Scientists get first glimpse of transformed seafloor around White Island

Scientists are gaining a first-time glimpse at how the ocean environment around Whakaari/White Island was transformed by December's deadly eruption.

Niwa scientists carrying out a seafloor survey from research vessel Tangaroa this week have already been struck by what they've observed around the offshore volcano in the Bay of Plenty.

That included large amounts of sediment being blasted into undersea canyons, which would have wiped out any life within them.

But so far, there's no sign that erupted material from the December 9 disaster has heightened the risk of a tsunami-generating ocean landslide.

Mapped in 2005, and again last year, the area is renowned for its thousands of seabed gas seeps and intricate network of canyons and channels.

Niwa marine geologist Dr Joshu Mountjoy said the new survey was building a picture of how that landscape had shifted – and specifically how much sediment had been dislodged during the eruption and where it had gone.

"What is visible from sea level is only a small part of the volcano – most of it is under the ocean so we want to see how that submarine environment has changed," he said.

"We expected that we might not see anything huge and catastrophic – but what we can see is there has been change in the canyon that leads off the island. It looks like all of the ash and sediment has gone down there."

While he couldn't yet say how many tonnes of material had been pumped in, the impact on seafloor ecosystems would have been deadly.

"Certainly from the satellite footage at the time, you could see a big sediment plume running out from the island.

"That will have just collapsed under its own density, and floated down through the canyon - and my guess is that most of the erupted material will have gone down this channel.

"So if there was anything living there, it would have been decimated. But then this hasn't been seen over a large area."

The volume of material that flowed through likely would've been small compared with the equivalent 100 million dumptruck loads of mud and sand that gushed through the Kaikoura Canyon during November 2016's 7.8 quake, he said.

Another focus of the voyage was getting a clearer picture of any changes in underwater volcanism.

The Tangaroa has been using a multibeam echo sounder that emitted sound beams which mapped the seafloor and objects in the water column like gas bubbles, all without disturbing the seabed.

Water samples were also being taken to measure methane and carbon dioxide gas levels, which could help detect the presence of underwater seeps linked to volcanic activity.

"There is also the potential to see new hydrothermal vents – particularly in shallow water close to the island," he said.

"No one has collected water column imaging close to Whakaari so this is an incredible opportunity to discover new areas of volcanic activity.

"So far, we've seen a couple of gas seeps - but they're reasonably modest-sized things and it doesn't look like there's any new volcanic activity on the seafloor that would be hugely dramatic."

Of 47 people on the island at the time, 21 were killed, including two who are missing and declared dead, and 26 suffered injuries, many severe.

Several inquiries into the disaster are still under way.