Jul 7 1995 - Ruapehu Volcano

Since the last Science Alert Bulletin (RUA-1995/06) of 28 June 1995, the largest volcanic earthquake (magnitude M_L 3.2) recorded at Ruapehu since 1988 occurred at 08:22 on 29 June. A smaller (M_L 2.4) volcanic earthquake occurred at 11:33 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 08:00 and 10:00 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 RUA-95/06).

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 46 °C in May 1995, to be at 33 °C 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.

Alert Status

Based on presently available information we recommend that the Alert Level at Ruapehu be maintained at Level 2, as should 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