Ruapehu - IGNS - Science Alert Bulletin V95/16

26 September 1995
17.50 hours
Ruapehu Crater Lake
 
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 ESE 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 1600 hours 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.
 
For further information contact:
Dr C J N Wilson, Programme Leader, Volcanology.
Ph: (07) 374 8211; Fax (07) 374 8199