In brief: Tangaroa records a scene of submarine havoc

A recent survey has revealed the ferocity of an underwater eruption north of New Zealand.

In October, NIWA research vessel Tangaroa mapped Havre, an undersea volcano 800 kilometres northeast of Tauranga, on the Kermadec chain of seamounts.

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Havre erupted on 19 July this year, leaving enough pumice floating on the sea to cover an area the size of Canterbury. "We found a new volcanic cone on the edge of the volcano," says NIWA Ocean Geology Scientist Dr Joshu Mountjoy, "towering 240 metres above the crater rim. It's fantastic to be able to record the change on the seafloor following these kinds of events."

Ejecta from the Havre eruption broke the ocean surface from a depth of 1100 metres, leaving a raft of pumice over 22,000 square kilometres. Satellites recorded clouds of ash. Several cubic kilometres of new material has been added to the volcano, and Mountjoy says a blanket of freshly-ejected pumice has now raised the caldera floor by up to 10 metres.

NIWA Marine Geophysicist Dr Richard Wysoczanski led the 23-day Tangaroa voyage, which set out to study the Kermadec volcanic chain stretching 1000 kilometres north from Bay of Plenty. On average, there is a volcanic eruption in the region once a year.

"One of the most exciting aspects of the cruise was the chance to remap Havre," says Wysoczanski. The seamount is a kilometre high, with a steep-walled, five-kilometre-wide caldera at its centre. Such structures are known to produce particularly spectacular eruptions.

Because NIWA had previously mapped Havre in 2002, says Wysoczanski, Tangaroa's multibeam echosounder could produce "a before-and-after comparison of the volcano, to determine the size of the eruption and the change it's made to the seafloor."

Mountjoy says Havre may not be finished yet: "One side of the caldera wall is bulging in towards the volcano's centre," he says, suggesting the site of a future eruption. "Or it might lead to an undersea avalanche."

Tangaroa's crew retrieved glassy volcanic rocks from the crater wall, and pebbles of pure sulphur, which will be analysed.

The Kermadec and Colville Ridges have rifted by more than 100 kilometres, and Wysoczanski wants to know where the new material is coming from.

He suspects that the Colville Ridge is the original feature, which has provided the parent material for the younger Kermadec Ridge.

The last four days of the voyage were spent studying tectonic processes on the Hikurangi margin subduction zone, off the North Island's east coast.

The work, funded by the Ministry of Business, Innovation and Employment (MBIE), is part of an international effort to understand the behaviour of the subduction system off New Zealand's east coast.

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