Sep 24 1995 - Ruapehu Volcano

Situation Summary

The eruptions at Ruapehu have continued through the past 24 hours, presenting scientists from the Institute of Geological and Nuclear Sciences with a wealth of new data and information. The conclusions below are based on four types of observation over this period and over the past week:

1. The Wairakei Research Centre is recording types of seismic activity which have never previously been seen at Ruapehu. These include extended periods of low frequency volcanic tremor and some unusual low-frequency earthquakes.
2. Our data show that a larger component of fresh magma (molten rock) has been erupted last week, than in any eruption since 1945.
3. There are increased levels of magmatic gas, specifically SO₂, recorded as chemicals dissolved in the water samples we have recovered from Crater Lake.
4. There have been 3 lahar-producing events in 1 week, accompanied by a high frequency of other explosions through Crater Lake.

We thus conclude that there is a high probability of events of a similar size (and likely accompanying effects) to the lahar-forming eruptions of Monday 18 and Saturday 23 September occurring in the future, spaced at intervals that are irregular (and thus unpredictable) but which would typically be days apart. Smaller eruptions (but still of a size to pose extreme risk to people on the summit area of the volcano) may be expected at irregular intervals of minutes to hours. We emphasise that the exact timing of all these eruptive events are unpredictable.

Our data also imply that there is a significant possibility that the activity to date is an introduction to a larger eruption, perhaps of the size and type last seen in 1945.

On the basis of available information we thus advise that the Alert Level at Ruapehu remain at Level 3.

Daily Summary

During the past 24 hours, the volcano has shown a variety of activity:

1. Small explosions, sending steam-rich plumes to heights of a few hundreds of metres have continued sporadically throughout this time period. The effects of these explosions have generally been confined to the area of the crater itself, and they are not powerful enough to register on our seismometer at our Chateau seismograph. Nonetheless, they represent an extreme hazard to anyone in the vicinity of the crater lake.
2. Medium-sized explosions, which have been a particular feature of the activity from about 18:00 until nightfall. These are ejecting steam-rich but ash-bearing plumes which have risen to heights of 500 to over 1500 m above the volcano. These explosions leave a distinctive, modest seismic signature as recorded by us at the Chateau seismograph. At the time of writing, this activity is a continuing feature of the volcano.

No further lahar-producing eruptions have occurred in the past 24 hours.

We are continuing to monitor the situation and advise the Ministry of Civil Defence and Department of Conservation. We emphasise again that Ruapehu is an actively erupting volcano, and people should stay clear of the upper slopes of the volcano and valleys (particularly the Whangaehu) which may act as paths for lahars.

C.J.N. Wilson
Volcanology Programme Leader

B.J. Scott
Volcano Surveillance Manager