Marine geologists investigating the past behaviour and hazard risk of volcanoes in the Kermadec Arc, northeast of the Bay of Plenty, have discovered two new submarine volcanoes near Raoul Island.

The scientists returned on May 23 from a successful collaborative expedition mounted by the National Institute of Water & Atmospheric Research (NIWA) and the University of Auckland. The 23–day voyage onboard NIWA’s deepwater research vessel Tangaroa investigated volcanoes on the two largest Kermadec Islands – Raoul and Macauley – and their submerged flanks.

At Raoul, two new submerged caldera volcanoes were found. Both volcanoes have relatively small craters, some 4 km in diameter (just under a fifth of Ruapehu’s crater diameter and half that of Wellington Harbour). One crater is very deep, measuring about 1000 m from the rim to the crater floor. Both volcanoes appear geologically young – in the order of thousands of years old – but laboratory analysis of sediments will be needed to properly assess 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’, says NIWA marine geologist Dr Ian Wright, who led the voyage. ‘We focussed on Macauley and Raoul because they represent the more explosive and potentially destructive volcanoes in the Kermadec Arc.’

The Kermadec Arc, on the Pacific Ring of Fire, has been the source of vigorous and explosive volcanic activity over the past several thousand years. Its most northerly island, Raoul, hit the headlines in March 2006, when it erupted suddenly.

‘Macauley and Raoul are interesting because at times their magma has been rich in silica and volatiles, making them prone to violently erupt and form calderas, posing significant hazards’, says volcanologist Professor Colin Wilson, who led the Auckland University team. ‘Scientists are finding that these explosive types of submarine volcanic eruptions are more common than previously expected.’

To get a picture of the volcanoes’ past eruption histories – including the age, size, and direction of the eruptions – the 20–member expedition took sediment samples and mapped the contours of the volcanoes both above and below sea level (the latter using sophisticated multibeam sonar technology).

A series of sediment cores taken from east and west of both islands revealed at least six eruptions from the two islands, recorded as centimetre thick layers up to 100 km from the islands.

At Macauley, new seafloor mapping has identified very large volcanic debris flows with waves of sediment up to 50 m in height near the island. These travelled 50 km or more along the seafloor from a very large eruption 6 300 years ago.

The University of Auckland team working on Raoul Island for a week have mapped inside the Raoul crater following the 2006 eruption. ‘This will give us clues as to how the eruption occurred’, says Professor Wilson.

The expedition also collected material (mostly pumice) thrown out by the volcanoes making up Raoul and Macauley Islands, and 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,’ explains Dr 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.’