Ruapehu - IGNS - Science Alert Bulletin V95/15

26 September 1995
Ruapehu Crater Lake
The Institute of Geological & Nuclear Sciences will be holding a Press
Conference at 15.00 hours today for an update on the status of the
volcano.  The venue is the Institute's Wairakei Research Centre, 400 m
east of Highway 1 on Karetoto Road, which leaves Highway 1 about a
kilometre south of Wairakei Resort Hotel.
Situation Summary
As of 10.00 the following information is available.
*    From about 16.00 hours (Monday) to 23.00 hours, low to moderate
tremor levels were recorded, accompanied by occasional explosions large
enough to be recorded seismically.  The explosions recorded on the
seismometers were only moderate in size, and not as large as those which
occurred yesterday morning.  The tremor levels were moderate from 23.00 to
01.00 and low from 01.00 to 09.00 hours, with minor fluctuations.  Only
one significant volcanic earthquake has been recorded since 23.00 hours
(at 06.20 hours; this corresponded with a minor explosion observed by our
field party).  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 Wahionoa 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.
Summary for Monday 25 September
Yesterday's activity resulted in the following features as seen by Institute
scientists and
other observers.
*    Explosive eruptions have continued at varying levels of size and
spacing in time.  None of the individual explosions was as large as that
which occurred on Saturday afternoon, but they were generally occurring at
such close-spaced intervals that they fed a plume which was up to 8-10 km
high.  The plume was blown towards an easterly quadrant and traces of ash
deposited to the east coast.  Total ash fall on the Desert Road was
roughly a millimetre, but the road was closed as a precautionary safety
measure in the afternoon and overnight.
*    In the area of the crater itself, Institute observers verified that
the lake outlet was dry, but could not determine whether the lake still
existed.  However, the nature of explosive eruptions observed yesterday
imply that some significant amount of water was left in the crater.  The
large blocks ejected by explosions are reaching no further from the crater
than in the Monday 18 or Saturday 23 September explosions (i.e. to a
maximum of about 1 km), but significant amounts of ash are continuing to
build up in the immediate area of the vent.
*    Lahars have continued to flow down the Whangaehu valley; and at least
one new lahar was observed down the Mangaturuturu valley.  No further
lahar activity affected the Whakapapaiti catchment.
*    A sample of ash collected from the Desert Road has been examined by
Institute scientists.  The particles range in size mostly between 10 and
250 microns, and about 30-60% of the ash consists of new magma disrupted
by the explosions.  Other material in the ash is identified as disrupted
material that originally formed sediments on the floor of Crater Lake,
that is, altered sediments, some hydrothermal minerals, and sulphur,
together with some fragments of rocks which have been torn from the walls
of the vent by the explosions.
Conclusion and Alert Status
The eruption involving new magma is continuing at Ruapehu.  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.
For further information contact:
Dr C J N Wilson, Programme Leader, Volcanology.
Ph: (07) 374 8211; Fax (07) 374 8199