Ruapehu - IGNS - Science Alert Bulletin V95/30

Wednesday 4 October 1995 
18.30 hours NZDT (UT +13)
Ruapehu Volcanic Activity 

Situation Summary

As of 18.00 hours the situation is as follows.

*     Volcanic seismicity remained at a low level, but with more activity
than Tuesday. Short periods of volcanic tremor occurred, mostly as bursts
of only a few minutes, but one episode of fluctuating, low to moderate
intensity occurred from 11.07 to 12.20 hours.  At least one of the periods
of tremor (around 09.50 hours) coincided closely with  eyewitness reports
of the formation of a visible plume.  Two discrete volcanic earthquakes
were recorded at 07.38 and 11.41 hours today, the former accompanied by
production of a vigorous steam plume (see next item).

*     Steam-rich plumes rising to heights sometimes exceeding 3-4 km were
seen above the volcano, and at least two of the plume-forming episodes
were linked to seismic events.  Minor amounts of ash were deposited from
the bases of these plumes, but ash fall was within only 1-2 km of the vent
and formed thin deposits.  A dusting of ash on top of overnight snow for
1-1.5 km to the NE of the crater, seen by airborne Institute observers at
around 09.30, is thought to have been generated by the plume accompanying
the 07.38 volcanic earthquake.  These observers also reported a 'wet'
appearance of one of the benches within the crater which suggested that
water may have 'sloshed' around in the crater during the 07.38 event.  At
09.30, the main vent contained sediment-loaded low fountains of water from
a narrow source in the middle of the crater lake.  An Institute scientist
reported at about 11.00 that the lake in the crater was very shallow,
partly a mud slurry, with numerous fumaroles (steam vents) discharging
through it.

*     Institute field parties were active on the eastern, southern and
northern flanks of the volcano, continuing the work reported yesterday.
Preliminary results of the COSPEC flight from yesterday (tentative and
subject to confirmation) indicated that the sulphur dioxide output was
about 200 tonnes per day, which is only about  5-10 % of that recorded on
26 September.

Cautionary note

A large amount of loose ash and debris is now present on the upper slopes
of the mountain.  This material is easily eroded by heavy rainfall, and
this process may generate lahars, quite independent of activity at the
volcano or the presence of water in the crater.  The weather forecast for
the next 24 hours or so indicates the likelihood of significant rainfall
to quite high elevations on the volcano, and there is the possibility of
some lahars being generated by rainfall re-mobilization of material.

Conclusions and Alert status

Mild eruptive activity is continuing on the volcano.  Small explosive
eruptions of the size seen yesterday and today are likely to continue, and
we advise that the Alert Level remains at 3.

Dr C J N Wilson, Programme Leader, Volcanology. 

Ph: (07) 374 8211; Fax (07) 374 8199