Ruapehu - IGNS - Science Alert Bulletin V95/10

21 September 1995
Ruapehu Crater Lake 

BACKGROUND
 
On September 20 two scientific parties visited Mt Ruapehu to assess the
affects of the recent eruptions and the status of activity at Crater Lake,
Mt Ruapehu.  Weather conditions had prevented any earlier visits to Crater
Lake.
 
The present activity at Mt Ruapehu is a continuation of an eruptive
episode which commenced in January 1995. Small phreatic (steam) driven
eruptions started about January 11 and continued to about February 27.
The Alert status at that time was level 2.  This was on the old system
which started at level 1.  The lake cooled during March and on April 4 the
alert status was reduced to level 1.  The lake temperature started to rise
again on April 16.  It rose rapidly April 24-26, with further phreatic
eruptions from April 27.  The Alert status was raised to level 2 on April
27, and minor eruptions continued until late May, when again the lake
started cooling.
 
On June 29 and July 3 vent clearing eruptions accompanied by volcanic
earthquakes occurred.  These were the largest discrete eruptions from
Crater Lake for some years.  The temperature of Crater Lake was falling
during late July and August.  On 12 September the Alert Status was lowered
to 1 as the lake was cooling and seismic monitoring showed lower levels of
activity, although results from chemical analysis of lake waters indicated
fresh lava was available to the Crater Lake system.
 
UPDATE
 
On Monday September 18 at 0807 h a moderate vent clearing explosive
eruption occurred through Crater Lake.  The eruption generated a flood and
lahar down the Whangaehu River.  A small mudflow also occurred on the
upper reaches of the Mangaturuturu Glacier. A second smaller event
occurred at 0122 h on 20 September and deposited fine muds about the
summit region and produced another, smaller, flood down the Whangaehu
River.  Both of these events were accompanied by volcanic earthquakes.
Observations at Crater Lake on September 20 confirmed the extent and size
of the two explosive eruptions.  The temperature of Crater Lake has risen
about 20-C to 48-C, and frequent small phreatic eruptions are occurring
within the lake (15 observed on September 20).  New scoria bombs were
found on the lake margins, and a very large overflow was occurring.  Lake
water chemistry show increased reaction between fresh lava and lake
water.  Geodetic data has also shown an increase in the diameter of the
crater.
 
Taken together, the above observations indicate that the vent beneath
Crater Lake is relatively open, and that new lava has/is being emplaced at
shallow depths beneath and within the lake.  Further relatively small
phreatic eruption will occur (similar to those in Jan-Feb and April- May)
with the possibility of ejecta being thrown out of the lake.  At this time
we have a change in the level of ongoing activity, with minor phreatic
eruptions and occasional moderate explosive events.
 
There is an increased likelihood of larger, possibly damaging eruptions.
 
ALERT STATUS
 
Based on the presently available information, in particular the scientific
observations of 20 September and data reduction 21 September we recommend
that the Alert level at Ruapehu be raised from level 1 to level 2.  A
further increase in Alert level may result if present trends continue at
the volcano.
 
Please note that this formal advice of the change in Alert Status was
preempted by a Press Release from DOC (Tongariro-Taupo Conservancy) on 20
September following informal radio communications between DOC staff, the
GNS staff on the volcano, and GNS staff at Wairakei, about the level of
hazard to parties intending to climb Mt Ruapehu on September 20.
 
Dr C J N Wilson
Volcanology Programme Leader