Oct 20 1995, 10:00 am - Ruapehu Volcano

Science Alert Bulletin RUA-1995/50 - Update

Situation Summary

As of 10:00, the following information is available:

  • Tremor levels have remained low at Ruapehu since the last bulletin at 17:00 on 19 October. The number of degassing eruption type events (without sudden onsets) has declined in size and frequency since about 00:00 last night. The volcano has been seismically quiet this morning.
  • A brown gas plume from Ruapehu has been drifting north over Taupo. Taupo Airport was temporarily closed this morning by the plume overhead. Ashfall(?) has been reported at Rotorua Airport.
  • Data from COSPEC measurements of SO2 gas output from Ruapehu made during flights on Friday 13 and Tuesday 17 October have now been analysed by Dr B.W. Christenson. SO2 output from Ruapehu on both these days was >10,000 tonnes/day. This is a large discharge when compared with other erupting volcanoes such as Pinatubo and Etna, where discharges around 5,000 tonnes/day have been measured during eruption periods. The Ruapehu data may be obscured by effects of ash in the measured plumes but this will not alter the fact that SO2 discharge has been large during the last week since the Crater Lake was removed.
  • The SO2 discharge probably results from at least two sources:
    1. Remobilisation of sulphur contained in the upper part of the volcano conduit system as elemental sulphur deposited over at least the last 50 years.
    2. SO2 discharged as magmatic gas derived from new magma involved in the present eruption episode.
    The proportions of gas from these two sources cannot be determined.
  • The gas discharge indicates that a substantial volume of magma is freely degassing beneath Ruapehu volcano. This is apparently occurring today, during a period of low seismicity, indicating that free gas streaming is occurring under open vent conditions. A further COSPEC measurement is about to be made today.
  • SO2 is a toxic gas which could produce a significant gas hazard on the upper slopes of Ruapehu, and possibly beyond, in suitable wind conditions.

Conclusions and Alert Status

The present eruption episode is continuing at a reduced level, with weak ash but strong gas emissions. We advise that the Alert Level remains at 3 for Ruapehu volcano.

I.A. Nairn (for C.J.N. Wilson)
Volcanology Programme Manager