Increased Activity at Mt. Ruapehu

GNS Science issued an alert bulletin for Ruapehu volcano yesterday, the 23rd of April 2008, reporting increased activity.

Mt Ruapehu last erupted on the 25th of September 2007. On that occasion a single hydrothermal eruption of 7 minutes’ duration occurred, and an eruption earthquake of magnitude 2.9 was recorded. Material was thrown up to 1½ kilometres from the crater, and a lahar flow was reported down the Whakapapa skifield and down the Whangaehu catchment. Roads around the mountain were closed as a precautionary measure, but re-opened before midnight. One climber was seriously injured when a volcanic missile plunged through the roof of Dome Hut.

Further investigation showed that the hydrothermal eruption ejected about 500,000 cubic metres of water, lake floor sediment and blocks up to a metre across from the crater lake. The eruption caused two small lahar flows, one into the top of the Whakapapa skifield, the other down the Whangaehu Valley. The eruption was similar to events in 1969, 1975, 1988 and 2006.

A volcanic earthquake at 11:05 p.m. on September 29th was shorter than that recorded during the eruption, lasting only 3 minutes. Two further periods of weak volcanic tremor of 5-10 minutes’ duration followed later the same night.

Following the eruption, the temperature of the crater lake rose by 20 ºC and peaked at 37.5 ºC on the 21st of December 2007. Since then it has remained above normal, fluctuating between 34 ºC and 36.8 ºC. The chemistry of the hot crater lake has also changed since the September eruption. Scientists believe that temperatures are increasing deep within the hydrothermal system, and the source of the heat is magma (molten rock).

This is supported by an increase in the levels of various gases measured at the volcano. Sulphur dioxide has increased since the September eruption and carbon dioxide, which has varied, has shown a recent increase. Hydrogen sulphide gas has become measurable since March 2008.

There has also been an increase in the background level of volcanic tremor and some short periods of stronger volcanic tremor have been noticed. But Brad Scott, Volcano Surveillance Co-ordinator at GNS Science, points out, “volcanic tremor and changes in the level of tremor are very common at Ruapehu. It is not clear if these changes are signs that further eruptions will occur. If further eruptions do occur, they may occur without warning.”

The Alert Level for Ruapehu volcano remains at 1 (signs of unrest).

[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST.]

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