Ruapehu - IGNS - Science Alert Bulletin V96/14

June 20 1996 (Thursday) 1600 NZST (UT +12)

Ruapehu Volcano


Overnight the intensity of seismicity at Ruapehu decreased to
levels that are lower than those recorded since prior to the
September-October eruption. Based on observations during an over
flight this morning the low levels are related to the very open
vent situation.  The crater now has a flat floor at about
120-150m deep below pre-1995 lake overflow level. Volcanic gases
are freely streaming through the rubble on the crater floor,
to feed the eruption column. As the gas velocity changes so does
the amount of ash in the eruption column. As the vent area is so
open there is no necessity for gas pressures to build very high,
hence the low level of shallow seismicity.

An eruption column of dark grey ash was generated at 1144h to
over 600m and only the weakest of seismic signals was recorded at
Dome. The active vent is continuing to produce weak-moderate ash
emissions, which are being blown off down wind and continue
to reach over 100-150km from the volcano.

There is no evidence that this week's eruption has altered the
nature of the eastern wall around the active crater.  In places,
especially on the northern slopes of Ruapehu, some large volumes
of loosely compacted scoria and ash have formed, and may well be
remobilised during rainfall. The debris has not frozen into place
at this time. Usually rainfall at this time will fall as snow
above 15-2000m. However there remains a high possibility that
these deposits could remobilise. The catchments most likely to be
affected are the Whakapapaiti clockwise around to the

A COSPEC flight to monitor the levels of sulphur dioxide in the
plume was completed late on Wednesday, and data reduction
indicates a SO2 flux of 4100 T/day.

The alert level remains at 3.

BJ Scott