Ruapehu - IGNS - Science Alert Bulletin V95/11&12
24 September 1995
Ruapehu Crater Lake
Situation Summary
The eruptions at Ruapehu have continued through the past 24 hours,
presenting scientists from the Institute of Geological and Nuclear
Sciences with a wealth of new data and information. The conclusions below
are based on four types of observation over this period and over the past
week.
1. The Wairakei Research Centre is recording types of seismic activity
which have never previously been seenat Ruapehu. These include extended
periods of low frequency volcanic tremor and some unusual low-frequency
earthquakes.
2. Our data show that a larger component of fresh magma (molten rock)
has been erupted last week, than in any eruption since 1945.
3. There are increased levels of magmatic gas, specifically SO2,
recorded as chemicals dissolved in the water samples we have recovered
from Crater Lake.
4. There have been 3 lahar-producing events in 1 week, accompanied by a
high frequency of other explosions through Crater Lake.
We thus conclude that there is a high probability of events of a similar
size (and likely accompanying effects) to the lahar-forming eruptions of
Monday 18 and Saturday 23 September occurring in the future, spaced at
intervals that are irregular (and thus unpredictable) but which would
typically be days apart. Smaller eruptions (but still of a size to pose
extreme risk to people on the summit area of the volcano) may be expected
at irregular intervals of minutes to hours. We emphasise that the exact
timing of all these eruptive events are unpredictable.
Our data also imply that there is a significant possibility that the
activity to date is an introduction to a larger eruption, perhaps of the
size and type last seen in 1945.
On the basis of available information we thus advise that the Alert Level
at Ruapehu remain at Level 3.
Daily Summary
During the past 24 hours, the volcano has shown a variety of
activity.
1. Small explosions, sending steam-rich plumes to heights of a few
hundreds of metres have continued sporadically throughout this time
period. The effects of these explosions have generally been confined to
the area of the crater itself, and they are not powerful enough to
register on our seismometer at our Chateau seismograph. Nonetheless, they
represent an extreme hazard to anyone in the vicinity of the crater lake.
2. Medium-sized explosions, which have been a particular feature of the
activity from about 18.00 hr until nightfall. These are ejecting
steam-rich but ash-bearing plumes which have risen to heights of 500 to
over 1500 m above the volcano. These explosions leave a distinctive,
modest seismic signature as recorded by us at the Chateau seismograph. At
the time of writing, this activity is a continuing feature of the
volcano.
No further lahar-producing eruptions have occurred in the past 24 hours.
We are continuing to monitor the situation and advise the Ministry of
Civil Defence and Department of Conservation. We emphasise again that
Ruapehu is an actively erupting volcano, and people should stay clear of
the upper slopes of the volcano and valleys (particularly the Whangaehu)
which may act as paths for lahars.
Correction
In Media Release V95/11 of 23 September 1995, reference was made to two
smaller lahars generated on the Summit Plateau of Mount Ruapehu entering
branches of the Whakapapanui Valley. This should have read the
Whakapapaiti Valley. Apologies for the error.
For further information contact:
Dr C J N Wilson, Programme Leader, Volcanology
Mr B J Scott, Manager Volcano Surveillance
Ph: (07) 374 8211; Fax (07) 374 8199