While mapping the Rumble III volcano, they found a marked change in the shape of the summit. A map made in 2007 showed an 800m-wide crater near the top of the submarine volcano.

The new map shows the crater has been filled and the nearby summit cone has been reduced in height by about 100m.

"This suggests there has been a major eruption that collapsed the summit cone and filled the adjacent crater," said Co-Chief Scientist on the voyage Cornel de Ronde of GNS Science.

Images taken by an underwater camera towed by the research ship show strewn lava boulders covered by black volcanic ash near the summit of the volcano, Dr de Ronde said. Rumble III volcano is about 350km northeast of the Bay of Plenty and sits at a depth of 1.4km.

The eruption is consistent with the fact that a number of the 90 submarine volcanoes along the Kermadec Arc are highly active. Some of the volcanoes along this 2000km underwater volcanic chain are as big as Mt Ruapehu.

The Kermadec Arc runs northeast of New Zealand between the Bay of Plenty and Tonga.

The two-week voyage, a joint United States and New Zealand project, ends today with the ship berthing in Auckland. The ship, R/V Thomas G Thompson, is one of America's top ocean-going research ships and is owned and operated by the University of Washington.

The purpose of the voyage was to enable 13 final-year oceanographic students from the University of Washington and four students from Southampton University in England to complete their theses. It also offered the opportunity for scientists from the US and New Zealand to gather fresh information on submarine volcanoes.

At Brothers volcano, underwater equipment towed by the ship showed vigorous hydrothermal venting expelling high-temperature, mineral-rich plumes into the ocean.