Ruapehu - IGNS - Science Alert Bulletin V95/07

7 July 1995
Ruapehu Crater Lake
Since the last SCIENCE ALERT BULLETIN (V95/06) of 28 June 1995, the
largest volcanic earthquake (magnitude ML 3.2) recorded at Ruapehu since
1988 occurred at 0822 h on 29 June.  A smaller (ML 2.4) volcanic
earthquake occurred at 1133 h on 3 July.  Both these seismic events were
accompanied by relatively small eruptions in Crater Lake.   Heavy cloud
cover prevented observation of the 29 June eruption, but its occurrence is
inferred from cessation of transmission from the Crater Lake ARGOS
monitoring installation at sometime between 0800 and 1000 h on 29 June
(and the subsequent discovery of its total destruction).  Steam columns
above Crater Lake accompanied the 3 July event, and its minor effects were
observed from the air shortly after the eruption.  The 29 June eruption
was the largest to have occurred certainly in the last 2 years, and
probably since 1988. It followed a period of very intense volcanic tremor
on 26 - 27 June (see SAB V95/06).
Chemical analysis of Crater Lake water samples, the latest collected on 4
July, shows that concentrations of magnesium and chloride have both
increased since April 1995, as has the Mg/Cl ratio, to reach the highest
values for several years.  The >100% increase in Mg concentration over the
last three months is consistent with injection of new lava into the Crater
Lake vent.  Such injection may have occurred during the periods of strong
volcanic tremor recorded in April, May and June of this year.
Crater Lake surface temperatures have decreased from a peak of 46oC in May
1995, to be at 33oC on 4 July. This result is unexpected from the tremor
levels, and is not presently understood.  A deformation survey of the
Crater Lake area carried out on 4 July found no evidence of inflation of
the crater area, suggesting that no significant overpressures existed in
the volcanic system beneath Crater Lake at that time.
Taken together, the above observations suggest that the vent beneath
Crater Lake is relatively open, with a strong possibility that new lava
has been emplaced at shallow depths beneath the lake.  Further relatively
small eruptions can be expected, similar to those which have occurred in
the last few months, and possibly throwing ejecta to several hundreds of
metres from the lake margins.   Larger eruptions which could generate
lahars cannot be precluded, but these can occur at any time at Ruapehu,
including during quiescent periods.
Based on presently available information we recommend that the Alert Level at
Ruapehu be maintained at 2, as should be restrictions on access to Crater Lake
basin.  Large eruptions at Ruapehu can occur without useful warning, and it is
important that the lahar Early Warning System be kept operative at all times.
C J N Wilson
Volcanology Programme Leader
Contact (07) 374 8211