From: Andy Soos, ENN
Published May 15, 2012 12:24 PM

Collapsing Volcanos

The bottom of the sea to many must be perfectly calm and placid. Well maybe not so... Volcanoes on the seafloor can grow or collapse tens of meters in just a few days, a new study has found, suggesting that the seabed is much more unstable than previously thought. Researchers, led by Professor Tony Watts of Oxford University’s Department of Earth Sciences, report in Nature Geoscience how they surveyed the topography of the active Monowai volcano, a submarine volcano on the southwest Pacific Ocean floor near Tonga, in May and June 2011.

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The University of Oxford team used sonar to map the seafloor around Monowai near Tonga before leaving to study another area. Monowai is a volcanic sea mount to the north of New Zealand. It is one of the most active volcanoes in the Kermadec volcanic arc. The most recent eruptions were in 2008[2] and 2011.

The summit is approximately 433 feet below sea level, considerably above the level of the nearby Tonga and Kermadec Trenches. The summit's position and depth changed between 1998 and 2004, due to a landslide and eruptive regrowth.

A week later the researchers discovered that a seismic station on Rarotonga in the Cook Islands had detected an intense five-day period of seismic activity at Monowai beginning shortly after they left. When the team returned further sonar mapping of the seafloor around the volcano revealed that while some parts of it had collapsed by almost 19 meters elsewhere fresh eruptions of lava had raised the volcano’s height by up to 72 meters in other portions of the sea floor.

Because they are underwater, and so difficult to study, little is known about how submarine volcanoes evolve or the frequency with which they erupt and grow. This new research suggests that these volcanoes could be rapidly pulsating, making the seafloor environment much more dynamic than anyone suspected; a finding that has implications for understanding what causes tsunamis.

Submarine volcanoes are underwater fissures in the Earth's surface from which magma can erupt. They are estimated to account for 75% of annual magma output. The vast majority are located near areas of tectonic plate movement, known as ocean ridges. Although most are located in the depths of seas and oceans, some also exist in shallow water, which can spew material into the air during an eruption. Hydrothermal vents, sites of abundant biological activity, are commonly found near submarine volcanoes.

"I've spent my career studying the seabed and have generally thought it pretty stable so it's stunning to see so much change in such a short space of time," said Professor Tony Watts of Oxford University’s Department of Earth Sciences.

"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. 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."

A report of the research, entitled "Rapid rates of growth and collapse of Monowai submarine volcano in the Kermadec Arc", is published online in Nature Geoscience.

For further information see Collapsing Volcanoes.

Monowai image via University of Oxford.

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