Deep monster: Researchers have been studying the Havre volcano in the Kermadecs since it erupted again in July 2012. This image dates from an October 2012 voyage by Niwa.

When the Havre undersea volcano erupted in 2012, scientists were way ahead and slightly behind the game.

They were way ahead because a 2002 expedition to the Kermadec Arc northeast of New Zealand had collected good data on the Havre volcano, meaning researchers had a rare opportunity to compare its shape and composition before and after the eruption.

They were slightly behind because they didn't realise there had been an eruption about 800 kilometres north of Auckland for almost two weeks.

The July 2012 eruption was about 10 times bigger than the 2010 Eyjafjallajokull eruption in Iceland, which famously disrupted air travel across parts of Europe. Havre was probably the largest and deepest undersea volcano ever documented.

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Most of the world's undersea volcanos are unstudied, yet they comprise about 70 per cent of the Earth's magma output, according to Tasmanian volcanologist Rebecca Carey, who has led much of the post-eruption research into Havre.

The first clue that "something extraordinary had happened in the Kermadec Arc" came from a civilian flying between Tonga and Auckland. She noticed a "peculiar large mass floating on the ocean" on July 31, 2012.

She snapped photos and shared them with a volcanologist, who confirmed the mass was almost certainly pumice, a lightweight rock that is ejected from volcanos and can float. There were rafts of the pumice, which mostly floated away to the northeast but some of which eventually washed up in New Zealand and along the entire east coast of Australia including Tasmania.

The volcanologist alerted colleagues around the world that a volcano has almost certainly erupted in the Kermadecs, according to Carey,

Scientists with access to freely available satellite data soon spotted the telltale signs of an undersea eruption – not lava flung into the sky but an atmospheric plume and a thermal hot spot on the ocean's surface. Data from the Polynesian Seismic Network also pointed to the same broad location – Havre.

Niwa The Havre volcano is about 800kms north of Auckland.

The eruption must have lasted about a day, and was soon understood to be significant because of its great size (1.2 cubic kilometres) and depth – 700 metres to 1200m below sea level.

Few volcanoes at this depth have been studied because of the cost and often their remoteness. Previous research has typically been done on ancient, uplifted volcanos.

But with Havre, researchers such University of Otago PhD student Arran Murch, his supervisor James White and Carey from Tasmania had the 2002 data. This was supplemented by an October 2012 research voyage by Niwa that included underwater topography mapping and rock dredging.

There was a second visit to Havre, in October 2015, by an American research vessel that included a "near-bottom" topography survey by an autonomous underwater vehicle of the entire caldera and rim. In addition, they got 250 hours of data, including photos and video, from a remotely operated vehicle, plus samples from 290 locations across the site.

The wealth of new data showed the eruption was a complex and spread lava, ash and pumice over a wide area. Carey reported a year ago that giant pumice clasts​ (boulders) 1m to 9m across spread over 35 square kilometres.

Otago's Murch studied ash layers and found there must have been several events days or even weeks apart, including a lave dome that collapsed and spread ash.

Apart from increasing human knowledge, the research could have impacts for human safety. When lava contacts sea water, there can be an explosive effect. More importantly, understanding how water pressure suppresses eruptions could one day prove critical if one of these underwater monsters goes off.