Damaged chopper shows power of eruptive current that blasted White Island

A picture posted to social media shows the 1.2 tonne Eurocopter AS350 Squirrel helicopter blown off a landing pad on the island, but still upright. Photo / Supplied

KEY POINTS:

• Six people now confirmed dead, grim task of autopsies and formal identification continues today
• Eight people remain missing on the island, presumed dead
• 30 are in hospital - 24 are in four regional burns units and the other six will be transferred as soon as possible
• Three patients have been discharged
• 47 people were on or near the island when the volcano erupted on Monday afternoon - 24 were Australian, nine were from the United States, five were Kiwis, four were Germans, two were Chinese and one was Malaysian

A damaged, ash-caked helicopter with buckled rotor blades has highlighted the "incredible force" of an eruptive current that swept across White Island.

A picture posted to social media showed the Volcanic Air helicopter blown off a landing pad on the island, but still upright.

The helicopter is a single-engine Eurocopter AS350 Squirrel, weighing around 1.2 tonnes.

"It's a fairly heavy chopper, so it's taken some incredible force to move it off the platform," aviation commentator Peter Clark said.

He expected the blast from the eruption had been so powerful it had snapped the machine's main rotor blades at their weakest sections in the middle.

University of Auckland volcanologist Professor Phil Shane presumed a "hurricane-style" current had struck the helicopter side-on.

"The windscreen is not broken and the body appears to still be in good condition, except for being smeared in ash – so I'd expect it was a strong, sideways current that has come straight out of the crater.

"And I do understand the eruption involved a strong, lateral blast."

Impossible to predict

Despite their devastating force – as dramatically shown at the Bay of Plenty volcano – hydrothermal, or phreatic, eruptions are almost impossible to predict.

At White Island, hydrothermal explosions triggered by super-heated steam and gas could be triggered without warning.

Often, this steam and gas built up behind a rock and mineral seal, and when the strength of that seal was exceeded by the gas pressure, an explosive eruption was unleashed.

The gas driving the short-lived eruption likely came from a deeper source of magma, but the magma itself was not thought to have been directly involved.

Regardless, the result was lethal.

"The expansion of water into steam is supersonic in speed - and the liquid can expand to 1,700 times its original volume," University of Auckland volcanologist Professor Shane Cronin said.

There was evidence to show that more than 60 of these blasts had occurred at New Zealand volcanoes within the last century.

Along with White Island, recent examples included the 2012 blows at Mt Tongariro's Te Maari Crater; the 2007 explosion at Mt Ruapehu that cost Auckland man William Pike his right leg; and a 2006 eruption on Raoul Island that killed conservation worker Mark Kearney.

"These are all volcanic systems that can produce sudden, steam-burst eruptions – and they're some of the most dangerous ones," Cronin said.