Mar 28 1996, 1:00 pm - Ruapehu Volcano

Situation Summary

On Tuesday and Wednesday, 26 and 27 March GNS and DoC staff have been working in or around the summit crater. They have observed a very small island which has appeared in the crater lake near its south-western shoreline. Observations were hindered by steam and fume emissions, but the island appears to be the top of a small lava extrusion (spine), which is presently emerging onto the floor of the crater. Observers have reported vigorous steam and gas emissions and sediment-laden geysering about the margins of this extrusion. Its visible extent above lake level is very small and is estimated to be about 15 m high and 20 m across. It may be part of a larger body, which until now has been concealed beneath the lake. The lake level remains about 80 m below overflow and the temperature is 50°C.

Up to present the extrusion process has been passive and is not clearly reflected in the parameters monitored at Ruapehu. The mean daily amplitude of the seismic energy as derived from Dome has been constant during the first 3 months of 1996. The period of low levels in early January reflects a time when the Dome installation was malfunctioning, rather than low seismic energy. The dominant frequency of the mean daily signal is about 6 - 7 Hz, which has been the dominant frequency since late October 1995. The spikes reflect the discrete high-frequency volcanic earthquakes which occur at rates of 20 - 60 per day. There has been an apparent increase in these since about day 56 (February 25). These are the type of earthquakes that could be expected to accompany an intrusion-extrusion. If this is so intrusive-extrusive activity has been occurring passively since late February, but until now has been concealed beneath the lake.

Tilt measurements at Dome were last made on March 9, showing a small decline from the previous measurement on January 18. No significant trends are apparent in the tilt data. Other data from Knoll also show no significant changes. The crater rim EDM network was reestablished this week. The flank EDM lines have not been occupied since late November.
A COSPEC flight on 28 February obtained a result of 500 tons per day. Down on other post eruption measurements.

Alert Status: Level 2

Activity at Ruapehu volcano has been at a low level for some months, by comparison to September - October 1995, but has been higher than it was pre-September 1995. As a result of this we have maintained the Alert Level at 2 and stated that this should not necessarily be interpreted to mean either that the present eruption has ended or that there are no hazardous areas on the volcano. The possibility of isolated explosive eruptions without useful warning signals has always been present.

The appearance of this island, which may reflect processes which have been ongoing for 4 - 5 weeks, could indicate two scenarios:

• passive intrusion-extrusion will continue as is; or
• further explosive eruptive activity could develop.

The data available from the scaled down monitoring programme in place at this time indicates no significant departures in any of the data sets. In particular, at present there is no deep-seated volcanic tremor which was observed to accompany the explosive activity in September - October 1995. This would tend to suggest the status quo will continue, at least until parameters change. However, with hot lava being extruded into the lake at the present time, the occurrence of relatively small-scale, shallow-sourced explosive activity is possible.

Hazard Zones at Level 2

We draw the readers attention to the current Hazard Zone map (issued 29 November 1995) which shows hazard zones that have applied over the summer. These reflect the situation at Alert Level 2 and are not affected by the appearance of the extrusion.

We have subdivided the hazards in the RED and ORANGE zones into two sub-zones. Sub-zone 1 reflects processes associated with explosive eruptions and gas emissions from the active crater, while sub-zone 2 reflects the lahar and debris flow hazards. In some places both can exist together.

The hazard zones are as follows:

1. RED 1 is a zone within 800 metres radius of the centre of the active summit crater. Within this zone there remains a high risk of ballistic block impact, and the possibility of pyroclastic fall and surge activity. Also, within this zone there is a significant hazard from volcanic gas.

   RED 2 is based on the Whangaehu catchment and reflects the lahar hazard in that area. This hazard is related to expulsion of the re-forming lakes and/or slumping of material erupted into this catchment during earlier phases of the eruption.

2. ORANGE 1 reflects a zone of lesser hazard due to ballistic block impact, pyroclastic fall and gas hazard about the active summit crater.

   ORANGE 2 reflects a risk from secondary lahars and debris flows which exist in the Wahianoa and Mangatoetoenui catchments.

3. YELLOW zones represent hazards from the thawing out of erupted ejecta, the specific level of risk varying with ice and snow volumes, slope and thickness of volcanic debris. Minor secondary lahars and debris flows could occur in the Mangaturuturu, Whakapapaiti, Whakapapanui catchments and on the steep slopes above Turoa.

Other areas may be affected by ashfall under conditions of uncommon wind speed and direction. Areas affected by the volcanic gas hazard are very dependent on wind direction and velocity. The hazard zone could extend beyond the mapped area under favourable conditions. Caution should be exercised in all sectors down wind of the active summit crater. Certain secondary hazards are largely unrelated to discrete explosions from the active crater and will continue to be the dominant hazards.

We advise that the Alert Level remains at Level 2.

B.J. Scott (for C.J.N. Wilson)
Volcanology Programme Leader