Residents in 'state of readiness'
A number of commercial helicopters have this morning hovered above the Te Maari crater on Mt Tongariro - now lying in relative calm after belching into life earlier this week.
The helicopters have been ferrying a number of media and scientists to the crater throughout the morning with most reporting little or no action from the site.
But the surrounding area shows the battle scars of Monday night's eruption with volcanic rock and ash clearly visible down to ground level.
Residents around the base of Mt Tongariro remain in a "state of readiness" should the crater suddenly burst into life.
But the small scale eruption has not deterred thrillseekers heading up to Ruapehu ski-fields.
They included young Dannevirke couple Phillip and Amanda Naus who are on a week's holiday in the area with friend Marcel Henman from Wellington.
"To be honest we are not sure what all the fuss is about," Mr Naus said. "We can't see any issues around the ski-fields and we have been staying in Kiritau around by Turangi and we never even heard the bloody thing on Monday night," Mr Naus said.
"I honestly think its a big drama over nothing," he said.
The first the trio heard of the eruption was a text message from the other side of the world.
"My father is in America and he text me to see if we were all ok - how weird is that," Mrs Naus said.
"We haven't seen any ash, we haven't even smelt the sulphur - so how are they suddenly smelling it down in Wellington when we are right here," said Mr Henman.
Tourists like Hiro Yashima from Japan were also heading to the skifields, unphased by the acitivity reported on the neighbouring peak.
"I heard that the mountain went boom, so I ready with my camera in case it go boom again," he smiled.
Mt Tongariro has remained calm overnight as some residents surrounding the mountain spent a tense night back in their homes.
Others returned to emergency accommodation set up on a Turangi Marae which would remain accessible for residents over the coming days.
Heavy rain fell overnight dampening down the ash from Monday night's eruption, however the clean-up will continue in earnest today as rooftops, cars and properties are cleared of the corrosive substance.
GNS scientists were hoping to fly to the top to Tongariro today to get a closer look at the damage and cause of the eruption, however consistent rain and low cloud is hampering those efforts.
SUPLPHUR SMELL IN WELLINGTON
Gas and sulphur smells from the Mt Tongariro eruption have travelled as far as Wellington this morning.
The crater spewed rocks and ash when it came to life for the first time in more than 100 years at 11.50pm on Monday night.
Many people have reported a smell of sulphur in the air in the capital.
GNS Science duty volcanologist Craig Miller said it was most likely the smells were caused by the eruption and there had been quite a few reports from the lower North Island of a sulphur smell.
"It's a little bit surprising it has gone as far as it has. It will be dissipating as it goes."
Hutt Valley resident Heidi Parker said she thought the smell was just her damp front yard at first but she could smell it all the way to work in Newtown. She said her colleagues had noticed it too.
Titahi Bay resident Robert Gibson said he and his colleagues at work had noticed the smell and were using air freshener to deal with it.
"It was a sulphury sewage-like smell. I thought my drains were blocked at first."
MetService forecaster Micky Malivuk said wind conditions were right for the smell to travel south.
Northerly winds of between 20kmh and 50kmh had been prevailing since yesterday afternoon.
EARTH SHAKES BENEATH VOLCANO
There has been little activity at Mt Tongariro since it rumbled to life on Monday night, but the earth has been shaking deep underneath it.
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.
There has been no volcanic activity since Monday night's eruption, but there were a few small earthquakes in the area overnight, GNS vulcanologist Craig Miller said.
Scientists yesterday warned that a sequence of bigger eruptions was possible.
GNS scientists were hoping to observe the crater by air this morning, and obtain airborne gas samples. However, heavy rain and low cloud in the area may hamper their efforts.
If no more eruptions occurred then they would hope to be able to get to the crater within a few days or weeks, Miller said.
They have already collected ash samples, which would be tested to determine whether magma, or molten rock, broke through to the surface.
Meanwhile, some residents surrounding the mountain spent a tense night back in their homes.
Others returned to emergency accommodation set up on a Turangi Marae, which would remain open for residents over the coming days.
DETERMINING WHAT HAPPENED
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.
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".
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.
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".
The most likely cause for the eruption 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, 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, 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, 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, Rosenberg said.
- Shabnam Dastgheib, Matt Stewart, Michelle Cooke, Jody O'Callaghan and Maryanne Twentyman