Could the rare submarine volcano erupting in PNG blow big?

A scientist monitoring a submarine volcano that has erupted in Papua New Guinea waters says there's little way of knowing whether it could blow big.

The volcano's activity has been described as rare and unusual, and coastal communities living around PNG's Bismarck Sea have been advised to be on heightened alert in the region for potential tsunamis

Located around 125 kilometres southeast of PNG's Manus Island, the volcano has emitted a large steam plume since earlier this month, while built-up lava has produced masses of pumice floating to the surface.

The eruption of the volcano, known provisionally as the Titan Ridge Volcano, was first detected on 8 May after a series of small to medium earthquakes were felt and underwater acoustic signals were detected.

In recent days, local fishermen have reported hearing deep rumbling in the area where the undersea volcano is located, while reports have emerged of dead fish floating in the area's waters.

PNG's Rabaul Volcano Observatory principal geodetic surveyor Steve Saunders said the volcano was located on a spreading ridge where two tectonic plates are pulled apart, causing basaltic magma to rise up.

He told RNZ Pacific that the current activity is very rare.

"There's eruptions there all the time, but they're quiet on the seabed, nobody notices. But this kind of thing, on the geological time scale, every few 1000 or even a few 100 years, you'll get some of these. So it's unusual."

A number of earthquakes have struck in the area this month, including some around magnitude-5. Asked whether the volcanic activity being seen now might precede a major explosion, Saunders said there was no clear way of saying.

"It could get a little bit bigger, it could stop. That's the million dollar question. It depends on how big the source is, the chamber that's feeding it," he said.

"It could carry on for years, just as it is at the moment. It could breach the surface, where we get some locally explosive activity, and if that happens, we may get some small tsunamis being produced off it.

"The other thing is because it's probably built a cone underneath, if we get a collapse of part of a cone again, we may get tsunamis, but again, it's not a definite, it's just a heightened risk at the moment. But, of course, it could stop tomorrow. We don't know how big this."