Some volcanoes can blast sideways and kill without the usual seismic warning signals.


- A strange eruption almost killed two climbers on Mount Ruapehu, New Zealand.

- Geologists traced the eruption to a mineral seal that failed after it reached critical pressure.

- The volcanic seals can create dangers at volcanoes worldwide.

On the quiet moonlit night of Sept. 25, 2007, two climbers were bedding down in a hut atop New Zealand's Mount Ruapehu when the one climber who was still awake noticed a strange, low rumbling. He thought it was perhaps a snow avalanche. A few seconds later the door was blasted open by a sudden gale force wind. In the moonlight he could see the air was full of water and boulders.

"Everything was like a shotgun blast," said geologist Geoff Kilgour of GNS Science's Wairakei Research Centre in Taupo, New Zealand. Kilgour has studied the unusual eruption and the climbers' detailed accounts. He and his colleagues have published a paper on what probably caused the surprise eruption in the March issue of the Journal of Volcanology & Geothermal Research.

The climbers' hut was just 500 meters from a lake, which covered a volcanic vent which had just belched a lot of ashy gas into the lake, creating a mini tsunami and a rain of rocks.

"So as these guys were being inundated with water, they essentially started drowning in the acidic waters," Kilgour explained. One climber had about 10 seconds when he thought about how strange it was to be drowning at the top of a mountain. Only the collapse of the floor, which drained the water into the hut's basement, saved them, Kilgour said.

Then it stopped. The entire bizarre outburst lasted less than a minute. In that time part of a nearby ski area was destroyed and one of the climbers lost a leg that was trapped under a rock. But both men lived.

The eruption was strange, said Kilgour because it came without any seismic warning. Instead of rumbling a lot before exploding hot gas and ash skywards, it skipped a step and let loose a very peculiar sideways blast from its water-filled crater, without any warning at all.

But the eruption was not unique. Something very similar happened on March 17, 2006 on Raoul Island in the Kermadec chain. A sudden burst of gas and debris exploded from the crater called Green Lake, killing New Zealand Department of Conservation ranger Mark Kearney.

What both eruptions have in common is that they were very brief, violent and came with little or no warning. Both were, in fact, more like artillery barrages, with refrigerator-sized projectiles, according to witnesses and geologists who have studied the aftermaths.

New Zealand geologists now also suspect that both eruptions are the result of the same process in which the vent of a simmering volcano can get gradually sealed by minerals, then build up gas pressure until the seal suddenly and unexpectedly rips open like a very large, bottle of hot, boulder-loaded champagne.

What's more, there is no reason to believe this sealing process is exclusive to these two volcanoes, said geologist Bruce Christenson of the GNS Science National Isotope Center in Lower Hutt, New Zealand. Christenson originally developed the seal model to explain the deadly Raoul Island eruption.

"If the chemistry processes are focused appropriately, these things can form in pretty much any volcano-tectonic environment," said Christenson.

At Raoul Island the minerals that formed the seal were from sea water moving through the volcano-powered hydrothermal system at Green Lake. At Ruapehu it was elemental sulfur carried by hyper-acidic waters.

Regardless of the mineral making the seal, however, the final step is triggering the eruption.

"What serves as the trigger is not clear at this time," Christenson said. When the seal is near critical pressure, even the slightest pressure such as from seismic waves passing through could cause the seal to fail and the gas to shoot upwards.

"The real problem is that they are almost impossible to predict," said Kilgour. "By seeing this eruption and also looking at Raoul Island, it's raising a string of questions for a string of research that needs to be done."