article: Jan 14 2008 - Volcanic Activity in New Zealand 2007

As in 2006 New Zealand’s volcanoes were again active in 2007 with a moderate to large eruption from Ruapehu and signs of unrest at two others (Ngauruhoe and White Island). The dam break event at Ruapehu was also a key event during the year.

Blast and ballistic fall field north of Crater Lake, and lahar flowing east.

The twelve active volcanoes in New Zealand are monitored by the GeoNet project using a variety of techniques. The most common and useful is volcano-earthquake monitoring, along with direct observation and water and gas chemistry. Ground deformation surveys are also conducted at some of the volcanoes.

A moderate to large eruption through the active Crater Lake at Mt Ruapehu on September 25 generated a blast and ballistic fall field to the north of the lake, across much of the summit area. It also generated lahars into two catchments draining off the summit of the volcano. The lahar to the north down the Whakapapa Valley reached into the skifield, while the floods down the Whangaehu Valley reached to just beyond the National Park Boundary. The Crater Lake started the year warm, ranging around 23 - 28 ºC cooling through the middle part of the year. The eruption occurred from a cool lake, but the lake has been heating since the eruption, reaching 37 ºC in December. Volcanic tremor and volcanic earthquakes are often recorded at Ruapehu, and were recorded through 2007. Following the September eruption the level of volcanic tremor increased and remained high at year's end.

In March, the tephra dam blocking the natural outlet of the Crater Lake failed during a storm and the much anticipated break-out lahar occurred. This created a unique opportunity for scientists to study a volcanic lahar and test contingency plans.

In late May 2006, a period of seismic unrest began at Ngauruhoe with a several-fold increase in the rate of volcanic earthquake activity. On June 6 2006, in response to the volcanic earthquakes, the Scientific Alert Level was raised from 0 to 1, signifying that there had been a departure from typical background activity and that there were consequently signs of volcano unrest. The signs of volcanic unrest have continued this year; however the program of volcanic gas and temperature monitoring has shown no changes to date. There have been no eruptions at Ngauruhoe since 1977. Volcanic earthquakes are considered a common precursor to volcanic eruptions, particularly when there is a significant increase in the number and/or size of events.

Also since 2000 we have also observed significant numbers of shallow volcanic earthquakes at Tongariro volcano without any other changes in monitored parameters and without an eruption.

New Zealand’s other active volcano is White Island, which has not erupted since 2000, when a scoria eruption occurred, marking the end of an eruption episode that ran from December 1975. Following the eruptions in 2000 the active crater has cooled, allowing a crater lake to form and fill the crater. The water level rose in this lake to reach within about 1 m of overflow in February 2006. Later that year the lake temperature rose to over 60 ºC and the lake level started to fall and by March 2007 the water level had receded 25 m. The fall is due to evaporation of the hot lake water. Through the remainder of the year the water level in the lake remained low.

Only two small hydrothermal eruptions occurred in Rotorua during 2007. The first on May 27 at the mouth of the Puarenga Stream, and a smaller one at the former site of Malfroy's geyser in the Government Gardens in late June. These were smaller than events last year.