Poring over comprehensive, high-resolution satellite images, the scientists subsequently confirmed the presence of gigantic rafts of silicic pumice, covering an area spanning roughly 400 square kilometres. They traced the rafts back in time to their source on July 18, when there was also evidence of an atmospheric plume, emitted from a point source, and a thermal hotspot. As no pumice had been observed before July 18, this was taken to mean that the eruption – at a depth of more than 900 metres below sea level, and possibly up to 1500 metres down – had probably taken place over the course of less than one day. "We were very excited," says University of Tasmania earth scientist Rebecca Carey. "We had discovered evidence of what later turned out to be the largest subsea volcano in recorded history." Fathoms down In its own right, a submarine volcano is nothing new. Dotted across stretches of ocean floor, chains of volcanoes may extend for hundreds to thousands of kilometres. But most of these tend to be less than 1000 metres down.

Crucially, says Dr Carey, the tremendous weight of overlying water will act to suppress a volcano's explosivity. "So explosive eruptions from great depth, like the one at Havre, are rare," she says. The Havre volcano and surrounding ocean floor had been mapped in 2002; following the eruption, the floor was remapped. "Our survey found multiple new vents that ranged in depth between 700 and 1500 metres below sea level," Dr Carey explains. A preliminary estimate suggested that up to two cubic kilometres of volcanic pumice had been erupted – roughly equivalent to 1000 times the volume of the MCG stadium. "As the largest and deepest submarine silicic eruption ever documented, this was likely to be a 'once in a century' event," Dr Carey says. "It was likely twice the size of the renowned Mount St Helens eruption of 1980, and perhaps more than 10 times bigger than the 2010 Eyjafjallajokull eruption in Iceland." Expedition

The Havre discovery reveals that explosive eruptions at depths of at least 900 metres below sea level are possible. "Before this event, the deepest eruption of a viscous-magma type of volcano had been at 300 to 250 metres below sea level," Dr Carey says. "So this eruption is unique and an exciting opportunity for frontier science." To find out more about the mechanics of plate tectonics in general and volcanoes in particular, a research expedition to Havre, funded by the US National Science Foundation, is planned for March, 2015, explains Dr Carey, who has been appointed co-chief scientist on the expedition. She says a remotely operated vehicle and an autonomous underwater vehicle will map the eruption vents and products and collect samples. Plate tectonics Beneath the Earth’s relatively thin, rigid crust lies a mantle of solid crystalline rock. At a depth of more than 100 kilometres or so, the rock is so hot that it's malleable and squishy and can flow like plasticine.

Over aeons, the covering crust has broken into massive brittle sections called plates. They comprise seven main ones and several smaller crustal plates, all moving and shoving against each other as they slide across the hot, soft mantle. The plates are continuously colliding and being forced under one another, a process called subduction. Occasionally, they fracture, causing earthquakes. The melting of the sub-ducting crust provides magmas for volcanoes. Eruptions explained Volcanoes, such as the one at Havre, are the surface expression of magma "plumbing" systems that lie deep within the planet's crust. Most of the world's large volcanoes, occurring on plate boundaries, are fed by magmas from the Earth's upper mantle and lower crust. Magma originating from Earth's mantle gets stored in chambers several kilometres below the surface. Pressure builds as more magma enters the system, resulting in fracturing of the crust. Eventually the pressure gets too great and magma accelerates to the surface where it erupts.

That said, the eruption of magma deep beneath the ocean remains something of a mystery. "This is because many of the world's submarine volcanic arcs remain largely unexplored," Dr Carey says. "So this historic eruption, from what is called a caldera volcano, provides a rare opportunity to further our understanding of deep submarine volcanic processes." Caldera volcanoes are typically the source of the largest eruptions on Earth, she says. "They spew out vast quantities of very viscous magmas, which drain underlying magma reservoirs. The inward collapse of a reservoir can re-pressurise the remaining magma and boost explosivity and the rate at which magma discharges." Types Volcanoes come in several forms: active (erupting regularly, including today), dormant (no sign of activity for many years. but likely to erupt again) and extinct (no sign of activity for ages). Some experts say there should probably be a fourth category, "erupting", to indicate dangerous and currently ongoing volcanism.

