Jan 14 1995, 12:00 pm - Ruapehu Volcano

Small phreatic explosions commenced by Friday 13 January, as part of the current heating cycle at Ruapehu Crater Lake. This change to an open vent situation was forecast by institute staff in mid-December, on the basis of temperature data from the lake. Two small disturbances were observed during a routine inspection yesterday, and scientists found evidence for earlier events recorded in strandlines of sulphur on the lake shore.

Current Cycle of Events

This current cycle began in the middle of November 1994. Data from the ARGOS satellite link indicates that the lake temperature has risen steadily from 15 °C in November to reach a current value of 48°C, a rise of 33 °C in 50 days. In addition levels of acoustic noise in the lake, also recorded by the satellite, have dropped as heating commenced and remain low.

Seismicity at Ruapehu has remained at a low level which is typical for periods of increased heat flow into the lake. This is typical of the cycles of activity at the volcano since 1984.

A survey of the crater diameter on Friday indicated no measurable inflation of the crater which implies no significant movement of fresh magma beneath the lake.

All these indicators suggest that the current cycle of activity is a typical one and there is no indication that a major eruption episode, as recorded in 1969 and 1975, is yet under way.

Background

Over the last 10 years there have been 15 discrete heating episodes in the lake, 11 of which have culminated in periods of small eruptions lasting a few weeks, as observed yesterday. The last period of such eruptions was in January - February 1994.

Cycles of heating and cooling at Crater Lake generally last 9 to 12 months. The typical pattern of cycles since 1984 has been for heating to be followed by small geyser-like eruptions that pose little threat beyond the summit of the mountain. Less commonly in the past heating episodes have been followed by larger explosive eruptions, for example in 1971.

During cycles since 1984 the area of significant volcanic risk has been confined to the summit of the mountain generally to within the immediate vicinity of Crater Lake.

Alert Status

The volcano remains at Scientific Alert Level 2 but we recommend that people avoid the basin of Crater Lake while the disturbances continue. While a dramatic increase in the level of activity remains unlikely, Ruapehu's behaviour prior to 1984 has been characterised on occasion by sudden unpredictable changes in Alert Status and a conservative approach to hazard zonation is justified on these grounds.

C.J.N. Wilson
Volcanology Programme Leader