A Kiwi scientist has unexpectedly discovered what appears to be a huge undersea volcanic eruption near the main island of Tonga.
For the past two years, University of Auckland coastal geomorphologist Dr Murray Ford has been using satellite images to track the evolution of a kilometre-wide island of ash and large rock fragments, created by a large eruption in December 2014 and January 2015.
But when scanning images today, he observed a vast plume measuring 30km long and 20km wide, not far from the newly-formed island and just 33km off the coast of Tonga's main island Tongatapu.
The plume, appearing as a greenish cloud in the ocean, was captured by the open-source US Geological Survey/NASA Landsat 8 satellite on January 27.
"It's come out of nowhere," said Ford, who immediately contacted volcanologists at the university who had recently travelled to Tonga.
"It was pretty unusual to get an image without any cloud in it - and then I saw this big, turbid plume of volcanic debris, so I checked in with our volcanologists to see what was going on."
GNS Science volcanologist Brad Scott said scientists were now making contact with colleagues in Tonga and notifying the Volcanic Ash Advisory Centre, although the erupted material appeared to be contained to the ocean.
Satellite images analysed this afternoon showed the plume emerging early last week and growing larger.
"We are now relatively confident that these images are representing a submarine eruption," Scott said.
The plume appeared to have originated from a site that last erupted between December 1998 and January 1999, and which previously erupted in 1911, 1923 and 1970.
"So it's in an area of known volcanism and often, naturally there will be a hazard to shipping, locally."
Scott said a check with the Ozone Monitoring Instrument (OMI), part of Nasa's EOS-Aura satellite showed it had not detected any signatures of volcanic gas in the atmosphere.
Undersea volcanic eruptions are not rare occurrences in the Pacific - and are sometimes picked up long after the event.
In November, a Royal New Zealand Air Force crew spotted a huge pumice raft across a 100km area of ocean west of the Minerva reef, about 500km southwest of Tonga, pointing to a large undersea eruption.
Scott said this month's eruption appeared to be about 420km from the pumice seen on November 15-16.
"It's not likely to be related, but as we couldn't back-track that pumice, we also can not exclude a connection."