Volcano grows at astounding rate

19:25, May 16 2012
Monowai volcano
AWESOME FORCES: A still from sonar images showing dramatic changes on the Monowai volcano. <b><a target=_blank href="http://www.bbc.co.uk/news/science-environment-18040658#?utm_medium=referral&utm_source=pulsenews">Watch animation on BBC</a></b>

An astounding pulse of destruction and growth at an underwater volcano north of New Zealand has provided a new insight into the behaviour of submarine mountains.

The Monowai seamount, which lies at the intersection of the Pacific and Indo-Australian tectonic plates at the Tonga-Kermadec subduction zone, underwent one of the fastest episodes of volcano growth documented on Earth.

It added about 8.75 million cubic metres of rock to its summit - a volume equal to 3500 Olympic-size swimming pools - in just five days.

New lava flows raised that area by 79.1 metres, while part of the volcano's summit collapsed by as much as 18.8 metres. Most striking was the creation of an entirely new volcanic cone.

The changes were measured by crew aboard the research vessel Sonne who had set out on a routine mapping expedition in the South Pacific last autumn.

Their findings have been published online in the journal Nature Geoscience. The article's authors include GNS Science geologist and geochemist Cornel de Ronde.

As they surveyed the seafloor near Monowai seamount in mid-May 2011 the crew noticed yellow-green water and gas bubbles rising above the volcano.

As the ship was leaving the area, near Tonga, it went through a patch of discoloured water with a strong smell, like rotten eggs, Oxford University geologist Anthony Watts said.

While the ship was surveying another area, a seismic station in the Cook Islands detected an intense five-day swarm of seismic activity and traced it to an eruption at Monowai seamount, MSNBC reported.

Watts and the ship returned in early June to find that part of the Monowai volcano had collapsed and another part had grown in dramatic fashion.

The new material lifting the peak was most likely magma that had erupted and hardened the week before, Watts, who led the study, said.

The rapid changes at Monowai suggest that the volcano grows and collapses in dramatic pulses.

Submarine volcanoes such as Monowai were much more difficult to study than volcanoes on land. Because so little was known about submarine volcanoes, it was unclear whether others also grew in rapid pulses, Watts said.

Had the ship been in the area of the volcano at the time of the eruption it could have been potentially dangerous, Watts told the BBC.

"If we had been over the volcano during the eruption, rocks could have hit the hull of the ship."

The researchers believe the changes are larger than at most other volcanoes. Only Vesuvius and Mount St Helens have recorded larger growth rates.

The paper said the speed of growth and change was "a reminder of how rapidly geological processes such as submarine landsliding and volcanism can occur".

"Any movement on the seabed has the potential to create a tsunami. An earthquake suddenly dislocates the seabed. Here a violent disturbance lasted five days with magma oozing out which might be too slow to trigger a tsunami - but it's unknown," Watts said.

"This is a violent exchange of rock into the water - it could destabilise the cone and cause a landslide which in principle could cause a tsunami.''

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