Underwater volcano ready to blow

The US Geological Survey’s Hawai’ian Volcano Observatory has been keeping a close eye on an undersea volcano in the Northern Marianas that has been showing signs of activity in recent days.

According to a weekly summary issued by the observatory, recent seismic activity “almost certainly heralds an eruption”.

According to the US Geological Survey report, seismometres on the islands are recording high levels of seismicity and the most likely source of the volcanic activity is the Ahyi seamount, which is about 20km southeast of Farallon de Pajaros.

Ahyi rises to within 64 m (210 ft) of the ocean surface and is associated with several reports of possible eruptions in historical times, the most recent in 2001.

“The seismic signals almost certainly herald an eruption. In fact, submarine explosions were heard by scuba divers who are conducting coral reef research in the area,” the report says.

“The divers even felt the shock waves from the explosions, and one of the most powerful ones reverberated through the hull of the NOAA base ship, Hi’ialakai, leading the crew to think something had happened to the ship. Shipboard personnel also reported a large sulfur slick on the southeast coastline of Farallon de Pajaros.”

The CNMI emergency management office and the volcano observatories of the USGS have been working together for over 30 years to establish volcano monitoring networks and assess volcanic hazards in the Mariana Islands.

Interspersed among the volcanoes that rise above sea level to form the Northern Marianas arc are many submarine volcanoes.

These arcs, such as the Aleutians and the Japanese archipelago, are formed at subduction zones—boundaries where one tectonic plate plunges beneath another.

Reports of discoloured water throughout the Mariana arc are common, indicating that the area may experience frequent submarine eruptions.

A dramatic, recent example is the 2010 eruption of South Sarigan seamount, which sent an eruption plume up to 12km above sea level.

The plume intersected many commercial air traffic routes, raising concern that the abrasive ash fragments could damage aircraft or even stall their engines.

In addition, the eruption posed a potential hazard to ocean-going vessels, as it produced a large area of discoloured water, possibly including a raft of pumice – a type of rock that can be produced in explosive volcanic eruptions.

A recent submarine eruption of Havre seamount north of New Zealand in 2012 created a 20,000 square-kilometre raft of pumice – about twice the area of the island of Hawai’i – that eventually spread to about 4 million square-kilometres as it broke up.

The USGS says it is possible, but not certain, that the current unrest near Ahyi seamount will escalate into a vigorous eruption, with the creation of pumice rafts, and even an explosive eruption column rising above sea level.

If this happens, there are possible threats of local tsunami sand ash fallout from the eruption plume.