Ruapehu - IGNS - Science Alert Bulletin V95/50

Friday 20 October 1995
1000 hours NZDT (UT +13)
Ruapehu Volcanic Activity

Situation Summary

As of 1000 hours, the following information is available.

*     Tremor levels have remained low at Ruapehu since the last bulletin
at 1700 h on 19 October.  The number of degassing eruption type events
(without sudden onsets) has declined in size and frequency since about
0000h 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 BW Christenson. SO2 output from Ruapehu on both these days was >10
000 T/D.   This is a large discharge when compared with other erupting
volcanoes such as Pinatubo and Etna, where discharges around 5 000 T/D
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.

IA Nairn  for 
Dr CJN Wilson, Programme Leader, Volcanology