Rumblings under Tongariro

Scientists have raised the Volcano Alert Level at Mt Tongariro after a series of quakes beneath the volcanic complex.

GeoNet said typically Tongariro experienced two such quakes - magnitude 2.5 or less - a year, but there had been more than 20 since July 13.

In a Volcanic Alert Bulletin issued this afternoon, it said the sequence started on July 13 and declined for a period, before restarting on July 18 and increasing in number yesterday.

‘‘These indicate unrest at Tongariro and give reason to change the Volcanic Alert Level to level 1 [from 0] and the aviation colour code to yellow [from green].’’

‘‘These earthquakes are small (magnitudes <2.5) and have only been well recorded by a few of the seismometers in our permanent network. The earthquakes cluster in a zone between Emerald Crater and the Te Mari craters at 2-7km depth.

GeoNet said to better understand the significance of the quakes it was planning to put portable seismic recorders around the epicentres and conduct sampling of selected hot springs, crater lakes and fumaroles in the area.

It described Tongariro as a massive complex of volcanic cones and craters formed by eruptions from at least 12 vents over more than 275,000 years. Erosion during the last Ice Age had worn away what was once a substantial mountain into the hiking destination it was today.

Five eruptions had been recorded at Tongariro between 1855 to 1897, with some unconfirmed activity in 1926-27.

The Te Mari craters, about  2km east of Ketetahi hot springs on the north side of Mt Tongariro, were the last craters confirmed as active on Tongariro.

In 1869 a large eruption accompanied by an earthquake at Tongariro  formed the upper Te Mari Crater during an explosive eruption. Maori descriptions talked of "bright red flame through the smoke that would burst and fall like snow", GeoNet said.

In November 1892 Te Mari again belched forth an immense quantity of steam, mud and boulders. The ejected material rose 600 to 900 metres before rushing down the mountain side. The last eruption began in November 1896 and continued until the end of that year.

GNS Science volcanologist Brad Scott said the increased activity "just means that something has upset Tongariro, and some subterranean process is occurring".

"It may or may not develop into anything further," he said.

"It's a little bit unusual," but Tongariro was "an active volcano in an active geothermal system".

In the past 4000 years Tongariro had been completely dominated by activity at Ngauruhoe, which had built a 750-metre high cone in that time.

No seismicity was being seen at Ngauruhoe, which geologically was part of Ngauruhoe, although it  tended to be treated as a separate volcano because, given its height, that was how the public viewed it, Scott said.

Some seismic activity had been recorded at Tongariro in 2001 but it was not sustained. Then the alert level had remained at 0, although it had come close to being changed.

Other than that and the latest activity, nothing unusual had been seen at Tongariro since recording equipment was installed, probably around the late-1980s.

The weather looked to be on the side of the scientists', so it was hoped to be able to take the extra monitoring equipment in tomorrow.

Alert bulletins are published whenever volcanic activity changes significantly. Alert levels range from 0 to 5, with 1 indicating signs of volcanic unrest in a departure from typical background surface activity.

The four aviation colour codes start at green, meaning the volcano is normal, then progress to yellow when a volcano is showing signs of elevated unrest. As activity ramps up the colour code can be lifted to orange and finally red.

White Island and Ruapehu are also at alert level 1, but both have a green aviation code.