Over The Horizon

A Fiery Birth

Two New Zealand scientists witnessed the dramatic birth of a new volcanic island near the Solomon Islands. The rare observation was made during an investigation of seafloor volcanic activity and associated mineral formation in the Bismark and Solomon seas.

Marine geochemist Gary Massoth and mineral geologist Cornel de Ronde, both of the Institute of Geological & Nuclear Sciences, were part of an international team on the CSIRO research ship Franklin who made a comprehensive study of the island-building activity.

The scientists found the shallow Kavachi seamount, which had been dormant for nine years, had started a new phase of eruptive activity. Kavachi, in the Solomon Island chain of volcanoes, is only 30km from the boundary of the Indian and Australian tectonic plates.

A roughly conical feature rising from a seafloor depth of 1100 metres, Kavachi is about eight kilometres in diameter at its base and has produced ephemeral islands at least twice in the past century.

"When we arrived at Kavachi, we found violent eruptions taking place every five minutes," Massoth says. "The eruptions were ejecting molten lava up to 70 metres above sea level, and sulphurous steam plumes rose to about 500 metres. At night we were treated to a spectacular fireworks display with the red glow of eruptions continuing."

The peak of the volcano was forming a sandy ashen beach two metres below sea level with regular violent bomb-like eruptions.

The ship approached to within 750 metres of the eruption centre and found that the volcano had grown substantially since it was last surveyed in 1984. The scientists were able to sample freshly formed volcanic rocks from the volcano's flanks.

"This was an unprecedented opportunity and has given us valuable geological information. We also systematically sampled gases and seawater at various depths around the perimeter of the volcano -- something that has not been achieved before with an erupting submarine volcano.

"We detected particle and chemical plumes from the eruption at least five kilometres from the centre of the volcano. This has provided valuable information about the impact of active volcanoes on ocean chemistry."

Massoth says Kavachi differed from Brothers volcano, the largest and most active submarine volcano north east of White Island, in that Brothers was deeper and hydrothermally active while Kavachi was shallow and volcanically active.

Hot rock, or lava, predominates at Kavachi, while hot water predominates at Brothers. Hydrothermal fluids were venting from Brothers volcano at about 300oC against 100oC at Kavachi. Hotter fluids react with the volcano host rocks more efficiently and are more heavily laden with dissolved minerals.

Observations at Kavachi showed that lava being quickly quenched in seawater did not produce a strong chemical plume in the ocean, unlike the active volcanoes northeast of White Island which vent large volumes of hydrothermal fluids and heat into the ocean.

"Kavachi has confirmed our observations that forearc volcano chains, such as the Kermadec chain north east of White Island, contribute significantly to the global inventory of heat and chemical emissions entering the oceans.

"The work we have been doing in New Zealand waters is effectively re-writing the textbook on submarine volcanism."

About 80% of the world's volcanism occurred in the ocean and only a small proportion of all submarine volcanoes had been systematically surveyed with scientific equipment, Massoth says.

At another location, the scientists dredged up what they believe is a world-record size "black smoker" -- a 2.7m-high chimney prised from an active volcanic vent at a depth of 1,700m. Black smoker chimneys are packed with minerals -- typically 1000 to 10,000 more concentrated than background levels in seawater. The chimney is expected to be rich in silver, zinc and gold, Massoth says.