image). Blue indicates where material was lost. This mainly comes from the neck of the volcano. Researchers say 2-3 cubic km have moved away from the upper parts of HTHH.

In addition to the sonar survey, the Tangaroa crew also studied the local ocean ecosystem.


On the seamounts south of HTHH, marine life continues to thrive

Unsurprisingly, the volcano’s flanks are now devoid of biology, but the team only had to travel about 15 km to find fish and mussels thriving on other seamounts.

“Both of these examples imply resilience of animal populations in the region,” said NIWA fisheries expert Dr Malcolm Clark. “And that’s important because it can provide insight into how the eruption may affect surrounding marine life and what the chances of recovery might be.”

The researchers also tested the water column for physical and chemical characteristics, including temperature, nutrients and oxygen concentration. In places, the ashfall had a fertilizing effect and triggered planktonic blooms. But the flip side is that researchers could also identify areas where oxygen in the water has become depleted.

The team took thousands of photos and collected hundreds of samples during the cruise, including 115 sediment cores and 250 kg of rock, some of which was newly formed during the eruption.

The RV Tangaroa did not inspect the Hunga-Tonga opening or caldera directly.

That will be left to a robot boat developed by British company Sea-Kit International. The 12-meter unmanned surface vessel, called Maxlimer, is currently in Singapore en route to Tongatapu, the main island of the Tonga archipelago.

Since the boat can be controlled remotely, it will be allowed to sail for long periods above the caldera. Caution is warranted as the volcano appears to still be active.

NIWA marine ecologist Dr Sarah Seabrook said this was evident from a lingering layer of ash near the volcano at a depth of around 200m.

“Our initial analyzes of the origin of the ash layer suggest that it is not a remnant of the January eruption, but rather may show that the volcano is still escaping. ‘that is, it actively releases volcanic ash, albeit on a much smaller scale. she explained.


Sea-Kit’s USV Maxlimer will fully map the caldera

The information provided by Maxlimer’s multibeam sonar equipment should be particularly instructive. A navy ship that passed through the caldera recently found that the water depth above the caldera had increased significantly.

Before the eruption there was only 150-200m of sea water. After the eruption there is now 800m or more of water. The HTHH magma chamber has been dug.

“Maxlimer definitely has the ability to measure that depth,” Mackay said. “And what we really hope is that once we get a really accurate map of the caldera, we can confirm the volumes of material that we already initially indicated.”

RV Tangaroa’s month-long mapping project was funded by the Nippon Foundation of Japan and organized by NIWA, in collaboration with Seabed2030, which is an international effort to properly map the Earth’s ocean floor.