Explosive undersea eruptions tracked

6 November 2008

The vast majority of the world's volcanoes erupt and collapse out of sight beneath the oceans.

Volcano map

A seafloor map showing the peak of the Monowai volcano in 1998

Perhaps the most active volcanic region on the planet is the 'ring of fire' around the edge of the Pacific Ocean.

Mount St Helens in the US and the Andean volcanoes of South America form part of this chain and are well monitored. Working out what is happening below the ocean surface is more challenging, but an international team of oceanographers have done just that. They have produced a sequence of seafloor maps going back almost a decade around the Monowai submarine volcano in the Pacific.

The researchers found the height of the volcano's summit changed by over 100 metres during this time. This is the largest depth change of a sub-sea volcano yet recorded.

Map of volcano

The Monowai volcano. A large flank of the cone fell away in 2004 following an explosive eruption. The crater in the background is around 10km in diameter.

Monowai is a cone-like volcano, about the same size of Mount St Helens, 1500 kilometres north-east of New Zealand. It rises from a depth of 1400m and makes up part of a chain of volcanoes in the region called the Kermadec arc.

The oceanographers found that the depth of the summit, which was just 40m below the surface in 1998, dropped to 130m below in 2004, then grew to less than 69m below the surface in 2007. This indicates the build-up and removal of significant volumes of material.

But depth changes on the upper slope of the volcano were even more interesting. Between 1998 and 2004, an 85,000,000 cubic metre section of the south-east facing slope collapsed causing the depth to drop 176 metres and leaving a vast scar. During this time the west side of the summit rose 51m, probably due to debris from an eruption.

The team, including Professor Ian Wright from the National Oceanography Centre, Southampton, based its work on three surveys taken in 1998, 2004 and 2007.

Map of site

The Kermadec arc north-east of New Zealand

"Monowai is going through a prolonged eruption cycle," says Wright, who undertook the surveys when he was working in New Zealand. "It has been erupting almost continuously for the last ten years."

"If the peak of the volcano was in deeper water, say 3000m instead of 300m, the hydrostatic pressure would suppress these explosive eruptions. We would get more effusive eruptions that are inherently more stable."

Monowai is a large volcano - its crater is about nine kilometres in diameter. It has been one of the most active volcanoes in the Kermadec arc in the last 30 years. With the summit so close to the surface, the sea above it bubbles violently during eruptions. Fishermen have reported steam rising from the surface and sulfur slicks.

The new research, published in the American Geophysical Union journal Geochemistry Geophysics Geosystems, is the first attempt to measure exactly what is happening beneath the waves.

It has been erupting almost continuously for the last ten years.

- Professor Ian Wright, National Oceanography Centre

The changes in altitude correspond to seismic activity detected by monitoring stations run by the Polynesian Seismic Network. The network indicated that between 1998 and 2004 there were nine periods of intense seismic activity. One particularly powerful and unusual spike in the data probably marks the massive collapse on the south-east slope.

It seems clear that the volcanic debris from eruptions builds up, causing the summit to rise. Eventually the upper slopes get too steep - this is known as volcanic loading - and become critically unstable, leading to collapse.

This cycle of events is common in volcanoes above sea level. But more often spans hundreds or thousands of years rather than the more rapid cycles observed here.

High-grade ores

The surprisingly short cycle period is in part, according to the team, caused by the high number of eruptions combined with the slope materials being less stable, leading to smaller, but more frequent slope collapses.

The knowledge that slope build-up and collapse may be regular at continuously active submarine volcanoes may be crucial in the future prediction and detection of tsunamis.

But there is another reason for charting these undersea volcanoes. Some contain important deposits - possibly the size of Wembley Stadium - of high-grade ores, including copper, zinc, gold and silver.

Wright and his colleagues are beginning to use geophysical techniques to image these deposits. Commercial mining in the oceans has not been tried before, though a mining company planning the world's first sub-sea sulfide mining hopes to start production in the Pacific in 2010. Any research would also need to investigate the ecological and biological effects of such extraction.