Rising magma sparking earthquake swarms and superheating Tongariro's geothermal systems in recent weeks is the most likely cause of Monday's steam-driven eruption.

Scientists are still trying to determine the cause and precise location of a new vent in the Te Mari crater cluster on the northeastern flank of the volcano.

Although a steamy plume has been spotted, pinpointing the vent has been hampered by bad weather which obscured aerial photography of the eruption zone yesterday.

GNS Science vulcanologist Nico Fournier said volcanic gas and periodic swarms of about 100 earthquakes had been recorded since scientists first noticed tremors on July 13.

Tongariro had experienced only one or two shakes a year in the past decade.

The earthquakes were centred two to seven kilometres beneath the mountain and had probably been triggered by magma "looking to find a way out".

Colleague Brad Scott said that, although scientists were aware of unrest in the past month, there were no typical warning signs immediately before the eruption.

"The volcano's been putting its hand up saying, 'Pay me attention, I'm upset'.

"But it didn't suggest it was going to pop. That's where the surprise came from."

Canterbury University hazard and disaster management lecturer Thomas Wilson said the volcano's hydrothermal system had probably exploded in a sequence known as a phreatic eruption.

"It's existing rock and material underneath the volcano that are fragmented and erupted. So in a purely steam-driven eruption, there's usually no new magma."

Two other types of eruption had yet to be ruled out - a phreatomagmatic eruption, where water and molten rock mixed, causing the magma to fragment in an explosive eruption; and a purely magmatic eruption, involving no water.

Dr Wilson said the rumbling could either stop completely, continue with same-size eruptions, or it could be the start of a "bigger eruptive sequence, which would probably mean larger eruptions, with more ash produced, probably leading to more widespread deposition of ash, to a thicker amount".

Seismic data shows the eruption lasted for up to two minutes, followed by a series of discrete small earthquakes in the next few tens of minutes, Mr Scott said.

The most likely cause was rising molten rock 1km to 5km beneath ground level heating and destabilising the hydrothermal system.

Hydrothermal eruptions tended to recur and could escalate into larger, energetic molten eruptions which spewed out lava and rocks.

Lack of rain, the small size of the eruption and mild ash fall meant there was no lahar.

ASH CAN PRODUCE HEALTH RISKS

Inhaling tiny shards of razor-sharp glass is one of the biggest dangers to public health from volcanic ash.

Ash is usually non-toxic and its effects are usually restricted to the tiny ground-up pieces of rock aggravating pre-existing respiratory conditions, according to GNS vulcanologist Graham Leonard.

Much of the ash cloud produced during Monday night's eruption had subsided yesterday, but civil defence services advised people to stay indoors and cover eyes and mouths if ash began to fall again.

The sandy ash was abrasive and would damage car windscreens if the wipers were turned on, Dr Leonard said. Vehicles, including air filters and radiators, should be regularly cleaned to flush out the ash.

GNS Science was testing overnight the components of a couple of ash samples collected around the northern side of the volcano.

Scientists would also look at the soluble chemicals that came out with the ash, including toxic chemicals such as fluorine. It was fluorine that killed stock around the site of the 1996 Mt Ruapehu eruption, GNS vulcanologist Michael Rosenberg said. "That can be very toxic to people and livestock."

However, Dr Leonard said that ash usually made drinking water too cloudy for both animals and humans to want to drink it well before it actually made the water at all toxic.

Rainfall about the time of the latest eruption meant there was less of a respiratory hazard.

People reported smelling sulphuric acids in the Tongariro area, but the volcanic gases were merely a nuisance and discomfort.

The Business, Innovation and Employment Ministry sent guidelines to businesses situated in areas of ash fall.

While short-term exposure to ash was not known to pose a significant health hazard, staff working in ash-filled air could face some risks, it said.

Short-term abrasion, inflammation, and irritation injuries to eyes and the respiratory system meant workers should be protected with dust masks or respiratory equipment.

A HISTORY OF VOLCANIC ACTIVITY

Mt Tongariro has a well-documented history of volcanic activity, GNS vulcanologist Michael Rosenberg says.

Eruptions occurred in 1869, 1892, 1896 and 1897.

Newspaper articles from the 1800s describe previous eruptions in the area.

"The scene was continually changing, the most interesting outbursts being best seen from Taupo," a 1896 newspaper clipping from Hawera & Normanby Star said.

"One day it was Ketetahi, next it would be Te Mare [sic], while Ngauruhoe sent up a fairly regular quantity of white steam, the other two being erratic."

Another from 1897 describes a "state of desolation, strewn with large quantities of mud, stone and sand".

"The eruption had formed a new crater higher up the mountain, blowing up some of the overhanging cliffs.

"Rocks estimated at about 4 tons weight had been hurled up, ascending 600ft above the mouth of the crater and falling over half a mile distant. The old crater has been filled up with debris, but it continues to steam. Liquid mud has flowed into Lake Rotoaira," the Evening Post report said.

A Bay of Plenty Times story from 1892 said the eruption did not appear to damage the mountain, but had ejected "an immense quantity of stones, ashes, etc . . .

"The sight is described as being terribly grand."

There were 10 to 20 craters in the area that had been active in the past 10,000 years, Mr Rosenberg said.

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