Sep 26 1995, 5:00 pm - Ruapehu Volcano

Situation Summary

As of 17:00 the following information is available:

• Seismic activity, including volcanic tremor, has remained at a relatively low level (though still significantly higher than that present last week before the current activity began). All our seismographs (including that at the Dome Shelter) are operating and providing valuable data.

• A field party of observers from the Institute visited the volcano between 06:00 and 07:00 hours and reported a plume drifting east-south-east being fed by numerous weak explosions. There has been continued minor ash fall overnight to the east of the volcano. The deposit of a very small lahar was observed in the Wahianoa Valley, and this is inferred to have been formed at some earlier stage in the eruption. There was a reduced level of lahars (when compared with yesterday) down the Whangaehu valley at that time.

• We have begun (weather permitting) measurements of the sulphur dioxide (SO₂) content of the plume using an instrument known as a COSPEC. A flight yesterday at about 16:00 determined the SO₂ discharge in the plume as 2600 tonnes per day (with a possible error of plus or minus 400 tonnes). This information confirms that there is significant involvement of magma in the eruption, but the amounts of SO₂ cannot yet be used to infer the amount of magma that might be present below the vent. This is because large amounts of sulphur were known to be present in the Crater Lake sediments and they may still be contributing significantly to the total SO₂ discharge. Further measurements will be carried out tomorrow (weather permitting).

• Observations during the day have been limited by the cloud cover. We would infer from the relatively low level of seismic activity that the explosions have continued to be modest in scale during the day, with a small plume. Lahars have continued in the Whangaehu valley, but at a reduced scale when compared with yesterday.

Conclusion and Alert Status

The eruption involving new magma is continuing at Ruapehu. Despite the lower intensity of activity, in the light of our observations and the seismic data available to us, we advise that the Alert Level for Ruapehu remains at Level 4. Any media reports to the contrary are incorrect and have not originated from the Volcanology Programme. We will review and consider the Alert Level each day at our morning and evening debriefings.

C.J.N. Wilson
Volcanology Programme Leader