The Kermadec Arc hosts numerous caldera volcanoes and is particularly active. Pumice from the eruption has been landing on Australian shores since April, 2013, says Dr Carey. "It has been slowly making its way down the Australian east coast and finally, in late March, it arrived in Tasmania." By the time it reached Tasmanian shores, some of the pumice had acquired a biological cargo of up to 80 marine "hitchhiker" species, including a colourful collection of barnacles, molluscs, corals, crabs, anemones and bristle worms. "As a result, the pumice is sometimes white, brown or black," Dr Carey explains. Watch-list Roughly 1900 or so of Earth's volcanoes are active. So which are the ones to watch? Volcanologists, who study the vents or fissures in the crust where molten lava erupts on to the surface, say there is evidence for magma accumulating beneath Yellowstone and Mount St Helens in the US, for example.

The last super-volcanic eruption occurred about 1200 years at Lake Taupo in New Zealand. Before that, Indonesia's Lake Toba erupted 73,000 years ago. Predictions Volcanologists use a volcano's eruption history to get some idea of when and how it might erupt again. The best way to predict this is through intensive monitoring of earthquake activity, changes in heat flow and distribution, the build-up of stresses in overlying rocks, groundwater temperatures and gas emissions, says Museum Victoria earth scientist Bill Birch. Although it is an inexact science, volcanology may gain some predictive power as a result of new research, reported in the British journal Nature.

It seems that the subterranean plumbing system opens up and allows a relatively rapid and efficient transfer of magma from the zone of production in the lower crust and upper mantle to the near-surface magma chambers beneath the volcano. The new work shows that this transfer of magma occurs on timescales of months, years or decades as opposed to the multi-centurial and multi-millennial scales envisaged by previous generations of geologists. "This means that continual and careful monitoring of ground surface shape and the background low-level seismic movements near dormant volcanoes might help pinpoint magma moving in the subsurface," Dr Birch says. Large volcanic eruptions on land are usually preceded by uplift of the ground-surface as magma from chambers tens of kilometres down is transferred to higher chambers, two or three kilometres beneath the surface. Volcanoes often signal their reawakening from a dormant state with steam blasts, and small eruptions of ash which increase in intensity over months to years. As the ground swells, the volcanic plumbing system usually unclogs as well. Steam blasts open up the conduits linking the near-surface magma chamber to the volcano.

Rhythmic earthquake activity, known as harmonic tremor, records magma movement in the Earth’s crust and often accompanies such events, Dr Birch explains. "These processes build-up and die-away and then build-up again, several or even tens of times before an eruption takes place." Links Learn more about the historic Havre eruption at: www.niwa.co.nz/news/first-sighting-of-volcano-responsible-for-undersea-eruption Read the Nature Communications article detailing the eruption research at: www.nature.com/ncomms/2014/140422/ncomms4660/full/ncomms4660.html Find out more about Dr Carey and her ground-shaking work at: http://www.utas.edu.au/earth-sciences/people/academic/rebecca-carey

Check out the work of Museum Victoria’s Dr Birch at: http://museumvictoria.com.au/collections-research/sciences/mineralogy-and-petrology/staff-and-students/william-bill-d-birch/ Learn more about volcanoes at IMAX Melbourne Museum, which periodically screens Forces of Nature, as part of its education programme. Details: http://imaxmelbourne.com.au/#/movie_56/synopsis/forces_of_nature/ Discover more about the world of submarine volcanoes at: http://pubs.usgs.gov/gip/volc/submarine.html A volcano to end all volcanoes might bring about the end of life, as we know it. Find out more about these scary scenarios in Bill McGuire's book, A Guide to the End of the World (Oxford University Press). VCAA links

VELS Levels 5 and 6 Science: http://vels.vcaa.vic.edu.au/vels/science.html Levels 5 and 5 Thinking Processes: http://vels.vcaa.vic.edu.au/vels/thinking.htm Please send bright ideas for new topics to pspinks@fairfaxmedia.com.au