KEY POINTS:
Auckland University researchers have mapped inside the crater of the Raoul Island volcano which erupted in March last year killing Department of Conservation worker Mark Kearney.
"This will give us clues as to how the eruption occurred," said volcanologist Colin Wilson, who led the university team on a joint expedition with the National Institute of Water and Atmospheric Research aboard research vessel Tangaroa.
The small eruption of gas and steam was at its most intense for only five minutes but coincided with Mr Kearney's arrival at the Green Lake inside the crater to take measurements.
The eruption followed a two-day earthquake swarm on the island, 1000km northeast of New Zealand.
Scientists believed the tremors 10km-13km from Green Lake were caused by tectonic plate movement rather than magma moving under the crater.
GNS Science had asked for monitoring of volcanic activity to be stepped up after the earthquakes.
The eruption flattened trees, threw boulders into the air and buried the surrounding area in mud up to 5m deep. Mr Kearney's body was never found.
The expedition also collected material such as pumice thrown out by the volcanoes making up Raoul Island and Macauley Island, 120km southwest of Raoul, and the nearby Healy submarine volcano to test theories about the effects of water pressure on the explosiveness of the eruptions.
"The theory is that eruptions occurring in shallow water (such as around Macauley) are more violent because the dissolved gases in their magma expand violently in the low water pressure," said expedition leader and Niwa marine geologist Ian Wright.
"Analysis of the material thrown out by these three volcanoes will indicate how explosive their eruptions have been in the past.
"This work will help us understand how and when large hazardous eruptions might occur in the future."
The scientists also discovered two new submarine volcanoes near Raoul Island. Both had relatively small craters 4km in diameter - just under a fifth of Ruapehu's crater diameter - and one crater was very deep, 1000m from the rim to the crater floor. Both volcanoes appeared geologically young but laboratory analysis of sediments will be needed to decide their age.
"We wanted to get a better understanding of the processes of construction and destruction of submerged volcanoes to underpin assessments of the hazard risk they pose," said Dr Wright.
"We focused on Macauley and Raoul because they represent the more explosive and potentially destructive volcanoes in the Kermadec Arc [part of the Pacific's ring of fire]."
- NZPA
