An area of floating pumice 250 nautical miles in length and 30 nautical miles wide in the South Pacific ocean. Picture: AFP / New Zealand Defence Force

(Story from News.com.au)

A giant floating island of pumice was created when a previously dormant volcano erupted amid more than 150 earthquakes over two days last month, scientists say.

The eruption of the Havre Volcano, about halfway between New Zealand and Tonga, is believed to have caused the 7500 square kilometre pumice “raft”, which was encountered by a New Zealand navy ship last week.

Cornel de Ronde, principal scientist of the Institute of Geological and Nuclear Sciences, told Radio New Zealand the source of the pumice had been identified in cooperation with French researchers in Tahiti who monitor earthquakes in the south-west Pacific.

“When they looked at their physical records they saw that on July 17th and 18th, there were some 157 earthquakes of magnitudes between 3.0 and 4.8,” he said.

De Ronde said they occurred near the time of the first sighting of the pumice raft, and when the institute looked at its database it found the Havre volcano, which it had previously surveyed.

It was a caldera volcano, like White Island, 50 kilometres off the west coast of New Zealand’s North Island, which erupted last week.

But the Havre, 1100 metres beneath the Pacific Ocean, was not thought to have erupted before, he said.

De Ronde said the pumice island was so light that it had floated several hundred kilometres from the volcano when it was encountered by the HMNZS Canterbury, which took samples last week.

Scientists were also analysing samples of rock ejected from Mount Tongariro, on New Zealand’s North Island, to try to find out why it erupted a week ago for the first time in 115 years.

A caldera is a cauldron-like volcano formed by the collapse of land after a volcanic eruption.