Ruapehu - IGNS - Science Alert Bulletin V95/40

Friday 13 October 1995
1630 hours NZDT (UT +13)
Ruapehu Volcanic Activity

Situation Summary

As of 1630 hours, the following information is available.

*     Tremor levels have been relatively low all day, accompanying mild,
continuous emission of ash and steam.  No volcanic earthquakes have been
recorded in the  past 24 hours.

*     Activity at the volcano has been reduced all day, with continuous
weak eruption of ash and steam.  An observation flight this afternoon
showed that there were two  vents active, one in the north and the other
in the south part of the crater lake basin.  The sites of these vents seem
to coincide with what had previously been observed as areas of upwelling
in the former crater lake.  Ash and steam were being continuously emitted,
with more ash coming from the southern vent. A low-level plume of haze was
seen drifting to the NE or NNE.

*     Results of our field studies suggest that some initial reports of
ash thicknesses  given yesterday were overestimates, often because strong
winds had caused drifting of ash.  The volume of the ash fall is still
estimated to have been between 0.01 and 0.05 cubic kilometres.

*     The style of activity at Ruapehu over the next few days at least is
likely to alternate between two styles

Low level activity (like that seen today), with weak, often continuous
emission of steam and limited amounts of ash, forming plumes which rise
only hundreds of metres to 1-2 km above the volcano.  Ash deposition is
confined to the immediate area of the volcano summit.

High level activity (like that seen on Wednesday night/Thursday morning),
with episodes lasting tens to possibly hundreds of minutes of powerful,
continuous activity generating plumes up to 10-12 km high and depositing
ash fall to tens or more kilometres from the volcano.  The areas affected
by the ash fall will depend on the wind direction at that particular
time.

While the general levels of volcanic tremor remain above the background
levels seen when the volcano was not active, and the vents remain
unblocked, it is most likely that these two styles of activity will
continue to alternate for at least some days to come.

Summary and Alert Status

The present eruption episode is continuing at a reduced level. We advise
that the Alert Level remains at 3.

Dr CJN Wilson, Programme Leader, Volcanology