Oceans

‘Surprising’ findings from Niwa’s ocean survey trip to Tonga

NIWA's research vessel RV Tangaroa has returned from Tonga with some surprising findings. Pictured: Tangaora with the peaks of the Hunga Tonga-Hunga Ha'apai volcano in the background.

Rebekah Parsons-King / Supply

NIWA’s research vessel RV Tangaroa has returned from Tonga with some surprising findings. Pictured: Tangaora with the peaks of the Hunga Tonga-Hunga Ha’apai volcano in the background.

A science voyage to Tonga to map the impact of January’s volcanic eruption returned some “surprising and unexpected” findings, scientists say.

The Niwa vessel, RV Tangaroa, has returned from its month-long expedition funded by the Nippon Foundation in Japan, to study the effects of the eruption of Hunga Tonga–Hunga Ha’apai.

Due to the power of the explosion, researchers expected to find dramatic changes to the volcano – instead, they found it largely intact.

Niwa marine geologist and voyage leader Kevin Mackay said he was completely taken aback. “With an explosion that violent – ​​the biggest ever recorded – you would expect that the whole volcano would have been obliterated.”

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Voyage leader and marine geologist Kevin Mackay was completely taken aback by what the Niwa scientists found.

Rebekah Parsons-King / Supply

Voyage leader and marine geologist Kevin Mackay was completely taken aback by what the Niwa scientists found.

However, they did find as much as seven cubic kilometres of displaced material – the equivalent of five Wellington Harbours or 3 million Olympic-sized swimming pools.

They canvassed an area of ​​22,000 km² of sea floor, and found changes covering an area of ​​8000 km².

“There is fine sandy mud and deep ash ripples as far as 50km away from the volcano, with gouged valleys and huge piles of sediment,” Mackay said.

The broken internet cable, which meant that communication with Tonga was cut off for five days post-eruption, was found buried under 30 meters of ash and sediment. There were strong indications it would need to be fully replaced.

The Hunga-Tonga-Hunga-Ha’apai volcano eruption on January 15 caught on camera. (Video first published April 2022).

Yet areas only 15km away were found with abundant populations of fish and other animals. Scientists speculated they were out of the eruption flow’s pathway, or far enough away to avoid thick ash.

Niwa marine biologist Dr Malcolm Clark called it “a positive sign”.

“Although the seafloor on the volcano is largely barren, surrounding seamounts have pockets of normal biodiversity, such as corals, sponges, starfish, and mussels, indicating the resilience of such marine ecosystems and giving some hope for recovery.”

Water quality was still recovering, with some ash yet to completely settle on the seafloor. There was also evidence that the volcano might still be erupting, with a dense ash layer found in the upper water layer near the volcano.

Niwa biogeochemist Dr Sarah Seabrook said ash had impacts on the ecosystem.

Deployment of the multicorer used to collect seafloor sediment samples.

Rebekah Parsons-King / Stuff

Deployment of the multicorer used to collect seafloor sediment samples.

“Volcanic ash fertilises microscopic ocean algae thanks to the ash’s concentration of nutrients and trace metals. In this case, there was a bloom of life so big that we could see it from space.”

Ash had also been linked to areas of low oxygen in the water, which could impact food production and carbon sequestration.

Tonga’s Deputy Secretary for Lands and Natural Resources, Taaniela Kula, said this study provided “invaluable knowledge” for a quick recovery and future preparedness.

The second part of the mission, scheduled for the middle of next month, will see the caldera mapped by an unmanned surface vessel – for safety reasons – built by Sea-Kit International, which is on its way from the UK. It will be piloted from Essex, 16,000km away.

Sea-Kit's USV Maxlimer preparing for dispatch to Tonga.

Sea-Kit International / Supply

Sea-Kit’s USV Maxlimer preparing for dispatch to Tonga.

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