Sep 28 1995, 9:00 am - Ruapehu Volcano

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.

C.J.N. Wilson
Volcanology Programme Leader