article: Jan 16 2007 - Volcanic Activity in New Zealand 2006

A summary of New Zealand's volcanic activity in 2006.

In contrast to the last few years, New Zealand's volcanoes were more active in 2006 with small eruptions from two of them (Raoul Island and Ruapehu) and signs of unrest at one other (Ngauruhoe). The filling of crater lakes at White Island and Ruapehu has also been changing the potential hazards at these volcanoes.

The twelve active volcanoes in New Zealand are monitored by GNS Science through the EQC-funded GeoNet project (www.geonet.org.nz) with a variety of techniques. The most common and useful method is volcano-earthquake monitoring, along with water and gas chemistry. Ground deformation surveys and monitoring are also conducted at some of the volcanoes.

Although only a small eruption, the most significant event in 2006 occurred at Raoul Island on March 17 when a phreatic (steam and gas driven) eruption occurred at Green Lake. The most intense part of the eruption lasted for only 5 - 10 minutes, with the entire eruption only lasting about 30 minutes. No precursory changes had been recorded in the Green Lake area during the previous few days. There had been an earthquake swarm on March 13 - 14 about 10 - 13 km from Green Lake. Tragically a Department of Conservation employee, who was making measurements at Green Lake when the eruption occurred, is missing and presumed dead as a result of the eruption.

On Mount Ruapehu, a smaller hydrothermal eruption, which hardly escaped the active Crater Lake, occurred on October 4. The eruption generated waves within the lake basin that washed up to 5 m above the lake surface; this was not high enough to flow over the tephra dam and form a lahar. However the lake level rose about 1.3 m in response to the eruption. The crater lake started the year very warm, being 25 - 32ºC from January to June, before starting to cool, dropping to 15-16ºC in August-September. Following the small eruption in October the temperature rose to 22ºC and peaked at 27ºC in early November. Volcanic tremor and volcanic earthquakes were occasionally recorded at Ruapehu, with the strongest tremor occurring in late July.

Ngauruhoe volcano also started to show some signs of unrest this year, although no changes have been observed at the surface to date. There have been no eruptions here since 1977. In late-May 2006, a period of seismic unrest began with a several-fold increase in the rate of volcanic earthquake activity. On June 6, in response to the volcanic earthquakes, we raised the Scientific Alert Level from 0 to 1, signifying that there had been a departure from typical background activity and that there were consequently signs of volcano unrest. We further responded by installing three additional seismographs close to Ngauruhoe and by beginning a more intensive program of volcanic gas and temperature monitoring.

We have measured temperatures at fumaroles near the summit of Ngauruhoe and have collected gas samples for detailed analysis. We have also measured carbon dioxide release through the soil. Neither the fumarole temperatures nor the analysis of gas samples have shown any changes from data collected before the unrest began.

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, although eruptions do not always follow elevated earthquake activity.

New Zealand's other active volcano is White Island, which has not erupted since 2000. Following the eruptions in 2000 the active crater has cooled, allowing a crater lake to form and fill the crater. The water level has risen over 30 m and reached to within about 1 m of overflow in February 2006. Since August-September the lake temperature has risen from 45 - 50ºC to over 60ºC and the lake level has started to fall. The fall is due to evaporation of the lake now that it is so hot.

Only one significant hydrothermal eruption has happened in a geothermal system during 2006. That was at Kuirau Park in Rotorua, when spring 615 erupted on the evening of December 10. Mud, blocks of sinter and hot water were showed over the near by trees and reserve area. This event was much smaller than one in January 2001, which showered mud over the nearby hospital.