Since mid-2008 the number of volcanic earthquakes close to Ngauruhoe has declined to the background level. Regular measurements of volcanic gas levels and the temperature of a summit gas vent have failed to record any significant changes over the last two and a half years.

“The reduction in earthquake activity means that an eruption in the near future is unlikely without further earthquakes or other changes and the appropriate alert level is therefore zero”, said GNS Science Volcano Section Manager Gill Jolly.

A significant increase in earthquake activity in May 2006 prompted the alert level to be raised to one (signs of volcano unrest). Over the next two years GeoNet recorded an average of 5 to 30 earthquakes a day close to Ngauruhoe, though the maximum daily number was as high as 80. “Since mid-2008 the number of earthquakes has returned to a typical level of a few per week”, said Brad Scott, GNS Science Volcano Surveillance Coordinator. “This no longer constitutes significant unrest”, he said.

The regular programme of gas and temperature measurements at Ngauruhoe was boosted as a result of the earthquakes starting in May 2006. However, these measurements have consistently shown only small amounts of volcanic gases (sulphur dioxide and carbon dioxide) and no changes in the temperature of the one notable steam vent in the summit area (typically about 86 °C).

GNS Science Volcano Seismologist Art Jolly said, “The earthquake data we collected during the unrest probably points to a geothermal rather than volcanic source for the events, and we hope it will help us better understand the next unrest episode at Ngauruhoe”.

“Although the unrest that started in May 2006 hasn’t resulted in an eruption, raising the alert level was consistent with the observed activity. It emphasised that Ngauruhoe remains an active volcano and everyone needs to be aware of this” said Brad Scott.

Steven Sherburn
Duty Volcanologist

Many new springs are forming on the Main Crater floor, and older springs are starting to flow again. These changes are similar to those observed during the previous lake filling (2004 - 2006). The Crater Lake temperature remains hot at 57 °C and the lake colour has also changed to a light green, reflecting little suspended sediment.

On the south side of the Main Crater floor, about half way between the lake and the sea, is an area of high temperature (101 - 103 °C) fumaroles. Steam, gas and mud emissions creating acid rain have increased from the largest vent in the last couple of weeks, and this will continue as the lake rises. Fumarole activity in this area has been increasing for several years. Activity has fluctuated in these features and they have varied between being ‘wet’ and ‘dry’ dependent on the water level in the Crater Lake.

Other monitoring, including the deformation surveys and soil gas mapping, have also shown changes about this fumarole area as it has heated and increased in activity. The current changes represent changes in the shallow geothermal system related to the rising water levels on the island.

White Island’s Crater Lake occupies a crater that formed during eruptions in the 1970s – 1990s. The last eruption at White Island, in 2000, occurred from a vent that is now beneath the lake.

No other significant changes have been observed at White Island.

The Volcanic Alert Level for White Island remains at Level 1; there is no significant change in volcanic activity on the island.

Steven Sherburn
Duty Volcanologist

Ongoing monitoring at Ruapehu, including a recent Crater Lake visit to measure the lake temperature and collect water samples, data from airborne gas measurements and seismic activity, have shown changes in the last month. These changes indicate that the period of lake cooling and lower gas flux is over.

The Crater Lake temperature has now increased to 22 °C, up from 16 °C in August. This temperature change was accompanied by some volcanic tremor. From October 2007 to June 2008 the lake temperature ranged between 34 to 37 °C before declining to a low of 16 °C in August 2008. The airborne gas plume measurements have shown a similar trend with SO₂ and CO₂ gas emissions being high while the lake was hot, decreasing when the lake cooled and now increasing again. Cyclic activity in the Crater Lake is very common at Ruapehu.

These observations are consistent with the volcano-hydrothermal system now being more open to the Crater Lake. This type of volcano unrest should result in the lake temperature continuing to rise and the lake colour changing as sediments on the lake floor are disturbed by the increased gas flow into the lake. Ruapehu remains an active volcano and future eruptions may occur without warning.

The Volcanic Alert Level remains at Level 1 (departure from typical background surface activity, signs of unrest).

Brad Scott
Volcano Surveillance Co-ordinator

Ongoing monitoring at Ruapehu, including a Crater Lake visit to measure the lake temperature and collect water samples, data from airborne gas measurements and seismic activity, have shown declines in the recent measurements. This indicates that the current period of elevated unrest is passing.

The Crater Lake temperature has now dropped to around 23 to 25 °C, having ranged from 34 to 37 °C since October 2007. The latest measurement was 23.3 °C (July 15). The airborne gas plume measurements which had shown significant increases after April 2008 now show that SO₂ and CO₂ gas emissions have decreased to the typical background levels for Ruapehu.

Results from the Crater Lake chemistry are also indicative of cooling in the vent area. The slight increase in the level of volcanic tremor we previously reported has also declined to typical background levels.

These observations are consistent with the volcano-hydrothermal system now cooling. The current phase of volcano unrest appears to be over, however Ruapehu remains an active volcano. Future eruptions may occur without warning.

The Volcanic Alert Level remains at Level 1 (departure from typical background surface activity, signs of unrest).

Brad Scott
Volcano Surveillance Co-ordinator

* Previously known as Science Alert Bulletin and Scientific Alert Level.