Sep 26 1995, 10:00 am - Ruapehu Volcano

Situation Summary

As of 10:00 the following information is available.

• From about 16:00 (Monday) to 23:00, low to moderate tremor levels were recorded, accompanied by occasional explosions large enough to be recorded seismically. The explosions recorded on the seismometers were only moderate in size, and not as large as those which occurred yesterday morning. The tremor levels were moderate from 23:00 to 01:00 and low from 01:00 to 09:00, with minor fluctuations. Only one significant volcanic earthquake has been recorded since 23:00 (at 06:20 hours; this corresponded with a minor explosion observed by our field party). All our seismographs (including that at the Dome Shelter) are operating and providing valuable data.

• A field party of observers from the Institute visited the volcano between 06:00 and 07:00 and reported a plume drifting east-south-east being fed by numerous weak explosions. There has been continued minor ash fall overnight to the east of the volcano. The deposit of a very small lahar was observed in the Wahianoa Valley, and this is inferred to have been formed at some earlier stage in the eruption. There was a reduced level of lahars (when compared with yesterday) down the Whangaehu valley.

Summary for Monday 25 September

Yesterday's activity resulted in the following features as seen by Institute scientists and other observers:

• Explosive eruptions have continued at varying levels of size and spacing in time. None of the individual explosions was as large as that which occurred on Saturday afternoon, but they were generally occurring at such close-spaced intervals that they fed a plume which was up to 8 - 10 km high. The plume was blown towards an easterly quadrant and traces of ash deposited to the east coast. Total ash fall on the Desert Road was roughly a millimetre, but the road was closed as a precautionary safety measure in the afternoon and overnight.

• In the area of the crater itself, Institute observers verified that the lake outlet was dry, but could not determine whether the lake still existed. However, the nature of explosive eruptions observed yesterday imply that some significant amount of water was left in the crater. The large blocks ejected by explosions are reaching no further from the crater than in the Monday 18 or Saturday 23 September explosions (i.e. to a maximum of about 1 km), but significant amounts of ash are continuing to build up in the immediate area of the vent.

• Lahars have continued to flow down the Whangaehu valley; and at least one new lahar was observed down the Mangaturuturu valley. No further lahar activity affected the Whakapapaiti catchment.

• A sample of ash collected from the Desert Road has been examined by Institute scientists. The particles range in size mostly between 10 and 250 microns, and about 30 - 60% of the ash consists of new magma disrupted by the explosions.Other material in the ash is identified as disrupted material that originally formed sediments on the floor of Crater Lake, that is, altered sediments, some hydrothermal minerals, and sulphur, together with some fragments of rocks which have been torn from the walls of the vent by the explosions.

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