Volcanic activity at Mt Tongariro remains low but the volcano is still emitting a significant amount of gas, GNS Science says.



The volcano erupted yesterday, for the second time this year, and scientists are now keeping watch on both Mt Tongariro and Mt Ruapehu, warning that further eruptions are expected.

The eruption at Tongariro's Te Maari crater about 1.30pm was thought to have sent a two-kilometre plume into the air but GNS Science confirmed today that the ejection was three or four kilometres high and lasted for about five minutes.



Mt Tongariro had lay dormant for more than 100 years until an eruption early on August 6.

Scientists today used an observation flight around the mountain to assess the extent of yesterday’s eruption.

The volcano was producing a lot of gas, but with no ash. The odour was noticed in the Manawatu and Hawke's Bay, the primary downwind areas of Te Maari, GNS Science said.



Depending on weather conditions people may notice the smell of sulphur 100km or more from the volcano. However, any health impact was likely to be limited to minor irritation of the skin and eyes.



Historical evidence shows that there may be more eruptions to come.



"Progressive pressure may build up over time and we think that's what's happening," GNS vulcanologist Nico Fournier said.



"At the moment we can't make any judgement calls but the overall opinion is if there is an eruption it is unlikely to be quite big ... the bigger the eruption the more signals you should get beforehand and we haven't seen that."



Signals included earthquakes, gases emitted and the ground swelling slightly.



"We haven't had any evidence of any of those at the moment," Fournier said.



Scientists only had those historical events to compare the recent activity to, and Fournier said that a series of eruptions in the late 1800s indicated that more eruptions were to follow.



Known eruptions at Tongariro occurred in 1869 and intermittently in the years between 1886 and 1897.



Also, research based on volcanic events around the world showed that eruptions usually came in a series, Scott said.



"When you get an eruption it can stop, but most of the time there's further eruptions over months or years."



The point of origin of yesterday's eruption was similar to that from August, but two currents were formed, GNS vulcanologists said in a statement.



These currents, called pyroclastic density currents, consisted of a mixture of ash, volcanic gas and atmospheric air.



The two small currents were formed to the west and north of the crater and were believed to have been created because the ash column did not have enough energy to rise, which meant it probably collapsed at the base and the currents flowed downhill.



"There is no evidence at this stage of big blocks having been ejected far from the crater during the eruption."



There was also no evidence of magma reaching the surface.



With Ruapehu also recording activity, there was still an increased likelihood of an eruption there too.



GNS took samples of Ruapehu's Crater Lake yesterday and hoped to have a better insight once results were in next week, Fournier said.



While New Zealand vulcanologists didn't usually have to keep a close eye on two volcanoes experiencing heightened activity at the same time, they weren't letting anything slip, he said.



"We need to make sure we don't miss anything at Ruapehu while Tongariro is putting on a show."



ERUPTION DISRUPTION



The Tongariro Alpine crossing would remain closed until the middle of next week, unless the Department of Conservation was advised otherwise by GNS, programme manager for communication relations, Kim Alexander-Turia, said.



The main huts on the mountain, including Oterere and Mangatopopo were also closed.



The Whakapapa track to Tama Lake remained open but visitors should inform DOC's Ruapehu office when they planned to be on the track, Alexander-Turia said.



Civil Defence has cancelled its national advisory regarding ash fall as no ash is coming from the volcano.



The agency did not have any further advice for people in the area other than what was on their website.



All roads in the area were open but police urged anyone driving nearby to be cautious - not so much of the volcano, but of other cars in the area.



Area Commander Inspector Steve Bullock expected traffic in the area to become heavier over the next few days as many people would want to take advantage of the clear conditions to visit.



"That is completely understandable and whilst we don't discourage people from now travelling to the area we are asking them to put road safety and their own safety before sightseeing."



People were urged to pull over to the side of the road to view the mountain rather than trying to view it while driving.



Some Air New Zealand flights cancelled this morning were expected to resume this afternoon, including flights in and out of Taupo and Gisborne.

- MICHELLE COOKE, MIKE WATSON, MARTY SHARPE, BELINDA FEEK and ANGELA CUMING