The "pumice raft", which is around 25,000 square kilometres in size.

The "pumice raft", which is around 25,000 square kilometres in size.

A large pumice raft spotted floating in the Pacific Ocean was formed nearly a month ago when an unknown volcano erupted, scientists have determined.

An Air force Orion spotted the huge raft, measuring 463 kilometres by 55 kilometres, on Thursday, and samples were collected later in the evening by the navy's HMNZS Canterbury.

Vulcanologists had thought it might have been produced by New Zealand's third erupting volcano, the undersea mountain Monowai, but that theory has now been put to rest.

Navy officers with a piece of the pumice.

Monowai started erupting on August 3, whereas scientists have determined the pumice raft was first spotted on July 19. An Air New Zealand pilot also took a photograph of it on August 1.

Tahitian vulcanologists have determined that the raft became visible on July 19 and was caused by a volcanic eruption associated with a series of earthquakes in the days prior.

They have pin-pointed the origin of the raft to 72 kilometres south west of Curtis Island, one of the Kermadec islands, halfway between New Zealand and Tonga.

More than 157 earthquakes between magnitude 3 and 4.8 occurred in the area between July 17 and 18, the Laboratoire de Geophysique told their colleagues at GNS Science.

GNS vulcanologist Brad Scott said he was unaware of any volcano in the specific location where the raft was believed to have originated from.

"At this time I don't know if we could identify a sub-marine volcano at that location," Scott said.

It could have been emitted from a previously-unknown volcano, or it could be that others have knowledge of a volcano in the area while he doesn't, he said.

GNS would be liaising with colleagues to help determine what undersea volcano the pumice came from.

They would be assessing samples obtained by the crew on board the Canterbury, and should have results within the next couple of weeks.

Pumice rafts were not uncommon, GNS vulcanologist Michael Rosenberg said.

They could float for thousands of kilometres, be washed ashore or become saturated and sink, Rosenberg said.

Some rafts from Indonesia have ended up in Hawaii, while others from eruptions near Papua New Guinea have ended up at the Marshall Islands.

This raft from near Curtis Island was last spotted by the navy on Thursday, approximately 85 nautical miles west-south-west of Raoul Island.