Ruapehu - IGNS - Science Alert Bulletin V95/19

28 September 1995
09.00 hours
Ruapehu Crater Lake
Situation Summary
As of 09.00 the following new information is available.
*     Levels of volcanic tremor have remained moderate for most of the
past 24 hours, with some minor fluctuations.  Several volcanic earthquakes
have been recorded overnight, three of them between 02.15 and 03.40, and
these are thought to reflect larger, discrete explosions.  The most recent
volcanic earthquakes were recorded at 07.36 and 08.39, but both of these
were much smaller than yesterday evening's event
*     Observations this morning show that the activity is generally mild,
with puffs of ash-bearing steam rising from the crater.  There was a
spectacular discrete explosion last night at 06.40, which was seen by
Institute observers and the assembled media, and several more probably
occurred during the night, spaced at intervals of a few tens of minutes to
several hours.  The Whangaehu River was reported by New Zealand Rail
observers as being back to a normal level but still carrying large amounts
of sediment.  The river bed has been scoured by the passage of the lahars
to a level estimated to be at least a metre below its old position.
*     Three flights carrying Institute scientists obtained close-up views
into the crater afternoon.  One group saw the remnants of Crater Lake
which is now at a level some tens of metres below the outlet and steaming
vigorously.  The nature of the eruption plume this morning implies that
the lake is still present at the time of this report.
*     Yesterday's direct observations by scientists of the southeastern
crater wall in the vicinity of Pyramid peak indicate that there has been
significant erosion, caused by the fallout of ejecta and water during the
explosions.  This portion of the wall has been recognised for many years
as a weak point of the crater rim.  There is now an increased possibility
of collapse of a small portion of the crater rim should this pattern
accelerate.  However, due to the lowered level of water in Crater Lake the
risk of lahars associated with such an event is minimal.
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