Ruapehu - IGNS - Science Alert Bulletin V95/18

27 September 1995
18.00 hours
Ruapehu Crater Lake
 
Situation Summary
 
As of 17.30 the following new information is available.
 
*     Moderate levels of volcanic tremor have continued all day (following
the increase which occurred around 04.00 hours this morning), though with
levels fluctuating somewhat on a 2-3 hourly basis.  Volcanic earthquakes
(up to a size equal to those recorded during Monday's eruption) occurred
during the moderate eruptive episode from 06.00 to about 07.00, but at
longer-spaced intervals.  Since then, no significant volcanic earthquakes
have been recorded.
 
*     After the period of moderate-scale explosive activity early this
morning, moderate to low levels of explosions have continued through the
day.  During an observation flight by Institute staff this afternoon, they
could see some details within the crater.  The outlet was dry, and the
water level had dropped by some tens of metres, exposing a terrace that
had been formed during the 1945 eruption but which had been concealed
underneath the Crater Lake until this eruption.  The actual water level
was not visible, but clearly the lake is very much smaller than it used to
be.
 
Cautionary note
 
The observations that Crater Lake has now largely vanished and the outlet
is dry should not be taken to indicate that the future threat from lahars
has disappeared or will do so shortly.  The loose ash and debris on the
upper slopes of the volcano are easily eroded by heavy rainfall, and this
process may generate further lahars quite independent of activity at the
volcano or the presence of water in the crater.
 
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.
 
For further information contact:
Dr C J N Wilson, Programme Leader, Volcanology.
 
Ph: (07) 374 8211; Fax (07) 374 8199