Niwa researchers have confirmed and mapped a volcanic eruption that occurred over a kilometre below sea level but was powerful enough to breach the surface of the ocean.
The eruption, 800km north east of Tauranga, produced clouds of ash visible by satellite when it erupted on July 19, 2012.
The Tangaroa, a Niwa research vessel, has just mapped the Kermadec volcano and found it produced a pumice raft the size of Canterbury.
The vessel recently embarked on a 23 day expedition in the Kermadecs, north of New Zealand, to study the volcanic chain that stretches 1000km north of the Bay of Plenty.
"When we mapped the area yesterday we found a new volcanic cone which has formed on the edge of the volcano, towering 240 metres above the crater rim," said Niwa ocean geology scientist Dr Joshu Mountjoy.
"It is fantastic to be able to record the change on the seafloor following these kinds of events."
The voyage, led by Niwa's volcanologist Dr Richard Wysoczanski, comprises geology and biology scientists from Niwa, GNS Science, as well as students from Victoria University of Wellington and The University of Auckland.
"One of the most exciting aspects of the cruise is the chance to map Havre volcano which, we have now confirmed, erupted in July," said Dr Wysoczanski. "We know the shape of the volcano from previous research. Using the multibeam echosounder, we made a before and after comparison of the volcano to determine the size of the eruption and the change it has made to the seafloor."
Niwa had previously mapped Havre volcano in 2002, showing a 1km high undersea mountain with a 5km wide, 800 metre deep central crater. This central steep-walled crater is a caldera, which is a type of volcano, like Lake Taupo, known to produce large and violent eruptions.
Over the last decade there has been one significant undersea eruption a year on average across this whole area.
Joshu Mountjoy's interest is in the changing undersea landscape: "One side of the caldera wall is bulging in towards the volcano's centre.
"The bulging may indicate where an eruption may occur in the future, or it might lead to an undersea avalanche."
In addition to the material erupted out into the sea and atmosphere, several cubic kilometres of new material has been added to the volcano. Large volumes of freshly erupted pumice have accumulated on the caldera floor, raising the seafloor by up to 10 metres.
Glassy volcanic rocks were sampled from the fresh crater wall, typical of newly erupted material.
"We found fresh volcanic rocks related to the eruption all over the area, and there are new volcanic cones in one area," said Dr Wysoczanski. "We have collected volcanic rocks, up to beach ball size, and analysis will take place on these when we are back at Niwa. The rocks vary in colour from black glassy material to pumice."
The voyage is part of an international effort to understand the earthquake behaviour of the subduction system off New Zealand's east coast. A 380km long seismic profile will be collected to image sediment on the Pacific Plate.